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Offline Scott S

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So, you want to run pods?
« on: April 30, 2020, 02:50:14 am »
 Since this is a question that gets asked on a weekly basis, often several times a week, I thought it deserved it's own thread/sticky.

 There are constantly people asking "I have pods and an open exhaust. What should my jetting be?".  Well...how long is a piece of string?

 The simple answer is, there is no simple answer. We don't know what brand pods, what brand exhaust, the condition/tune of the engine, your altitude, the amount of back pressure, the amount of wear on the carbs slides or needles, and probably a dozen other factors. Not all pods are created equal.

 The ONLY way to know the proper jetting for sure is  dyno time. Be it professional tuning on a "rolling road", or plug chops and the "butt dyno".
 Chances are "just go up on the main" ain't gonna work.  The stock intake is a well engineered system, meant to work throughout the rev range. It also offers some form of weather protection. Pods, in general, will always sacrifice somewhere, be it a flat spot in the mid-range, too rich down low, too lean up top, etc. Not to mention what happens if you get caught in the rain or crosswinds

 So many people mention the difficulty of removing and replacing the carbs with the stock air box. True, it can be a bit of a pain, but fresh intake rubbers helps immensely, and it's not like the carbs have to come off every weekend.

 People mention the "performance" factor. "I added them to help the engine breathe better", only to tape up half the surface of the pods.

 And, of course, just for style.

 I have asked a few local experts to weigh in on this subject. On how the stock system works and why. On what to expect when you make changes. These people have decades of experience with the SOHC4; as engineers, mechanics, racers and real world experience.
 As this thread gets going, feel free to add your personal experiences. Be sure to include as much info as possible about the engine combo, jetting changes, location, etc.

« Last Edit: May 01, 2020, 12:53:42 pm by Scott S »
'71 CB500 K0
'17 Triumph Street Scrambler
'81 Yamaha XS650

Offline HondaMan

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Re: So, you want to run pods?
« Reply #1 on: April 30, 2020, 06:42:44 pm »
Here's some info on how the carbs work, so you can see what happens when you try to use pod filters (or velocity stacks).
Refer to the picture(s) below, modified from my book.

The fuel mix depends on there being 3 distinct pressure zones in the carb, plus one intermixed zone that is made from 2 pressures in the airbox-to-carb throat area. In the accompanying pictures here, these pressure zones, relative to an imaginary 0 pressure, are:
ATMOSPHERE – about 14.7 PSI at sea level (BLUE in the pictures)
MIXED – about 9 PSI at half throttle, 12 PSI below ¼ throttle (BLUE+GREEN in the pictures)
AIRBOX – about 9 PSI at full throttle, 10 PSI at mid-throttle (GREEN in the pictures)
VACUUM – from less than 1 up to 3 PSI on the engine’s side of the slide (YELLOW in the pictures)

(NOTE: with apologies to those of you who are Blue-Green colorblind, sorry…)

Bernoulli’s principles are at work here: the faster air is moving, the lower is its pressure, and vice-versa. That’s why dead-still atmosphere air is the highest pressure involved here.

First thing to remember: the highest, ATMOSPHERE, pressure is fed to the float bowls via small vent hoses that draw their air pressure from a still, quiet spot on the bike, like behind the airbox, near the battery (some reach up under the seat). This is the force that pushes fuel up through the jets when the throat of the carb is at lower pressures.

Second thing to remember: the air filter must slightly restrict the air entering the airbox, to generate a slightly lower pressure for the venturi in the carb. This MUST happen so the higher pressure in the float bowl can push fuel up the jets and into the venturi. (This is why pod filters are trouble, more on that later).

Third thing to remember: the vacuum from the engine is PULSING at low engine speeds and smoother at higher speeds. This means the idle circuit mixes differently from the higher speed mainjet-needle system. They do NOT act the same.

Idle-to-1/4 throttle: the air-bleed screw adjusts the amount of air that is fed to the emulsifier chamber (those tiny holes) of the pilot jet, and corres[ondingly, how much air-fuel mix gets pushed up the jet. This air comes in through the bell area at the back of the carb, near the outside of the passageway where the air is most still (and therefore is at higher pressure than the moving air). When you close the air screw, it pushes LESS fuel up the pilot jet, and vice-versa. This pressure is lower than the float bowl pressure, so when a vacuum hits the top of the pilot jet hole, the high-pressure bowl pushes fuel up, the mid-pressure emulsifier air bubbles the fuel as it rises, and it FOAMS up into the throat of the carb, wetting everything with a mist. This mist gets sucked into the engine during the NEXT intake cycle, not the present one that supplied the vacuum. This is why there is a slight ‘lag’ when you blip the throttle: it is running one engine revolution behind your twistgrip.

Low Midrange - ¼ throttle to midrange, below the “wasp waist” zone in the carb body: by ¼ throttle, the air layer along the edges of the passageway are running slower than the air in the midst of the flow. The slow-moving layer is higher pressure than the moving air, so it matches the pressures on both ends of the pilot jet and that stops working. In effect, the air pressure at the air screw is now the same as at the pilot jet’s tip, so it quits pumping fuel up. Now, the taller needle jet under the slide is sticking up where the fast-moving, lower-pressure airbox air is streaking by, and the high pressure in the float bowl is pushing fuel up through the main emulsifier (also called the “mainjet holder”). The air into this emulsifier is fed on the opposite side of the carb’s bell area, across from the air screw, through a little brass metering hole. This is an airbox-pressure zone, so it is lower than the float bowl, but higher than the moving air in the needle jet’s tip: it has enough force to bubble the fuel in the emulsifier to aerate it as it gets pushed up the emulsifier. The needle in the slide controls how much of this fuel gets sucked out by the passing air, and the engine is running fast enough now that it is pretty much a steady vacuum.

Upper Midrange: Now the slide has raised above the wasp-waist zone of the carb throat, and the vacuum is very steady from the engine, so the whole throat of the airbox-side of the carb is at airbox pressure, which is starting to drop slightly as the filter restricts it: this is necessary in order for the fuel to aerate in the emulsifier, lest it be “clumpy” and rich, and not burn well. Most roadrace bikes with velocity stacks will not run at less than this throttle opening, which is why old movies show the bikes constantly blipping the throttle to keep it alive. (Watch a video of the RC66 Honda 250cc Six for examples). The ‘wasp waist’ in the carb throat causes some turbulence now, to help make the throttle feel linear: in effect, it disrupts the mixture a little bit from 45% to 55% throttle opening so the transition from low-speed mixing to high-speed mixing is not sudden, like a switch – or, like a roadrace bike with carbs. The needle in the slide is metering the amount of fuel to mix with the air at this point.

Full throttle: in the CB750 SOHC4 Honda carbs, there is almost no difference between ¾ and full throttle except for the amount of fuel the petcock can supply (to keep the float bowls full) at full throttle vs. ¾ throttle. The size of the mainjet determines the mix ratio with the passing low-pressure air, and the air must be at lower pressure in the carb throat than in the float bowl or the fuel will NOT rise to the venturi area. When a too-open intake airbox is used, the mixture gets lean above 5/8 throttle because there is no air pressure difference available to push the fuel up to the needle jet.

That is the basic operation of the 2-jet carb as is found in all of the SOHC4 Hondas.

The trouble with ‘pod’ filters or velocity stacks: removing the intake restriction that is intended as part of the mixing system also removes the device that generates the “middle” air pressure in between vacuum levels and atmosphere levels. This removes the ability of these carbs to push fuel up the emulsifiers and to also aerate the fuel, so the mixture becomes uneven and highly non-linear with throttle settings. There is no way for the emulsifiers to aerate the fuel, so the owner chases rich and lean spots, alternating between “flat” (lean) spots and too-rich mixtures that foul the sparkplugs quickly for lack of air. On racing bikes with velocity stack, the throttles are held more-than-halfway open most of the time, so the wasp-waist section of the carb throat generates enough turbulence to help aerate the too-rich fuel mix risng in the needle jet. The idle circuit will usually still work enough to let the bike run a ratty idle, but it requires frequent ‘blipping’ to blow out the fuel buildup that occurs, and the low-range throttle settings are always erratic. You can hear this when the bikes enter slow lanes (or the pits) and when they launch back onto the track from the pits: they burble and spit until they clean out and run again. On the street: this type of riding is impossible in traffic, and the result is fouled sparkplugs. In normal use the plugs should last at least 3000 miles in these bikes before cleaning the plugs: if yours don’t go that far, then something is ‘up’.

Another “anomaly” that confuses sometimes is: the pilot air screw has only a very narrow adjustment range. In the CB750K0-K6 “roundtop” carbs this is from ½ turn to 1-1/2 turns: less than ½ turn has already cut off most of the lifting air to the pilot jet, so the engine suction can only eke out a little bit of fuel at each suction, making it lean. Beyond 1-1/2 turns, the pilot jet’s size cannot pass any more fuel, so it is maxed out (presuming proper jet size). On a 4-cylinder engine, though, while you are adjusting ONE carb, there are 3 OTHER carbs pulling the air through that carb you are adjusting, but at THOSE carb’s speed. If you have a 4-1 pipe and an exhaust mixture analyzer, turning that one carb’s air screw inward too much finally leans out the mixture so far that it will no longer fire in the cylinder, and the unburned fuel dumps into the exhaust, raising the hydrocarbons. This “looks” just like what happens when you turn the air screw too far out, making the mixture too rich and unable to burn, causing too much hydrocarbons. Don’t let this confuse you: turning the air screw inward leans it out, just like in a car engine with a carb and a constant vacuum and an idle mixture screw (but that’s a whole ‘nuther mixing system, won’t go there just now…).

A side note 6/12/2022: Honda discovered in the lean-burning CB650 (with smog-controlled PD carbs as also found on the CB750F2/3 and K7/8) that the turbulence of air passing over the vent tubes for the bowls was causing the CB650 to suffer a "flat spot" in throttle response at hiway speeds when the throttle would get jerked open (like for passing another vehicle). So, they modified the vent tube air supply just like we did for racing, and I show in My CB750 Book, by plugging the bowl vent tubes into the bottom side of the airbox to pick up the most-still air that can be found on a moving bike. It's found in the Service Bulletin here:
http://manuals.sohc4.net/cb650/
as "HSB 650 #1", and it works just the same way. The turbulent, moving air that is supposed to be supplying higher pressure to the bowls becomes lower pressure from the bike moving thru the air, so the fuel gets less 'push' up the emulsifier tubes. Pod-filtered bikes suffer even worse, as there is no airbox to quiet the air.
;)

« Last Edit: June 12, 2022, 09:15:38 am by HondaMan »
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Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
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Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Offline slikwilli420

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Re: So, you want to run pods?
« Reply #2 on: May 01, 2020, 06:03:06 am »
Ill weigh in on this. Im running stacks and sock filters on stock carbs on my race bike, and it runs fantastic with only a main jet change. I would thing the high compression and bigger valves would exacerbate any poor settings on the carbs so I think I am pretty well dialed in. The caveat is that I am running a race bike, so it likes above the top half of the now 10K RPM range, really coming alive around 6,500.

The issue with pods/stacks on the street is multi-fold from what I have gathered. First, stacks and pods are not the same beast. Pods, largely, since most seem to choose the absolute CHEAPEST ones they can find, are not made to replace the internal velocity stack arrangement from our stock air box. Stacks, to me at least, are a far better alternative and can easily be filtered to give the filtration of pods with the flow more similar to the stock air box stacks. The last piece of the puzzle is the newby them self. Anecdotally from seeing how these new riders are building their bikes (skateboard seats, $99 shocks, bullsh*t tires) I am going to assume that they are not riding the bike in the upper half of the RPM range, where the intake signal is strongest and where the bike is designed to be run. Adding pods or stacks to a bike that is intended to putter around town at 2,500-3,500 RPM in a gear too high for the speed is going to make all of it work against the intended outcome.

I was convinced pods were the way for my '76F when I first bought it in my late teens (now nearly 20 years ago) and I put them on and did all the plug chopping I could afford, changing jets up and down, needles up and down, resyncing each time. I spent many full days trying to make it work. I would settle on a good setting that felt like it pulled decently, then the next time I ran it, it was off. It didnt idle, it didnt pull off idle, it only ran OK at above 3,000 RPM. What's a guy to do, put the stock air box with K&N filter in and go. Thing ran like a scalded dog right off the bat. Im convinced that unless you are racing, the stock air box is the only real good option to make this all work with stock carbs.
All you gotta do is do what you gotta do.

Vintage Speed Parts Mashup: http://forums.sohc4.net/index.php?topic=133638.0
Rickman CR Parts Kit Refresh: http://forums.sohc4.net/index.php/topic,154837.0.html
AHRMA CB750 Racer: http://forums.sohc4.net/index.php/topic,158461.0.html
AHRMA Superbike Heavyweight Racer: http://forums.sohc4.net/index.php/topic,173120.0.html
'76F CB750 Patina Redemption: http://forums.sohc4.net/index.php/topic,174871.0.html

Offline dkihlgren

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Re: So, you want to run pods?
« Reply #3 on: May 01, 2020, 11:33:37 am »
I translated this from a swedish site www.braigasen.se
The owner is a real carb-guy

Carburetor with fuel screw
Most common on 4-stroke carburetors (except Mikuni HSR). A fuel screw is always located downstream of the throttle and is pointed

Low speed jet

Start up and warm the engine. Find a comfortable idle turn using the idle screw. Set the fuel screw to 1 turn.

Slowly open the fuel screw (or mixer screw *) - one quarter turn at a time - wait a while for the engine to react. If the speed increases significantly, adjust this down using the idle screw. Continue to open the mixing screw until you notice that the rpm is starting to drop. If the rpm does not decrease even though you have reached 3-4 turns ... Change to a larger low speed jet!
If the speed instead drops as soon as you open the mixing screw more than 1 turn, then screw it in instead. If the speed does not drop even though the screw is almost closed ... Switch to smaller low speed nozzle!
With the right size of the low-speed nozzle, you should have found a rpm hump somewhere between 0.5 and 3 turns.

You have the right low speed nozzle if you find a rpm hump somewhere between 0.5 and 3 turns on the mixing screw

Needle jet

Replace the main jet with one that you drilled to 3mm. Move the throttle needle to the leanest position (clip at the top). Insert a button pin on the twist grip. Then you attach a piece of paint tape to the twist gripts fixed part and mark where you have 1/2, 3/4 and full throttle. Run on 1/4 throttle on any of the higher gears on medium rpm preferably in easy uphill. In this test, the engine will get too much fuel at a certain throttle. It is when this happens that gives us an indication of whether the needle jet needs to be replaced or not.

If you can give more than 3/4 throttle (... maybe even keep full?) The needle jet is too small. Switch to a larger one!
If the engine gets drowned already at 1/2 throttle, the needle jet is too large. Switch to a smaller one!
If the engine is drowned at 3/4 throttle, the needle jet is correct.

Needle

Finding the right throttle needle is perhaps the biggest challenge when it comes to jetting. Mikuni has manufactured thousands and thousands of size-specific needles over the years. What separates all these needles is 1) the diameter of the first straight piece 2) the length of the straight piece before taper, ie before it starts to taper and 3) the conicity, measured in degrees.
    In this situation, we are content with adjusting the needle we have. It has five positions where position 1 (top) gives the leanest mixture and where position 5 (bottom) produces the richest mixture. To test the best needle position, you can test how the engine behaves between 1/3 and 2/3 throttle at different speeds and loads. Another way is to see which needle position gives the best acceleration from about 2000 rpm up to half throttle. Both of these tests are best done in light uphill.

Main jet

Now that you have found the right low-speed and needle jet, it's time to move to the high-speed circuit. Choose a test road where you can stay on full throttle.

Give full from 1/2 throttle and see if it responds well and wants to rev. If it doesn't, try plugging the air filter to 2/3 (... or give it a little choke). If this gets better, you will mount a larger jet.
If the engine fails to turn on and you think it is doing well on full throttle then test to hold full and then turn off a little on the throttle, not much just a little. If you then feel that power increases, well, after all, the main jet is too small.
If you want to make sure that you have chosen the right main jet, you can take the help of a good friend who sits further away on the test road with a timer. When he gives the start sign, you accelerate from stationary up through the gears until you reach the point where he stands and watches you. You change the main jet and make another run. The fastest time gives you the right main jet.
Please complete with a plugchop…

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Offline HondaMan

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Re: So, you want to run pods?
« Reply #4 on: May 01, 2020, 07:08:54 pm »
I translated this from a swedish site www.braigasen.se
The owner is a real carb-guy

Carburetor with fuel screw
Most common on 4-stroke carburetors (except Mikuni HSR). A fuel screw is always located downstream of the throttle and is pointed


Umm...Keihin doesn't make a single carb that I have ever seen in which this is the case. All Kehins use an AIR screw for the idle circuits. A few Harley carbs (notably the old Linkerts) use an adjustable main jet that DOES adjust the fuel orifice, maybe this is what he is referring to? Or, maybe some of the Mikunis he is speaking to have adjustable mainjets, but I have not seen one like that.

Automotive carbs like Holleys, FoMoCo, and Carter, use fuel metering screws for the idle circuits (which is opposite the Keihin method), which confuses some folks who don't know about the differences.
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline seanbarney41

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Re: So, you want to run pods?
« Reply #5 on: May 01, 2020, 09:05:43 pm »
I translated this from a swedish site www.braigasen.se
The owner is a real carb-guy

Carburetor with fuel screw
Most common on 4-stroke carburetors (except Mikuni HSR). A fuel screw is always located downstream of the throttle and is pointed


Umm...Keihin doesn't make a single carb that I have ever seen in which this is the case. All Kehins use an AIR screw for the idle circuits. A few Harley carbs (notably the old Linkerts) use an adjustable main jet that DOES adjust the fuel orifice, maybe this is what he is referring to? Or, maybe some of the Mikunis he is speaking to have adjustable mainjets, but I have not seen one like that.

Automotive carbs like Holleys, FoMoCo, and Carter, use fuel metering screws for the idle circuits (which is opposite the Keihin method), which confuses some folks who don't know about the differences.
diaphragm cv carbs on v4 bikes have a fuel screw...probably others too, but not until the 1980's...sooo, doesn't really apply to sohc4's
If it works good, it looks good...

Offline dkihlgren

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Re: So, you want to run pods?
« Reply #6 on: May 02, 2020, 02:22:51 am »
I translated this from a swedish site www.braigasen.se
The owner is a real carb-guy

Carburetor with fuel screw
Most common on 4-stroke carburetors (except Mikuni HSR). A fuel screw is always located downstream of the throttle and is pointed


Umm...Keihin doesn't make a single carb that I have ever seen in which this is the case. All Kehins use an AIR screw for the idle circuits. A few Harley carbs (notably the old Linkerts) use an adjustable main jet that DOES adjust the fuel orifice, maybe this is what he is referring to? Or, maybe some of the Mikunis he is speaking to have adjustable mainjets, but I have not seen one like that.

Automotive carbs like Holleys, FoMoCo, and Carter, use fuel metering screws for the idle circuits (which is opposite the Keihin method), which confuses some folks who don't know about the differences.
diaphragm cv carbs on v4 bikes have a fuel screw...probably others too, but not until the 1980's...sooo, doesn't really apply to sohc4's
Well my carbs on my 650 has fuel screw, or so I thought... Change fuel screw to air screw and the rest is the same

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« Last Edit: May 02, 2020, 06:45:26 am by dkihlgren »

Offline PeWe

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Re: So, you want to run pods?
« Reply #7 on: May 02, 2020, 08:59:09 am »
My CB750 with 836, 4-1 and original carbs with pods

Pilot jet: 40  (not changed)
Needles: 4:th notch from top. Stock was 3.
My K2 got those carbs and need 4th notch too with stock airbox and HM300 replica. So this is more about the better flowing exhaust.

Main jet: Larger.
CB750 at least 125-130

As said before mostly main jet to change if needles are at 4th notch from top on CB750 K2-K6 carbs. I guess K1 are the same.

Air screw:
This will run bike on low cruising speed on 5th gear, around 60mph.
Open it too much can be easier to get too lean.
With airbox 1.5 turns out (correct 1 turn out) to find way too lean, pods will most likely be leaner.
Then close it in 1/8 steps. Have a screwdriver with you and take a ride and adjust after a while.

I have had a long adjusting period with other carbs where the needle jets had to be changed .
- CB750 stock carbs not.
This jet is also called emulsifier tube and affect at around 3 mm throttle lift over idle and transit to needle heights.

Needle jets must be correct!
I have later understood that my Mikuni Smoothbores were too lean and I tried to fix that with richer pilot that is not possible.
Needle heights can help but bring other issues.
« Last Edit: May 02, 2020, 09:56:24 am by PeWe »
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline seanbarney41

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Re: So, you want to run pods?
« Reply #8 on: May 02, 2020, 06:54:16 pm »
I translated this from a swedish site www.braigasen.se
The owner is a real carb-guy

Carburetor with fuel screw
Most common on 4-stroke carburetors (except Mikuni HSR). A fuel screw is always located downstream of the throttle and is pointed


Umm...Keihin doesn't make a single carb that I have ever seen in which this is the case. All Kehins use an AIR screw for the idle circuits. A few Harley carbs (notably the old Linkerts) use an adjustable main jet that DOES adjust the fuel orifice, maybe this is what he is referring to? Or, maybe some of the Mikunis he is speaking to have adjustable mainjets, but I have not seen one like that.

Automotive carbs like Holleys, FoMoCo, and Carter, use fuel metering screws for the idle circuits (which is opposite the Keihin method), which confuses some folks who don't know about the differences.
diaphragm cv carbs on v4 bikes have a fuel screw...probably others too, but not until the 1980's...sooo, doesn't really apply to sohc4's
Well my carbs on my 650 has fuel screw, or so I thought... Change fuel screw to air screw and the rest is the same

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actually no...if you have an air screw, the method of adjustment as described is totally wrong
If it works good, it looks good...

Offline HondaMan

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Re: So, you want to run pods?
« Reply #9 on: May 02, 2020, 07:08:57 pm »
I think what is important to remember about the adjustment screws is: in order for a fuel screw to work, there must be fuel PRESSURE. Honda introduced fuel pumps to the Goldwings and many of the later CV carbs, and in some of those there are fuel-adjust low-speed screws.

The old Harley Linkerts had a screw that adjusted the height of the metering needle in the carb venturi, sort of like having manually-adjustable needle clips that you could turn while riding.
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline PeWe

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Re: So, you want to run pods?
« Reply #10 on: May 03, 2020, 12:38:39 am »
My Mikunu TMR 32 have fuel screws. I have adjusted them a lot the last month ;)
They have their area separate from pilot jets.

Too lean will be heard as misfires at idle and decelerations. Also as "sneezing" carbs at throttle increase from low lift.

I usually start lean. A ride with screwdriver, adjust in 1/8 turns out until it disappear. Rides usually 60-90 km.
Open screw more fuel.

I'll soon have a ride since I have adjusted rhe needles 0.5mm by a washer to raise rhem just a little. From too light colored plugs. The clip richer sooty plugs at low lift.
I'll see uf I need to close fuel screws just a little due to richer needles.
Those carbs works as the Mikuni chart show. I guess valid for all carbs. It is about throttle lift.
One thing that I misunderstood before was the cooperation of fuel screws and pilot jets. The screws have their own area too.


One thing easy to determine.

If a richer main jet makes bike to accelerate better in the middle range but will be too rich at full speed:
Raise needles 1 step, decrease main jets.

I have noticed that a restrictive exhaust system accept wrong jetted carbs,  better flowing not that much
 I replaced 4-4 with a much better flowing 4-1 and found way too rich jetted carbs on the first quarter throttle lift.

« Last Edit: May 03, 2020, 03:13:00 am by PeWe »
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline Scott S

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Re: So, you want to run pods?
« Reply #11 on: May 03, 2020, 03:23:18 am »
 I think this is all good information and just goes to prove that "just go up two on the main jet" isn't a proper tuning method when it comes to pods, or changing the intake/exhaust system.
'71 CB500 K0
'17 Triumph Street Scrambler
'81 Yamaha XS650

Offline Jarrett_Honda

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Re: So, you want to run pods?
« Reply #12 on: May 07, 2020, 03:47:11 am »
You guys are way too smart. I'm not going to lie and say that I understand it all.

What I do understand from all of this is that the engineers that designed the bike knew what they were doing they they built the carbs and the airbox to work together. And that changing one piece of that formula messes everything up.

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Offline seanbarney41

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Re: So, you want to run pods?
« Reply #13 on: May 07, 2020, 09:23:59 am »
...exhaust too...they even put a warning sticker/stamp on the exhaust.  Very few have listened.
If it works good, it looks good...

Offline Jaroldoe10

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Re: So, you want to run pods?
« Reply #14 on: May 27, 2020, 07:19:03 pm »
Hello gentlemen after reading this my question is this. I am trying to put a chopper together from a CB550 and honestly I don't think the air box would really look decent sitting under the frame all open like. Is there a happy medium do they sell airbox to replace the stock one that isn't all bulky is there a way around this maybe a different type of pod filter. Any help is appreciated I need to do something other than an air box and I don't need it to be erased machine but I do need it to run decently.

Offline seanbarney41

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Re: So, you want to run pods?
« Reply #15 on: May 27, 2020, 08:32:47 pm »
Hello gentlemen after reading this my question is this. I am trying to put a chopper together from a CB550 and honestly I don't think the air box would really look decent sitting under the frame all open like. Is there a happy medium do they sell airbox to replace the stock one that isn't all bulky is there a way around this maybe a different type of pod filter. Any help is appreciated I need to do something other than an air box and I don't need it to be erased machine but I do need it to run decently.
in my experience, it is not really the air box itself that is the key to good running.  Its the velocity stack boots.  Pop those boots out of the box, clamp 'em on, up size the main jets 5 or 10 numbers, or until it runs good, and you are golden...only now you really should figure out some kind of filtration.  Try googling Henry Abe breadbox and you are on the right track.  Crafty fab guys have made their own incorporating the stock v stacks.  UNI foam and RamAir filter socks are gonna be your friend here.  This stuff really works.  Cheap ass Emgo pod filters that block off your emulsion air bleeds don't.
If it works good, it looks good...

Offline Tom C

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Re: So, you want to run pods?
« Reply #16 on: May 31, 2020, 04:08:18 am »
Here's a story I like to tell about pod filters v OEM air boxes :

A few years ago I took care of my buddies' 93 Ducati 900 Supersport - a beautiful bike.  He bought it just before leaving for Asia on an extended expat assignment.  I cleaned out the carbs and got her running, then took her out for a ride (because that's what you do when your buddy leaves his Ducati in your garage...). 

Not long into the ride I discovered a giant flat spot in performance at full throttle.  I couldn't figure out what was going on, didn't want to futz with the carbs and with no Ducati dealers nearby, I went to the the local powersports dealer here in Delaware for help.  The girl behind the counter told me they don't work on Ducatis, but added "My dad works on old motorcycles, but I'm not sure that yours is old enough for him to look at".  She gave me his name and number, I called Joel up and told him the story of the the bike, he agreed to take a look.

I rode the bike over to Joel's house and waited for him to arrive, which he soon did riding an old Vespa.  We chatted for a bit outside, he agreed to work on the bike.  At this point he opens his garage door and my jaw drops - the entire lower level of his house is wall to wall motorcycles!  Certainly the largest collection I have ever seen of vintage motorcycles: all makes, models and sizes.  Turns out Joel is the retired founder and owner of Powersports East, the largest dealer in Delaware, and an avid collector or motorcycles.  You can see some of his bikes here: (http://www.retrotours.com).  He as a small business organizing tours on which you ride some of his vintage bikes.   Very cool fellow!

Joel has the bike for a few days, and when I get back in touch with him, he explained that a previous owner had removed the stock Ducati air box, replacing it with K&N pods.  The carbs had not been re-jetted, and as a result the bike was running lean at full throttle, explaining the loss of power at WOT.

Joel said that he could rejet the carbs to restore the power at full throttle, but strongly suggested a different course of action.  Pods are great if you are expert rider on a track and you have the skills necessary to use every last bit of horsepower at high rpms.  The performance advantage at full throttle is achieved at the cost of performance at smaller throttle openings, where most of us spend our riding time. 

If you want this bike to perform well throughout the rpm range, Joel suggests,  find and reinstall an OEM air box.  The air box on the Ducati, in addition to being expertly designed to maximize overall performance, is a work of art that perfectly blends functionality, aesthetics and engineering.  Joel was somewhat passionate about this.

I was able to find a used air box for the bike (from Ducati racer of all people).  Joel installed it, after which the bike was an absolute dream to ride!  Sadly, my buddy's brother came up from Atlanta a few months later and rode the beautiful Ducati away.....



Long story short, while the OEM air box/filter assembly on the CB550 is perhaps not as aesthetically pleasing as pods, you certainly lose something in terms of rideabily and performance throughout the rpm range of your bike should you decide to go with pods.   
74 CB550K0

Offline PeWe

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Re: So, you want to run pods?
« Reply #17 on: May 31, 2020, 10:51:23 am »
Bad idea to change filtering system without any reason.

Pods needed when modified engine by cam and more, need of more airflow since exhaust flow better. No need of better inflow if exhaust restricts and vice versa

Or not std carbs, no box available that fits.
It takes time to figure out the air/fuel screws, pilot jet, needle jet, needle setting and main jet.

CB750 with K6 carbs, only needle heights and  much larger main jet needed after 836, pods and 4-1.
Mikuni Smoothbore or TMR, correct needle jet (emulsifier tube) the start point after clean idle by pilot jet. This cover the very first 1-5mm throttle lift after idle.  If wrong a never ending pilot and needle hight change loop with always something wrong.

Needle height comes next with matching main so something will happen when twisting from 4000rpm.

I have seen that a restrictive exhaust can hide bad carb jettings. Replace exhaust with better flow and the problems pop up as I saw last year, fixed now .
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Online grcamna2

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Re: So, you want to run pods?
« Reply #18 on: May 31, 2020, 11:33:52 am »
subscribed
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline mark ward

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Re: So, you want to run pods?
« Reply #19 on: June 02, 2020, 10:10:07 am »
All great info, as I've been re-tuning my VM29's after 836 rebuild, CYCLE X Rapid-Fire 4-1 exhaust; and still dealing with the 2000-4500 rpm choppy / rich scenario.
     Jet needle is in 3rd position. When moved to 2nd from top; it's smoother, but then is too lean after 4500. Need different jet-needle taper; or, perhaps it's time I replace the pods.

Offline PeWe

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Re: So, you want to run pods?
« Reply #20 on: June 02, 2020, 11:05:23 am »
My VM29 setup and jetting 836 and cam etc with 4-1

Needle jets?
(Emulsifier tube where main jet is attached to)
- Sudco speciify  O-8.

Mine had smaller  O-6 at delivery and explain why I increased pilot jets without solving anything.
Important to have right. Affect the low throttle lift around 2-5mm.
(I made mistake to increase pilot for this area causing sooting rich idle)

Pilot for clean idle.
- Sudco specify 17.5  which I think is OK

Needle in the middle was my last setting with 836.
Not leaner, clip sit higher up.

Main jet 120
Might be a step smaller too.

Air jet a small jet sitting at the rear where air filter is attached.
0.9   according to Sudco and probably right.

Increasing this caused leaner  around 3500- 5000 rpm on my setup.

Air screws often said to be 1/2 turn open, mine too .

Floats, verify with clear tube to be sure. 23mm a starting point.
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline mark ward

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Re: So, you want to run pods?
« Reply #21 on: June 03, 2020, 09:39:55 am »
Thanks Per. I have marveled at much of your past documenting, of your 836 tuning. I ordered a Jet needle with different taper; but perhaps I'll also order that size needle-jet, as well. It appears that your experience was similar.   I may also try my old velocity stacks, combined with some type of 'sock' foam filter, which would slip over the bellmouth. 

Offline Scott S

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Re: So, you want to run pods?
« Reply #22 on: June 04, 2020, 02:49:00 am »
 And PeWe just confirms that it takes work to get decent tuning. Just proves the whole "go up two on the main" won't cut it, usually.

 That was kind of the point of this thread: MOST people don't take the time or effort to make the necessary changes.
'71 CB500 K0
'17 Triumph Street Scrambler
'81 Yamaha XS650

Offline mark ward

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Re: So, you want to run pods?
« Reply #23 on: June 04, 2020, 10:42:38 am »
Oh how right you are. I also tried the air jets 1.0, 1.1, and 1.2.    Rechecked float-height, etc.     I hope replacing my pods with my old 1980's velocity-stacks, covered with RamAir foam sock-filters, will also improve that venturi turbulence scenario, at the lower rpm range.
      I must say, the CYCLE X Rapid Fire 4-1 exhaust does pull stronger from 2000- 6000 rpm; than my old RC exhaust. Now to just get it smoother from 2000- 4500.

Offline scottly

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Re: So, you want to run pods?
« Reply #24 on: June 04, 2020, 08:29:23 pm »
  The air box on the Ducati, in addition to being expertly designed to maximize overall performance, is a work of art that perfectly blends functionality, aesthetics and engineering.

Long story short, while the OEM air box/filter assembly on the CB550 is perhaps not as aesthetically pleasing as pods, you certainly lose something in terms of rideabily and performance throughout the rpm range of your bike should you decide to go with pods.
The stock 750 airbox, with it's generous air filter area, was a good design, although some performance may have been sacrificed for noise reduction.(Mr Honda didn't like loud motorcycles ;D) The 550 and smaller bikes had completely different type systems, with much  less filter area, which has a greater effect when switching to low restriction pods, in turn requiring re-jetting. There is a member on here who has ridden her pod equipped 550 from coast to coast and much more. ;D ;D   
Don't fix it if it ain't broke!
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