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So, you want to run pods?

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Scott S:
 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.

HondaMan:
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.
;)

slikwilli420:
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.

dkihlgren:
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…

Skickat från min Mi 9T Pro via Tapatalk

HondaMan:

--- Quote from: dkihlgren 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


--- End quote ---

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.

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