Topic: Needle Height Adjustment

[The Lone Builder]

Having resolved my synching problems – see this post for details http://forums.sohc4.net/index.php/topic,194668.0.html – I go the bike out on the road and have now done some 250 kms.

All was not well: the engine ran hot; slow riding was "jerky"; and tickler was uneven. The bike was running well otherwise, although I later felt it ran out of steam a bit after 140km/hr.

Then someone pointed out that the bike smoked out of the right side!! This was disaster for me as this was the reason I had replaced valves, valve guides and piston rings. However this only lasted for about ½ mile and then stopped! Weird.

On inspection of those pipes – Nos 3 & 4 – they were wet at the end while the other side was bone dry. Maybe it's running rich on those cylinders I thought, so I checked the plugs when I got home (see pics). To me that is too lean. Yes/no?

Before the strip down, the plugs on that side had been somewhat wet and sooty (see pic), so when I put the carbs back together, I moved the needle from the 3rd slot to the 2nd. This was based on Hondman's comments in his book.

It would see however, that this move has leaned out the mix too much. Would this cause my problems - heat and poor low speed running?

Next question is, can I change the love of the needles with the carbs remaining in situ? And once I've done this, do I have to rebalance the carbs again?

[BenelliSEI]

Did you do all four, or just two? I’d definitely put them back. Not sure how you will do that in place……

[HondaMan]

If the compression in the cylinders is within 15 PSI of each other (first test), then I'd suspect a vacuum leak in #3 and #4 intake hoses.
This will make #1 & #2 run slightly slower than they should for the slide opening of the carbs, which makes it lean there.

[PeWe]

I have tried to remove needles for adjustment without taking carbs apart. I could not get them out.
Carbs off and ruin the sync a must.

I have adjusted all 4 needles on my Mikuni TMR in 20 minutes, tank off- on included. Without ruin the sync. Top cover off, end plug in throttle off, lift needle.
CB750 carbs works fine but the needle removal should have been designed different.

CB750 stock exhaust and airbox stock. Needles must be stock too.
Carb model important too.
I have noticed that 064A carbs runs richer needles than 086A.
Aftermarket needles does not work at all.
I bought cheap kit, saw direct that needles differed a lot, lenght, thickness and taper so I did not try them.

[The Lone Builder]

BenelliSEI:     All four were done.

Hondaman:    The pic showing all 4 carbs is a "before" pic. It was burning oil on the right hand side 4>3. Since then valves and guides have been replaced; cylinders honed and new rings installed; and needles adjusted.

Carbs are 657B, 110 main, 40 pilot and currently needle in slot 2, going back to 3.

Interestingly this http://www.hondachopper.com/garage/carb_specs/carb_specs.html says the main jet should be 105 with the needle in no 4.

PeWe:     I was afraid of that. So here I go again. Glad I know about the ratchet strap trick! 

[BenelliSEI]

BenelliSEI:     All four were done.

Hondaman:    The pic showing all 4 carbs is a "before" pic. It was burning oil on the right hand side 4>3. Since then valves and guides have been replaced; cylinders honed and new rings installed; and needles adjusted.

Carbs are 657B, 110 main, 40 pilot and currently needle in slot 2, going back to 3.

Interestingly this http://www.hondachopper.com/garage/carb_specs/carb_specs.html says the main jet should be 105 with the needle in no 4.

PeWe:     I was afraid of that. So here I go again. Glad I know about the ratchet strap trick! 

That “105 Main” spec was a change for the cb750K6.

[HondaMan]

BenelliSEI:     All four were done.

Hondaman:    The pic showing all 4 carbs is a "before" pic. It was burning oil on the right hand side 4>3. Since then valves and guides have been replaced; cylinders honed and new rings installed; and needles adjusted.

Carbs are 657B, 110 main, 40 pilot and currently needle in slot 2, going back to 3.

Interestingly this http://www.hondachopper.com/garage/carb_specs/carb_specs.html says the main jet should be 105 with the needle in no 4.

PeWe:     I was afraid of that. So here I go again. Glad I know about the ratchet strap trick! 

That “105 Main” spec was a change for the cb750K6.

It was actually done in the K2 after 3/1972 production. Then in the K3 between about 1/73 and 3/73 the mainjets went to #110, then back to #105 again. Interestingly, this #110 mainjet corresponded to the worst-made intake ports (very little to no hand-finishing, as was done before), and after a good port relief job they run much too rich!

Today's internet forums claim all sorts of jetting for these bikes, most which don't match my notes from actually WORKING on the bikes in those years and keeping my own notes.


Lone Builder: go to #4 notch (2nd from bottom) for the needles unless you have a hotter-than-OEM cam, or very open pipes. The #2 notch was never used in these roundtop carbs, except by fiddlers like us.
The K3 heads are my favorite ones to port. They can pick up 20% volume flow by making them look like K4 heads.

[The Lone Builder]

Haha!

Too late Mark, I've just put them back on with the needles at 3. NOT taking them off again.

Interestingly, given what you said previously about an air leak, I noticed that the clamps on the manifold end of the boots for Nos 3 & 4 cylinders were not as tight as those on 1 & 2. The former boots could rotate a bit whereas the latter were fixed.

I used an old clamp with the ends cut off underneath the clamp and both boots are now tight, tight.

I'm running out of time here; I have to leave for London tomorrow week! 

[BenelliSEI]

How’s it running?

[The Lone Builder]

Not there yet.

A few minor issues en route that took longer to sort out than might have been expected.

I took the opportunity to check valve clearances also.

Tomorrow morning I'll get it going, and maybe even take a ride if it's not raining.

[willbird]

I have changed needle height in a friends backyard in a grassy area, living dangerously LOL. It has been since 1994 but I seem to recall being able to just take out the screws that hold the carbs to the rack and not pull the rack clear off. I would have been up sheet creek had I dropped an E clip in the grass.

[HughL]

Had a similar issue and moved my needles up to the 4th slot (from the bottom to top) and am still running crazy rich on a bone stock set-up. I noticed my airbox says CB750F on it so I'm wondering if that could be an issue - wanted some opinions or thoughts on it. Running 064A carbs. (76 750K)

[willbird]

Had a similar issue and moved my needles up to the 4th slot (from the bottom to top) and am still running crazy rich on a bone stock set-up. I noticed my airbox says CB750F on it so I'm wondering if that could be an issue - wanted some opinions or thoughts on it. Running 064A carbs. (76 750K)

IMHO many (maybe all?) of the airboxes say that on them.

[PeWe]

Had a similar issue and moved my needles up to the 4th slot (from the bottom to top) and am still running crazy rich on a bone stock set-up. I noticed my airbox says CB750F on it so I'm wondering if that could be an issue - wanted some opinions or thoughts on it. Running 064A carbs. (76 750K)

IMHO many (maybe all?) of the airboxes say that on them.

My K2 has 064A carbs since september last summer.
That bike had K6 carbs (086A) before.

Stock airbox with K&N filter
Yamiya  no numbers 4-4 with rather good flow.

With 086A carbs
- Airscrews 7/8
- Pilot jet 40
- Needle 4th notch from top
(They had 3rd as stock with 341 pipes)
- Main jet 115

064A carbs got same jetting to start with. Way too rich.
- Airscrews 1
- Changed needle clip to 3 (middle)
-Main jets 115
This worked much better.
I'll test 112 main jet coming season.

It seems that good throttle response on 5:th gear from 4000-5000 rpm need a little rich.
That make the ride much better.

064A carbs were stock on 1975 F according to Honda CB750 Shop manual.

I have noticed that too low fuel levels in float bowls will cause too lean (starvation) when riding a little faster.
Increase main jet will not help, fuel starvation mixed with too rich will affect plug color and head scratching.

[HondaMan]

064A carbs were stock on 1975 F according to Honda CB750 Shop manual.

I have noticed that too low fuel levels in float bowls will cause too lean (starvation) when riding a little faster.
Increase main jet will not help, fuel starvation mixed with too rich will affect plug color and head scratching.

The low float bowls were (are?) a chronic issue in the 064a carbs. I always thought it was because of the new airbox with the single wide vent across the bottom of the box, which was done to make the intake noise quieter in response to [more] restrictions from the US Department of Transportation (DOT). The thought at the time was that a 'flutter' of air pressure across the opening of the airbox at higher speeds was disturbing the fuel in the float bowls, making them press harder against the float valve and reducing the fuel level. What we observed was: with the bike running in 4th gear at 6000 RPM, accelerating would be strong to 8000 RPM just once, then it took longer to repeat the test a second and third time. If the bike was then run again at about 5000 RPM for a mile or so and the test repeated, it did the same thing again. The float bowls were just not filling as deep.

On bikes with Vetter fairings and lowers, which fully enclose the airbox and carbs, things got MUCH better, often improving the top speed by as much as 8 MPH (from 102mph to 110mph in 2 cases I observed) in top gear, too. I resolved this in those carbs on normal bikes by staggering the floats 1mm, making the one closest to the sidestand 1mm 'deeper' than the other side (which I've posted about before). This seemed to help. It also helps in the 086a carbs, although those don't seem to have the same problem, all else being equal (airbox, fairing or not, etc.). I've not figured out 'why', though. 

[newday777]

064A carbs were stock on 1975 F according to Honda CB750 Shop manual.

I have noticed that too low fuel levels in float bowls will cause too lean (starvation) when riding a little faster.
Increase main jet will not help, fuel starvation mixed with too rich will affect plug color and head scratching.

The low float bowls were (are?) a chronic issue in the 064a carbs. I always thought it was because of the new airbox with the single wide vent across the bottom of the box, which was done to make the intake noise quieter in response to [more] restrictions from the US Department of Transportation (DOT). The thought at the time was that a 'flutter' of air pressure across the opening of the airbox at higher speeds was disturbing the fuel in the float bowls, making them press harder against the float valve and reducing the fuel level. What we observed was: with the bike running in 4th gear at 6000 RPM, accelerating would be strong to 8000 RPM just once, then it took longer to repeat the test a second and third time. If the bike was then run again at about 5000 RPM for a mile or so and the test repeated, it did the same thing again. The float bowls were just not filling as deep.

On bikes with Vetter fairings and lowers, which fully enclose the airbox and carbs, things got MUCH better, often improving the top speed by as much as 8 MPH (from 102mph to 110mph in 2 cases I observed) in top gear, too. I resolved this in those carbs on normal bikes by staggering the floats 1mm, making the one closest to the sidestand 1mm 'deeper' than the other side (which I've posted about before). This seemed to help. It also helps in the 086a carbs, although those don't seem to have the same problem, all else being equal (airbox, fairing or not, etc.). I've not figured out 'why', though. 

What year were the 064A carbs on?

[PeWe]

Last bowl issue was with the 064A carbs.
First too low. I tested not flowing over before I mounted them on bike.

Adjusting no 2 ended up in too high or too low. Finally not flowing over when on sidestand direct after a ride.

I bought my set cheap, recently cleaned in parts.

According to shop manual stock on 1975 F.

[HondaMan]

According to shop manual stock on 1975 F.

Yeah, that's what I thought: the F0 came with the 064a. They were much the same as the 657 series, except the needle changed to 271304 (and sometimes a different last digit). This was done to lean out the 2000-3500 RPM range to meet with the new cam opening the intakes at 0 degrees instead of before TDC.

Their main fuel-feed issue is the single fuel hose from the usual 750F0 tank, though. The 3-4 carbs fill up first and then the 1-2 carbs because of the fuel tee next to the 1-2 carbs: the movement of the fuel through the tee toward the far carbs makes a slight suction on the 1-2 side, making it fill last. By accident I found a way to make it more even, which was to install a fuel petcock at the tank that faces the spigot toward the back of the bike, not downward. This gets in the way of the choke lever a little bit, so the fuel line must run along the bottom of the tank 3" or so and then turn sharply: to do that I made a brass-tube elbow there to turn under the tank toward the carbs in one hose diameter's length. It stopped the #2 carb late-fill problem, though it looks a little odd. That bike is still here in Colorado. Every year the owner brings it in with a dead battery and dried-out carbs: it hasn't been ridden 300 miles in the last 10 years! Beautiful, bike, too...

[HughL]

Had a similar issue and moved my needles up to the 4th slot (from the bottom to top) and am still running crazy rich on a bone stock set-up. I noticed my airbox says CB750F on it so I'm wondering if that could be an issue - wanted some opinions or thoughts on it. Running 064A carbs. (76 750K)

IMHO many (maybe all?) of the airboxes say that on them.

My K2 has 064A carbs since september last summer.
That bike had K6 carbs (086A) before.

Stock airbox with K&N filter
Yamiya  no numbers 4-4 with rather good flow.

With 086A carbs
- Airscrews 7/8
- Pilot jet 40
- Needle 4th notch from top
(They had 3rd as stock with 341 pipes)
- Main jet 115

064A carbs got same jetting to start with. Way too rich.
- Airscrews 1
- Changed needle clip to 3 (middle)
-Main jets 115
This worked much better.
I'll test 112 main jet coming season.

It seems that good throttle response on 5:th gear from 4000-5000 rpm need a little rich.
That make the ride much better.

064A carbs were stock on 1975 F according to Honda CB750 Shop manual.

I have noticed that too low fuel levels in float bowls will cause too lean (starvation) when riding a little faster.
Increase main jet will not help, fuel starvation mixed with too rich will affect plug color and head scratching.

I've got 105 mains and 40 pilots and am running it with almost 2 or 3 turns out on the screw (I have no idea what's going on with that but it runs best at that)

I do have a K6 which is why I'm curious as to why the airbox says 750F or if it's irrelevant since I'm running the 064A carbs anyway.

Last thing that I'm thinking could be causing the rich condition is maybe my timing is too advanced.

Thanks

[The Lone Builder]

I am happy to report that with the change I made (2nd slot to 3rd) and with tightening up the clamps on the carb boots, has resulted in a remarkable difference.

The bike starts instantaneously, it idles nicely and runs much more smoothly at low speeds.

The thing that has surprised me the most however, is the increase in power. The bike is much more responsive now, and, whereas before at 4000 rpm I was doing just about 100kph, now I'm doing 120-130 kph. It's like a new bike!! 

Only thing is, it feels like it's running a bit hot; too hot to touch the cylinders even for a second.

[willbird]

The thing that has surprised me the most however, is the increase in power. The bike is much more responsive now, and, whereas before at 4000 rpm I was doing just about 100kph, now I'm doing 120-130 kph. It's like a new bike!! 

That does not make sense, in high gear engine rpm and actual speed are a fixed ratio unless the clutch is slipping.

Bill

[The Lone Builder]

That's as may be, but it's what's happened.

Maybe the speedo or the tach is off; something else to check .... someday.

[HondaMan]

Had a similar issue and moved my needles up to the 4th slot (from the bottom to top) and am still running crazy rich on a bone stock set-up. I noticed my airbox says CB750F on it so I'm wondering if that could be an issue - wanted some opinions or thoughts on it. Running 064A carbs. (76 750K)

IMHO many (maybe all?) of the airboxes say that on them.

My K2 has 064A carbs since september last summer.
That bike had K6 carbs (086A) before.

Stock airbox with K&N filter
Yamiya  no numbers 4-4 with rather good flow.

With 086A carbs
- Airscrews 7/8
- Pilot jet 40
- Needle 4th notch from top
(They had 3rd as stock with 341 pipes)
- Main jet 115

064A carbs got same jetting to start with. Way too rich.
- Airscrews 1
- Changed needle clip to 3 (middle)
-Main jets 115
This worked much better.
I'll test 112 main jet coming season.

It seems that good throttle response on 5:th gear from 4000-5000 rpm need a little rich.
That make the ride much better.

064A carbs were stock on 1975 F according to Honda CB750 Shop manual.

I have noticed that too low fuel levels in float bowls will cause too lean (starvation) when riding a little faster.
Increase main jet will not help, fuel starvation mixed with too rich will affect plug color and head scratching.

I've got 105 mains and 40 pilots and am running it with almost 2 or 3 turns out on the screw (I have no idea what's going on with that but it runs best at that)

I do have a K6 which is why I'm curious as to why the airbox says 750F or if it's irrelevant since I'm running the 064A carbs anyway.

Last thing that I'm thinking could be causing the rich condition is maybe my timing is too advanced.

Thanks

The rich condition is being caused by the air screws. Their active range is only from 1/2 turn to 1-1/2 turns, and beyond 1-1/2 turns they are simply wide open.
They should be set between 3/4 turn (if they have been 'bottomed' often) and 1 turn. The idle circuit stops supplying fuel at about 1/8 slide-open position, as there is no more laminar flow in the pilot jet's area above that speed.

[HughL]

Had a similar issue and moved my needles up to the 4th slot (from the bottom to top) and am still running crazy rich on a bone stock set-up. I noticed my airbox says CB750F on it so I'm wondering if that could be an issue - wanted some opinions or thoughts on it. Running 064A carbs. (76 750K)

IMHO many (maybe all?) of the airboxes say that on them.

My K2 has 064A carbs since september last summer.
That bike had K6 carbs (086A) before.

Stock airbox with K&N filter
Yamiya  no numbers 4-4 with rather good flow.

With 086A carbs
- Airscrews 7/8
- Pilot jet 40
- Needle 4th notch from top
(They had 3rd as stock with 341 pipes)
- Main jet 115

064A carbs got same jetting to start with. Way too rich.
- Airscrews 1
- Changed needle clip to 3 (middle)
-Main jets 115
This worked much better.
I'll test 112 main jet coming season.

It seems that good throttle response on 5:th gear from 4000-5000 rpm need a little rich.
That make the ride much better.

064A carbs were stock on 1975 F according to Honda CB750 Shop manual.

I have noticed that too low fuel levels in float bowls will cause too lean (starvation) when riding a little faster.
Increase main jet will not help, fuel starvation mixed with too rich will affect plug color and head scratching.

I've got 105 mains and 40 pilots and am running it with almost 2 or 3 turns out on the screw (I have no idea what's going on with that but it runs best at that)

I do have a K6 which is why I'm curious as to why the airbox says 750F or if it's irrelevant since I'm running the 064A carbs anyway.

Last thing that I'm thinking could be causing the rich condition is maybe my timing is too advanced.

Thanks

The rich condition is being caused by the air screws. Their active range is only from 1/2 turn to 1-1/2 turns, and beyond 1-1/2 turns they are simply wide open.
They should be set between 3/4 turn (if they have been 'bottomed' often) and 1 turn. The idle circuit stops supplying fuel at about 1/8 slide-open position, as there is no more laminar flow in the pilot jet's area above that speed.

My impression was the further out the screw the leaner the mixture - correct me if i'm wrong. I don't know how else to adjust the mixture on a completely stock bike at this point aside from turning them out as far as I can.

[HondaMan]

Had a similar issue and moved my needles up to the 4th slot (from the bottom to top) and am still running crazy rich on a bone stock set-up. I noticed my airbox says CB750F on it so I'm wondering if that could be an issue - wanted some opinions or thoughts on it. Running 064A carbs. (76 750K)

IMHO many (maybe all?) of the airboxes say that on them.

My K2 has 064A carbs since september last summer.
That bike had K6 carbs (086A) before.

Stock airbox with K&N filter
Yamiya  no numbers 4-4 with rather good flow.

With 086A carbs
- Airscrews 7/8
- Pilot jet 40
- Needle 4th notch from top
(They had 3rd as stock with 341 pipes)
- Main jet 115

064A carbs got same jetting to start with. Way too rich.
- Airscrews 1
- Changed needle clip to 3 (middle)
-Main jets 115
This worked much better.
I'll test 112 main jet coming season.

It seems that good throttle response on 5:th gear from 4000-5000 rpm need a little rich.
That make the ride much better.

064A carbs were stock on 1975 F according to Honda CB750 Shop manual.

I have noticed that too low fuel levels in float bowls will cause too lean (starvation) when riding a little faster.
Increase main jet will not help, fuel starvation mixed with too rich will affect plug color and head scratching.

I've got 105 mains and 40 pilots and am running it with almost 2 or 3 turns out on the screw (I have no idea what's going on with that but it runs best at that)

I do have a K6 which is why I'm curious as to why the airbox says 750F or if it's irrelevant since I'm running the 064A carbs anyway.

Last thing that I'm thinking could be causing the rich condition is maybe my timing is too advanced.

Thanks

The rich condition is being caused by the air screws. Their active range is only from 1/2 turn to 1-1/2 turns, and beyond 1-1/2 turns they are simply wide open.
They should be set between 3/4 turn (if they have been 'bottomed' often) and 1 turn. The idle circuit stops supplying fuel at about 1/8 slide-open position, as there is no more laminar flow in the pilot jet's area above that speed.

My impression was the further out the screw the leaner the mixture - correct me if i'm wrong. I don't know how else to adjust the mixture on a completely stock bike at this point aside from turning them out as far as I can.

All the way in cuts off the fuel mixture entirely. As the screw is backed out it begins to let in more of the mixture. As the passage is opened more than the cross-sectional area of the pilot jet, it gets no more rich as the screw goes further out: this limit is physically reached at 1-1/2 turns in the 750's roundtop carbs.

The 'size' (the number on the pilot jet) of the jet is what controls how "rich" this mixture is, and the screw allows 0-100% of that air/fuel mixture to be available to the engine during an intake stroke. The mixture in the pilot circuit is drawn up and atomized at the front edge of the carb's slide, into a mist.

Much of the confusion about these screws comes from confusing these carbs with Detroit's automotive carbs of the last century: those carbs fed a constant-vacuum system in the intake manifold of the engine, and the amount of fuel allowed was directly controlled by the idle mixture screw(s) in those carbs, up to the point of 1/4 throttle (prior to about 1986 or so). Many lawn mower carbs are similar to the old car carbs: they often have just one screw that adjusts the amount of gas entering a small orifice enroute to the engine, and those have a very limited range of RPM over which they actually work - like, idle 500 to 1800-ish RPM, max. These bikes have very different carbs from those types.

Another thing that didn't help much was Honda's old Shop Manuals, which, in their poorly-translated (from Japan) English used the phrase "for richness, turn the screw in". What this actually MEANS is: "If the engine is running rich, turn the screw in", as the context of the rest of the paragraph is describing WHAT TO DO with the carbs to solve certain situations. This statement was commonly found in the Honda CB90/100/125 bike manuals, and is seen in some versions of others as well: it was a reference to the way the early Honda engines would vibrate the carb screws outward, which would cause the idle speed to drop and the mixture to get richer over time. This was a common tune-up point on the Hawks/SuperHawks in particular: mine would turn the idle mix screws outward about 1/8 turn in a summer of riding. They vibrated MUCH more than these bikes, though!