Weird carb sync problem

[ptm]

I'm still having this problem syncing my carbs. Other than this it runs fabulously - from idle through redline - no bogging or hesitation - just full tilt boogie fun. However, when I try to sync my carbs I get the following readings:

cyl.....1000 rpm....2000.......2500

1..........26.............24...........26

2..........27.............24...........27

3..........27.............25...........27

4..........19.............31...........35

I know I can get 1-3 more even, although they're pretty close. The problem is #4. It's way under the other three at idle, but as soon as I start to increase engine speed, it goes much higher than the others.

It's been this way for a few thousand miles. The carbs are clean, boots to both the head and air box are pliable and clamped tight. The o-rings on the boots to the head are new and the boots are bolted tight (but not too tight). I get decent mileage - 35-40mpg, and the plugs look good, maybe a tad on the rich side, but all four look the same.

Any ideas? Thanks in advance.

 

[earlfor]

Re: Weird carb sync problem

Best guess...........the slide in #4 is sticking. Best guess as to why............
If you remove the slide, close to the top edge of the slide, you will see a slot with a spring loaded ball joint embedded in it. Gunk can build up in the slot, restricting freedom of movement of the slide.

Earl

I'm still having this problem syncing my carbs. Other than this it runs fabulously - from idle through redline - no bogging or hesitation - just full tilt boogie fun. However, when I try to sync my carbs I get the following readings:

cyl.....1000 rpm....2000.......2500

1..........26.............24...........26

2..........27.............24...........27

3..........27.............25...........27

4..........19.............31...........35

I know I can get 1-3 more even, although they're pretty close. The problem is #4. It's way under the other three at idle, but as soon as I start to increase engine speed, it goes much higher than the others.

It's been this way for a few thousand miles. The carbs are clean, boots to both the head and air box are pliable and clamped tight. The o-rings on the boots to the head are new and the boots are bolted tight (but not too tight). I get decent mileage - 35-40mpg, and the plugs look good, maybe a tad on the rich side, but all four look the same.

Any ideas? Thanks in advance.

 

[duaneage]

I agree that a sticking slide or a loose screw on top of the carb (the ones that seal the diaphram) is probably why it is acting up. Check that the little tubes you screw into the intake boots are sealed good, I have a problem with #3 on mine where the first two threads are bad and I cannot get the tube in tight sometimes.

Remember, #3 is the reference, then you sync 4 followed by 2 then 1 and check 4 again.

Lather rinse repeat until it squares off. The Clymers manual says 2 and 3 should be a tad below 1 and 4 but some like them all the same, especially with 4-1 headers.

 

[Einar J Skog]

Hi :lol: :lol:

I have always synced the carbs in another way.

1-2 with left adjuser,,, 3-4 with right adjuster,,, and then balanced those pairs with middle adjuster..and then set idle were I want it. 1200 rpm

GS1000GLX

 

[Hoomgar]

I was taught to sync at a higher RPM. I sync between 3000 and 3500 RPM and then give small blips to the throttle from there and get the sync close in that range. That range and higher are what is most critical as it is where you will operate most of the time. Anything below 3000 RPM is not as critical. I know this doesn?t help with your consistency issue but it is what I would recommend if you can?t solve the imbalance. Hopefully it is just a sticking slide like has been suggested but I would recommend syncing the way I have described either way.

 

[ptm]

Thanks for the replies - I appreciate the suggestions.

earl - I know the linkage you're talking about. Last winter when I cleaned the carbs I took those apart and cleaned them out. I put a dab of synthetic grease on them when I put them back together (at your suggestion, I think ), so I don't think it's due to them being dirty, but it's worth a look.

duaneage - These are the old VM style carbs, so there's no diaphram. The tubes are in pretty tight - the threads seem good and I tighten them until the o-ring on the tube is squished a bit.

Hoomgar - I agree, it's probably better to sync them at a higher rpm - I think earl talked about doing that in a thread not too long ago. I guess I could use the idle screw to hold the engine speed up there so I don't need three hands. I'm also concerned about sucking the mercury into the engine - especially with #4 getting so high. Plus, it just makes me crazy not knowing what's going on. There's something happening to make it work like it is.

How much should the vacuum vary with increasing engine speed? The others don't change more than a few cm between 1000 and 2500. It seems that the vacuum should increase more on all of them as the engine speed goes up. What kink of vacuum does it pull at 8000rpm?

Thanks again for the ideas.

 

[Anonymous]

You have to set the idle to 3000 RPM with the idle screw. Adjust one carb at a time. The idle will go up and down, readjust idle to 3000 after every carb adjustment. Once they are all within 1 to 2 cm of vacuum, you are done. Reset your idle and you are ready to go. Setting your carbs up at idle will do nothing, unless you drive at idle.

Andre

 

[KEITH KRAUSE]

I really doubt the slide is sticking. But still possible.
Easy to tell though, even with the carbs on the bike.
With the filter housing off, you can place your finger nail tip lightly against the slide and slowly turn the throttle. If the slide is binding at all, you will feels a sudden slip upward. If the carbs are in your hands, turning the adjusting screw along its entire range would easily show a bind too. You would feel it in your screwdriver and/or see it while observing the slide move. I lightly polish the carb walls with aluminum polish during each carb job, along with lubing the throttle arms. If it is binding, hopefully this will stop it.
I know the #4 level being so different from the others is bothering you, but if you see no significant difference in the plugs burning...
That's what bothers me. The plug should be showing that cylinder running richer, with that much more vacuum at 2,500.
This would open a new can of worms, so, because the mixtures are all the same apparently...
One thought about your reads above was a leaky valve. But I would think the read at #4 would be lower at BOTH the 1,000 and 2,500 rpm speeds, not just the 1,000 read. Anyone?
Another thought, have you correctly adjusted the pilot fuel and side air screws to that carb? Did you set all four carbs using the highest rpm method? Notice anything different about either screw settings compared to the other three?
Did you follow the correct synch order for these carbs? Setting the others to match #3?

 

[Einar J Skog]

Hi :lol: :lol:

Why is everybody talking about "Setting the others to match #3?"

In my world I adjust them even and then set the idle were I want it.

Nobody adjust at idle.

GS1000GLx

 

[duaneage]

Because unlike 1 ,2 and 4 , 3 does not have an adjustment. There are only three screws.

 

[Einar J Skog]

Hi :lol: :lol:

You are balancing them in pairs.
When carb 1 is going up ,carb 2 is going down..
When carb 3 is going up ,carb 4 is going down..
3 adjusters....
One between 1-2, One between 3-4 and One in the middle, balancing left-right pair.

Why cant everybody understand that ???

GS1000GLx

 

[ptm]

Thanks for the responses.

I know the #4 level being so different from the others is bothering you, but if you see no significant difference in the plugs burning...
That's what bothers me. The plug should be showing that cylinder running richer, with that much more vacuum at 2,500.
This would open a new can of worms, so, because the mixtures are all the same apparently...

Here's a shot of the plugs - they're due for replacement. They've got over 5000 miles on them, but the color's pretty good:

One thought about your reads above was a leaky valve. But I would think the read at #4 would be lower at BOTH the 1,000 and 2,500 rpm speeds, not just the 1,000 read. Anyone?
Another thought, have you correctly adjusted the pilot fuel and side air screws to that carb? Did you set all four carbs using the highest rpm method? Notice anything different about either screw settings compared to the other three?
Did you follow the correct synch order for these carbs? Setting the others to match #3?

The bike is completely stock - exhaust, carb jetting, and airbox except the air filter element itself is a Uni, but other than that it's all stock. Compression is 150, 140, 135, 152. A squirt of oil in #3 and it goes to 165, so it's the rings. The valves clearance is spot on. I did a complete overhaul of the carbs last winter (thanks to you and others on this site) - thorough clean, new o-rings, etc. I set all the pilot air screws to 1.5 turns out and all the pilot fuel screws to 1 turn out.

I have tried to adjust the pilot air screws for highest idle, but a full turn either way seems to make little, if any, difference. I ended up leaving tham all at 1.5 turns except #1 which is 1.75 turns out. I did do a bench sync before remounting them and when I first sync'ed them with the manometer (which I'm sure I did at 1000-1200 rpm) I believe they were pretty close (it's the one thing I didn't write in the service log).

Since then I've put on 5500 miles and as I said earlier, it runs really well - pulls smooth and strong up to 115mph @8000rpm 8O . I have no complaints except that it can be a little jerky coming on and off the throttle. I don't know if that's the carbs, throttle cable adjustment, or drive train. It seems better if I have the chain a little less loose and it's sometimes better after adjusting the throttle cables. I was going to chase that in a different post, but maybe it's related.

Thanks again for the ideas.

 

[Hoomgar]

Sorry if this is a hi-jack Paul but I need to ask a question.

Duane and Einar, what are you two on about here? Aren't his carbs VM like mine? It's a 70's GS so they would be the same right? Were talking about syncing. You adjust the float heights to do that. Each carb has it's own adjustment. What am I missing? What 3 screws are you talking about??? The need to learn is knocking.

 

[KEITH KRAUSE]

Sorry if this is a hi-jack Paul but I need to ask a question.

Duane and Einar, what are you two on about here? Aren't his carbs VM like mine? It's a 70's GS so they would be the same right? Were talking about syncing. You adjust the float heights to do that. Each carb has it's own adjustment. What am I missing? What 3 screws are you talking about??? The need to learn is knocking.

Mark, ptm does have 26mm VM carbs.
Einar has CV carbs, different synch procedure.
ptm's carbs have a master carb, #3. It has no adjuster screw for the slide. You set the others to match it.
I'm not sure what you mean by "We're talking about synching, you adjust the float heights to do that". ?? Synching and float adjustments are seperate things.

 

[KEITH KRAUSE]

I would first check the slide action. If good...
The bike being a "little jerky" coming on and off the throttle could very well be related to that difference in vacuum.
If not the vacuum, then try to determine if this symptom is a rich or lean condition so maybe you can tune it out with the pilot fuel/side air screws.
Does the problem get worse as the bike warms up? Suggests rich condition.
If you temporarily remove the air box lid, does it get worse (lean) or better (rich)?
Also, synching at such low rpm's isn't the way to do it, in my opinion.
I set them at about 3,000/3,500. I'm not saying you must set them there, but at least get the ignition advancer involved and set them at about 2,500? At least this is closer to where you spend time riding.

 

[Hoomgar]

Sorry if this is a hi-jack Paul but I need to ask a question.

Duane and Einar, what are you two on about here? Aren't his carbs VM like mine? It's a 70's GS so they would be the same right? Were talking about syncing. You adjust the float heights to do that. Each carb has it's own adjustment. What am I missing? What 3 screws are you talking about??? The need to learn is knocking.

Mark, ptm does have 26mm VM carbs.
Einar has CV carbs, different synch procedure.
ptm's carbs have a master carb, #3. It has no adjuster screw for the slide. You set the others to match it.
I'm not sure what you mean by "We're talking about synching, you adjust the float heights to do that". ?? Synching and float adjustments are seperate things.

I meant slides. Sorry about that. So now, why do his vm26 carbs not have an adjustment for the #3 slide? Both my sets do. I wasn't aware there were different animals out there?

 

[ptm]

I would first check the slide action. If good...
The bike being a "little jerky" coming on and off the throttle could very well be related to that difference in vacuum.
If not the vacuum, then try to determine if this symptom is a rich or lean condition so maybe you can tune it out with the pilot fuel/side air screws.
Does the problem get worse as the bike warms up? Suggests rich condition.
If you temporarily remove the air box lid, does it get worse (lean) or better (rich)?
Also, synching at such low rpm's isn't the way to do it, in my opinion.
I set them at about 3,000/3,500. I'm not saying you must set them there, but at least get the ignition advancer involved and set them at about 2,500? At least this is closer to where you spend time riding.

Thanks, Keith. A lot to think about.

I haven't noticed a difference with the throttle action when cold or hot. Unlike many it is not very cold-blooded, so it runs pretty well from the get go. I think the next rainy weekend I'll just pull the carbs; check the slide action; do a careful bench sync; remount them; adjust the pilot air screws; do a vacuum sync and see what I get. I'll definitely do the sync at a higher engine speed. The manual says 1500, but I've been doing it at the 1000-1200 idle speed (didn't look at the manual and forgot what I had read earlier...). It make sense to do it at higher rpms.

So now, why do his vm26 carbs not have an adjustment for the #3 slide? Both my sets do. I wasn't aware there were different animals out there?

I agree with Hoomgar - I'm a little confused about the #3 carb being non-adjustable. On both the ones on the bike and a set of '79 spares I have, all 4 carbs have the adjustment screw for the slides, including #3.

 

[Einar J Skog]

Hi :lol: :lol:

Sorrrrrry for the confusion..

CV carbs have 3 adjusters to sync in pairs .
They are a part of the gas linkage in front of carbs...

Older carbs have 4 adjusters ,on top of each trottle slide.



GS 1000GLx (Yoshimura 4-1,KN filter,DynoJet Stage 1,Wiseco 1100
pistons, Metzeler tires,++++++++++++++++)

 

[Hoomgar]

Hi :lol: :lol:

Sorrrrrry for the confusion..

CV carbs have 3 adjusters to sync in pairs .
They are a part of the gas linkage in front of carbs...

Older carbs have 4 adjusters ,on top of each trottle slide.



GS 1000GLx (Yoshimura 4-1,KN filter,DynoJet Stage 1,Wiseco 1100
pistons, Metzeler tires,++++++++++++++++)

Well that's what I figured but not according to Keith now??? He is saying that even the vm266 carbs on this 750 don't have an adjustment for the #3 carb so if that is true I simply want to learn. Why and what models? How do you know?

 

[KEITH KRAUSE]

Mark, I got caught up reading duaneage's post and because ptm didn't say otherwise in his next post, I assumed his carbs used a master carb. Many people visiting this site have said they have a 750 with no adjuster above the slide at #3. I have yet to see a set of these for myself, but I'm guessing they may be on the '76 or '76 and '77 models??
PS: I just read back and duaneage said "reference carb", not master. However, I've still read about owners saying they have these carbs.