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Discussion Starter #1 (Edited)
I suspect that my bike is running to lean.
  • The spark plugs looks like lean running plugs
  • One of the exhaust pipe has started to become yellow\blue
  • When engine warm and I open (edit: closed) choke while riding nothing happens, even at 6-7000 rpm.
Otherwise everything else seems fine. Revs up nice and run past 70 mph with no problems.

How can I achieve better mix if it runs lean? Jets are clean and float height is set to 26mm according to specs.
Will pilot screw influence mixture while running or only idle and at low revs? Should I try with lower float height (edit: I mean raise fuel level)?

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Time to start the wonderful journey of jetting. It's much safer to be a bit too rich than too lean. Else you'll be burning up internals.
 

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You got it, chief. What are your primary jets? 105? Try 110 or jump straight 115. Jetting is so dependent on your available fuel, weather, altitude, etc. There's no one size fits all. And yeah, the bike will run like a bat out of hell when lean, but you're also going burn up your motor.

You even noted it yourself. Choke makes no difference when the bike is warm. You're definitely on the lean side. Basically if you're running flat out and flip closed the choke and it does nothing go up in jets, if it stumbles but doesn't die...jets are probably good, if it dies then you're probably too rich. It took me awhile to get it, but my front wheel comes off the ground shifting into second and almost completely extends the forks when shifting into third on my little 71 cb350.
 

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Idle mixture screws are only for idle and just barely above, beyond that they have no function, it gets transferred to the primary and secondary main jets respectively as the revs rise. I need to help you clear up something though... the choke is CLOSED when you want to add it at higher revs to see if it helps the running situation. The choke is OPEN under normal riding conditions and only used (CLOSED) when the engine is cold to help enrich the mixture for cold starts. And to further illustrate, you may not have noticed this:
310389
 

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I suspect that my bike is running to lean.
  • The spark plugs looks like lean running plugs
  • One of the exhaust pipe has started to become yellow\blue
  • When engine warm and I open choke while riding nothing happens, even at 6-7000 rpm.
Otherwise everything else seems fine. Revs up nice and run past 70 mph with no problems.

How can I achieve better mix if it runs lean? Jets are clean and float height is set to 26mm according to specs.
Will pilot screw influence mixture while running or only idle and at low revs? Should I try with lower float height?
Confirm that the carbs are in sync. If so, it would suggest that one carb is not getting adequate fuel. Either a jet is partially plugged or the float level is incorrect.

You may need to remove the carbs and give them a thorough cleaning. Then check again that they are synced.

Gas with ethanol will make a carbed engine run lean compared to gas without.

Are both the plugs the same color?

A lower float height would make it run leaner.
 

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Discussion Starter #7
Idle mixture screws are only for idle and just barely above, beyond that they have no function, it gets transferred to the primary and secondary main jets respectively as the revs rise. I need to help you clear up something though... the choke is CLOSED when you want to add it at higher revs to see if it helps the running situation. The choke is OPEN under normal riding conditions and only used (CLOSED) when the engine is cold to help enrich the mixture for cold starts. And to further illustrate, you may not have noticed this:
View attachment 310389
I meant when I close. The choke is normally in open position (like in the picture) when driving.
 

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Discussion Starter #8
Confirm that the carbs are in sync. If so, it would suggest that one carb is not getting adequate fuel. Either a jet is partially plugged or the float level is incorrect.

You may need to remove the carbs and give them a thorough cleaning. Then check again that they are synced.

Gas with ethanol will make a carbed engine run lean compared to gas without.

Are both the plugs the same color?

A lower float height would make it run leaner.
Carbs are cleaned and o-rings replaced, kept original brass. I struggle to sync the carbs perfectly. Bench syncing with feeler gauge and guitar string didn't work so well. I think mix ratio is better in the right cylinder, see picture below.

Will it help if I adjust the float height for the lean carb by 1mm? If not then I will try doctorruckus suggestion and try different jet size.

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You've check your ignition system to insure the coils are good, timing is set, plug caps are proper resistance, correct NGK plugs are installed? 90% of carb problems are actually ignition related.
It's not possible to bench sync independent carbs, once you make any idle speed adjustment the "bench sync" is off. You need to sync the carbs to obtain the correct idle speed, then set the mixtures. Idle speed cannot exceed 1300 or the mixture cannot be set properly, spec is 1100-1300 so use 1200.
After the idle speed and mixture are set you can do the high speed sync where you adjust the cables so each carb is operating the same.
 

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Discussion Starter #10
You've check your ignition system to insure the coils are good, timing is set, plug caps are proper resistance, correct NGK plugs are installed? 90% of carb problems are actually ignition related.
Based on color of spark plug and the fact that closed choke doesn’t affect a warm engine while running that the issue most probably is fuel/air mix?
I have checked ignition but not cap resistance and coils, I’ll check them too.
 

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A closed choke on a warm/hot engine should flood the cylinder fouling out the plugs. Sounds like a massive vacuum leak.
 

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Discussion Starter #12
I had a vacuum leak that lead to high idle rpm when hot engine but I think I have solved that. Would it help adjusting float height by 1 mm?
 

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I had a vacuum leak that lead to high idle rpm when hot engine but I think I have solved that. Would it help adjusting float height by 1 mm?
It might, but it's a BandAid fix if they are both set properly now.

Something is out of adjustment, worn, or a passage/jet is partially plugged in the carb most likely. Bench syncing should have gotten you close, and if a significant adjustment is required when checking with vacuum gauges I'd say it's and indication of an issue somewhere.
 

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Discussion Starter #14
Today I thought I would sync carb according to this video
.
When adjusting idle mixture screw for left cylinder the engine didn’t not die no matter how much I turned the screw out, I could completely unscrew the idle mixture and the left cylinder would still run.
On the right side the engine died after 3 complete turns. Specs is 3/4 - 1 turn.
Both died when I tightened all the way in.
Does this say anything?
 

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Today I thought I would sync carb according to this video.
When adjusting idle mixture screw for left cylinder the engine didn’t not die no matter how much I turned the screw out, I could completely unscrew the idle mixture and the left cylinder would still run.
On the right side the engine died after 3 complete turns. Specs is 3/4 - 1 turn.
Both died when I tightened all the way in.
Does this say anything?
That would suggest the fuel flow in the left carb is being restricted compared to the other side.

Either the idle mixture passage or the Pilot Jet are partially blocked - is what I'm thinking.

This video also covers syncing, and was easier to follow for me -

 

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If I'm not mistaken (I have been before...) the idle mixture screw on those carbs adjusts fuel flow (not air flow like some others) so turning the screw OUT increases fuel flow and provides a RICHER mixture.

Bottoming them out blocks all fuel flow and should kill the engine (as it did for you)..

Someone please correct me if I'm wrong.
 

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The general rule is if the mixture screws are on the engine side of the float bowl, they're fuel screws so more turns out is more fuel. If they're on the air cleaner side, they're air screws so more out is more air mixed with the same fuel
 

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Discussion Starter #19
The general rule is if the mixture screws are on the engine side of the float bowl, they're fuel screws so more turns out is more fuel. If they're on the air cleaner side, they're air screws so more out is more air mixed with the same fuel
Thanks for the clarification. On 722a carbs the screw is on engine side.
 
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