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Hanging Idle But Seems Rich

rdalexanry

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Hello,

I just acquired a 1981 cb750 that I am trying to get running as the wife's first bike. I can get it to stay running but it idles very high. Cold starting on choke everything seems fine until the choke is off then it stalls unless given throttle. However if given throttle it has a hanging idle at 3k+ rpm. If the choke is engaged again the idle climbs farther. Riding down the road I can't get it to rev past 5k rpm in any gear, and to start it hot the throttle has to be held wide open. It ran fine for a brief minute for me to get a timing light on it and that checks out as far as I can tell. I plan on pulling plugs next to confirm richness, but looking for guidance one where to go from there.
 
Have you cleaned out the carbs at all?

Since this bike is new to you, you need to do quite a few things unless you have proof that the previous owner or a shop did them. You should, in this order:

Check the spark plugs and spark plug wires for condition. If the spark plug wires are stiff, replace with fresh copper-core wire of the same diameter which you can buy at your local auto parts store.
Check and, if needed, adjust the valve clearances.
Remove the airbox and the carburetors
Inspect the condition of the air filter, and the rubber boots and clamps that connect the airbox to the carburetors. If you find any cracks or they are misshapen (hole is not round when at rest) you should replace them.
Inspect the rubber insulators and clamps that hold the carburetors to the engine. Same deal -- any cracks or deformed shapes mean replacement.
Open up and thoroughly inspect the carburetors. Any stiff or cracked rubber parts? You guessed it, replace them. Any gasoline residue or other junk inside the carb that's not supposed to be there? Clean it out. The process isn't super complex but if you're inexperienced with carburetors, get experienced help to guide you through or take them to a pro.
Make sure your throttle tube, throttle cables, and choke cable move freely when not connected to anything.
Reinstall the whole intake and carburetor system per the factory service manual.

This sounds like textbook carb issues from sitting to me.

Carbs do not like to sit without being used (with gas inside) and unfortunately a lot of motorcycles sit.
Ethanol gas has made this problem worse. You prevent it by riding your bike frequently, putting ethanol-free gas in, or using Sta-Bil.
 
Thanks those were my next steps. I know my way around carbs pretty well, but this is the first multi cylinder multi carb bike I have messed with them on. Do these bikes fire cylinders 1&4 and 2&3 at the same time? Didn't see any way for the ignition to fire each cylinder individually when looking at it.
 
You've guessed right. The coils are wired that way and fire 1/4 and 2/3.

Very good to hear you're familiar with carbs -- you should have no issue with these. Just keep track of each set of parts as you disassemble them. You could take them apart one at a time if you want.

Also a piece of unsolicited advice for when you reinstall them -- a heat gun or a hair dryer used to warm up the rubber insulators and airbox boots makes reinstalling the carb rack much easier.
 
You've guessed right. The coils are wired that way and fire 1/4 and 2/3.

Very good to hear you're familiar with carbs -- you should have no issue with these. Just keep track of each set of parts as you disassemble them. You could take them apart one at a time if you want.

Also a piece of unsolicited advice for when you reinstall them -- a heat gun or a hair dryer used to warm up the rubber insulators and airbox boots makes reinstalling the carb rack much easier.
Never thought of that. I've used hair spray in the past to give some lube then dry try to make a seal.
 
Status Update:

Plugs 3&4 completely black
Plugs 1&2 norml

Compression......
Cylinder 4 125psi
Cylinder 3 125psi
Cylinder 2 125psi
Cylinder 1 85psi dry 125psi wet

Oh boy.
 
125psi seems low for the other three though and the fact that all four are consistent bothers me -- are you sure your compression tester is good, and did you hold the throttle open while measuring?
 
Throttle was open, not too concerned with the low reading as of now. Once it has run for a bit ill recheck it, and once I get my transducers back from fluke I'll check the accuracy of my gauge.
 
It's supremely unlikely for all four cylinders to be exactly identical, especially after one of them reads low and is then oiled up to exactly the same compression reading as the others. That makes me think your gauge is doing something like topping out at 125psi. If you have access to a source of known-PSI higher air pressure you should check the gauge against it.
 
Well carb rebuild was somewhat successful. Cold bike idles amazing and can use the whole rev range in neutral. In gear under load first 1/2 throttle is strong but dies off after that and just sputters. Once hot it doesn't want to idle either. Throttle response remains crisp in the pilot jet range just acts like it gets rich out of no where on later half of the throttle. Also has a distinct whooshing sound when hot from the cylinder 1&2 pipe. I'll open the engine this weekend to check valves and go from there. Hopefully documenting all this will guide someone else one day.
 
Well carb rebuild was somewhat successful. Cold bike idles amazing and can use the whole rev range in neutral. In gear under load first 1/2 throttle is strong but dies off after that and just sputters. Once hot it doesn't want to idle either. Throttle response remains crisp in the pilot jet range just acts like it gets rich out of no where on later half of the throttle. Also has a distinct whooshing sound when hot from the cylinder 1&2 pipe. I'll open the engine this weekend to check valves and go from there. Hopefully documenting all this will guide someone else one day.
Sounds like a vacuum or exhaust leak -- the "whooshing" noise is a fairly strong indicator of this.
 
Well findings for the day. I have 1 exhaust valve per cylinder I can't even get a .0016" feeler gauge in. Not good not good at all.
 
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