Carb rich no matter what

sdew99

CB750 Enthusiast
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Nova Scotia Canada
Gonna throw this out here once more. 79 cb750k, carb #4 running rich no matter what I do. What else could make the carb rich other than:
-dirty air filter/restricted air
-improperly float height
-float valve not seating/sealing correctly
-pilot screw out too far
-air passages clogged
-worn needle
I have checked all these things and am grasping at straws at this point. I am
99.9% sure it's not spark and my leakdown test on that cylinder was 7% leakage so I can't imagine it being oil that fouling the plug.
 

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You could still have flooding. Are you sure the needle and seat are perfect? If you have a new needle it can still leak if the seat is not right.
I mean.. I have tested the needle and when the needle is closed, no fuel is coming past the seat. I would say the seat could probably polished but if it's not allowing gas by when closed, then it must be doing it's job?
 
Loose Chain, do you know of a way to verify the choke is working properly on a single carb while it is on the bike? I can't think of any. Coincidentally, that air cut off on carb 4 could be enrichening the mixture too if the diaphragm has holes.
 
Loose Chain, do you know of a way to verify the choke is working properly on a single carb while it is on the bike? I can't think of any. Coincidentally, that air cut off on carb 4 could be enrichening the mixture too if the diaphragm has holes.
Diaphragm for air cut off is brand new. I have taken top end of carb off with carbs on the bike and it seems to be consistent with the others. No binding or anything.
 
Loose Chain, do you know of a way to verify the choke is working properly on a single carb while it is on the bike? I can't think of any. Coincidentally, that air cut off on carb 4 could be enrichening the mixture too if the diaphragm has holes.
Are you thinking the #4 carb has a choke butterfly sticking closed? I think if the cable is hooked up with the carbs off the bike he should be able to operate the cable and see how they are working.
 
I've done that test on a bench (and while the carbs are in a bike, too). I'll tell you it isn't a 100% perfect test. Our fingers are going to push that float up with more force than fuel in the bowl will. Meaning the needle will be forced, instead of 'gently pressed'. What difference does it make?
If the float's tang (the part the float needle sits on) is not pushing the needle straight into the seat, any deviation will be overcome by the force of our hands. But that deviation can't easily be overcome by the fuel lifting the float.
I just had this fight with myself for two weeks on my cx500. Constant overfill and drain from the overflow tube, but I would 'verify' the needle stopping the fuel doing what you did. Long story short, I had to find pictures of an original float so I could bend the tang to match. In the end, it worked. But ib fight that SOB forever. I had to take multiple breaks from it after getting extremely frustrated.
While we are on the subject, your float height matters. If the fuel is too high in the bowl, too much fuel is sucked into the carb and you have a rich mixture.
None of us know if you have a carb issue and the above may not fix your issue. Let me go out on a limb and suggest pulling spark plug #4, and holding a couple layers of paper towel over the spark plug well while starting bike. The engine will spit out whatever is in the combustion chamber on the compression stroke into the paper towel(s). Maybe you'll be able to identify what the problem is; fuel or oil.
 
I've done that test on a bench (and while the carbs are in a bike, too). I'll tell you it isn't a 100% perfect test. Our fingers are going to push that float up with more force than fuel in the bowl will. Meaning the needle will be forced, instead of 'gently pressed'. What difference does it make?
If the float's tang (the part the float needle sits on) is not pushing the needle straight into the seat, any deviation will be overcome by the force of our hands. But that deviation can't easily be overcome by the fuel lifting the float.
I just had this fight with myself for two weeks on my cx500. Constant overfill and drain from the overflow tube, but I would 'verify' the needle stopping the fuel doing what you did. Long story short, I had to find pictures of an original float so I could bend the tang to match. In the end, it worked. But ib fight that SOB forever. I had to take multiple breaks from it after getting extremely frustrated.
While we are on the subject, your float height matters. If the fuel is too high in the bowl, too much fuel is sucked into the carb and you have a rich mixture.
None of us know if you have a carb issue and the above may not fix your issue. Let me go out on a limb and suggest pulling spark plug #4, and holding a couple layers of paper towel over the spark plug well while starting bike. The engine will spit out whatever is in the combustion chamber on the compression stroke into the paper towel(s). Maybe you'll be able to identify what the problem is; fuel or oil.
Thank you for all the information. Very good points! I will double check to make sure float height is correct. I did perform the test you mention and wasn't able to tell if it was oil or gas. Very small amount was on then paper towel with little color.
I just acquired a set of spare carbs, and am going to clean them and try the new #4 and see what happens. I'll keep you updated.
 
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