Third carburetor has a mind of his own!!!

[nikolab]

Yamaha 25hp, 1992, 3 cyl, cdi...
In neutral it works just fine, when I put it in gear on accelerating it boggs. When I try to set the pilot screw (needle screw, control of fuel air mixture) on the first two carburetor the engine dies when I close either one of them (probably normal), but when I close the third one the engine just keeps on running like nothing happens. When I take it out it looks normal (like the other two). Can there be a problem with dirty carburetors, or something else. I am not a mechanic but I like to take care of my Yamaha. Thank you in advance.

 

[rodbolt]

Re: Third carburetor has a mind of his own!!!

test compression,test spark fix the fuel system.
loss of spark or low compression on a cylinder can mimic this issue.
once spark and cylinder compression are verified good remove and inspect the fuel pump or simply replace it as a maint item. they do fail.
then remove and clean the carbs replacing worn or damaged parts as nessasary.
the air/fuel screws you refer to are not actually air/fuel mixture adjusting screws they simply allow more or less of the mixture past the throttle plates.
the mixture is actually controled by the pilot fuel and pilot air jet.
its also why carb link sychronization is so critical on most yamahas.
while the carbs are off take a light and carefully inspect the reeds.
by closing off the screws while idling you shut off the avalible fuel.

 

[nikolab]

Re: Third carburetor has a mind of his own!!!

I will check all of the mentioned, but just to add one more issue, normal idle, when it reaches the WOT it goes very good only the simptoms occure when accelerating, has speed, has power (does this change your opinion). Thank you for replying.

 

[ianyoung77]

Re: Third carburetor has a mind of his own!!!

Your symptoms are very common but the problem can be in many places. It's probably not going to hit WOT without spark but doesn't mean compression is perfect. Most likely culprit is fuel but that doesn't narrow it down much. If you don't want to pull down the carbies the first thing to check is the adjustments, but usually these don't just come loose. For a bog on acceleration the main thing to look at is the lower cam roller connection. It should basically be touching the accelerator arm. If there is a gap adjust the collar remembering the adjusting screw is reverse rotation. If this doesn't help you will need to follow Rodbolts list - good luck