87 yamaha 150 starving for fuel HELP!!!

[jonj20]

Hello ALL, I am a marine tech and new to the forum..

I currently have a 1987 Yamaha 150 that is baffling me..

It is starving for fuel at top end after about 2 minutes of run time.. only at high rpm.
it does run perfect within two minutes or so then it starts starving. i can prime the bulb and it will catch back up.

i have already changed, primer bulb, cleaned and rebuilt carbs, changed fuel/water separator, pumped tank, changed fuel line, and also changed fuel pump. i am at losses on this one and need you guys's help here.

more info on the pump. i only got one.. all we had in stock so i threw it on the top.. didnt change anything.. so i switched it to the bottom.. still no change..

could of both pumps went bad? is that common?

if you need to know anymore info let me know.. if anyone can give me some help ASAP..

trying to get this customer back on the water..

Thanks you all again.

 

[99yam40]

Re: 87 yamaha 150 starving for fuel HELP!!!

monitor fuel pressure/vacuum at problem RPM

 

[jonj20]

Re: 87 yamaha 150 starving for fuel HELP!!!

monitor fuel pressure/vacuum are problem RPM

So what could be causing this?? I can hook up a gauge to pressure and vacuum tomorrow but if there low what could be the problem? And thanks

 

[99yam40]

Re: 87 yamaha 150 starving for fuel HELP!!!

bad pump, sucking air some where, or a restriction would be my guess.
Pick up tube or pinched line, any connection can leak air in on suction side of pump

 

[jonj20]

Re: 87 yamaha 150 starving for fuel HELP!!!

okay i took customers boat out again today and i got to looking and the middle carb was missing BOTH pilot air jets.. could this be causing this problem? what do they do? it seemed to run a little better today but was still having the same issue..

 

[99yam40]

Re: 87 yamaha 150 starving for fuel HELP!!!

You say you cleaned and rebuilt carbs along with a lot of other things already,
but just now you noticed it is missing the pilot adjustment screws!!!

Did you not take the carbs completely apart to clean properly and then adjust everything like the service manual calls for after reinstalling them?
seems you would have noticed that before if you tried to adjust to specs.

 

[jonj20]

Re: 87 yamaha 150 starving for fuel HELP!!!

The screws that are missing isn't really a adjustment screw though.. And when I cleaned them I took bowl float and needle and all gaskets off then bathed them then put all back together.. These screws go in the barrel of the carb..

 

[99yam40]

Re: 87 yamaha 150 starving for fuel HELP!!!

pilot screws are the idle mix and have passages and jets that feed them
Seem like you did not take carbs apart and clean all passages

 

[jonj20]

Re: 87 yamaha 150 starving for fuel HELP!!!

pilot screws are the idle mix and have passages and jets that feed them
Seem like you did not take carbs apart and clean all passages

i bathed them.. that crap goes through everything... but if its just for idle mix.. then thats not my issue... its only top end and idle is perfect.. hmmm...

 

[99yam40]

Re: 87 yamaha 150 starving for fuel HELP!!!

still says you did not take apart and clean properly.
You said jets and then screws are missing, so I have no idea what you have really done.
Yes some fuel and air goes through pilot jets and adjustment screws regulate that at low RPM and all of the fuel goes through the main jets to the motor.
If you block off some of it it will affect the overall.

Do not see how if the adjustment screws are missing it could ever run well at idle

 

[jonj20]

Re: 87 yamaha 150 starving for fuel HELP!!!

Gathered more info two top cylinders are low on spark.. i have a feeling its the stator.. after two minutes or so it heats up to much and then drops the spark.. will be pulling flywheel tomorrow to inspect it.