04 Yamaha f150 misfiring?

Josephmast94

Cadet
Joined
Nov 14, 2013
Messages
12
I have been battling this problem for some time now. my port side motor struggles to get above 30k rpm at wot, it shakes the entire boat. if i pump the throttle it will get up to around 35k but immediately fall back to 28-30k. here are some things I have tried.
compression is good, all 4 cylinders around 150
spark test, spark jumps a 7/16 inch gap and has new plugs
changed fuel filters including f/w seperator, inline, "f" filter, and vst filter.
gas is good because the other motor runs just fine off same tank.
switched injectors with the starboard side engine.
fuel pressure is good, 45 before starting with ingniton on then 37 at idle.


also the bottom cylinder is turning the plug black and its wet with gas. the engine shakes a little at idle compared the the other but seems to idle just fine around 750 rpm other than that. thanks in advance for any help!
 

Capt Ken

Commander
Joined
Jul 30, 2002
Messages
2,270
Re: 04 Yamaha f150 misfiring?

What is the fuel pressure when you lose power?
 

Capt Ken

Commander
Joined
Jul 30, 2002
Messages
2,270
Re: 04 Yamaha f150 misfiring?

It will tell you a lot. If it drops then you are losing fuel supply to the VST.
 

Capt Ken

Commander
Joined
Jul 30, 2002
Messages
2,270
Re: 04 Yamaha f150 misfiring?

To save some time. With the engine running on the hose, listen for light tapping in the valve cover. If you hear tapping, pump the primer bulb and see if the tapping goes away. If it does then the fuel pump is bad.
 
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