92 Yamaha 40 runs rough, misses

[PeteB88]

New, to me, Yam 40 3 cyl, oil injection, runs rough just above idle, middle power settings, but runs fantastic after plane and WOT. I have followed decarbon procedures and cleaned carberators with Seafoam, use good gas, etc and noticed improvements but cannot smooth out (rough and occasionaly miss) in low/mid cruise settings. It also seems to smoke more than it should at startup and idle to mid power. Suspect carbs are rich and/or timing. New plugs seem oily. Help? Advice? How to adjust carbs? <br /><br />thanks!!!

 

[ivar]

Re: 92 Yamaha 40 runs rough, misses

Does the plugs look similar, or is one of them different from the others? If the lower plug looks clear/whiter than the others, the lower crank seal may be going bad, allowing water into the lower cyl. <br /><br />Otherwise it can be that your mixture screws needs a little adjustment, they may change over time. <br />If you get a service manual, the mixture screw settings are listed there,along with a lot of other useful information.

 

[PeteB88]

Re: 92 Yamaha 40 runs rough, misses

Plugs all look the same, compression like 125, 125, 122 last week - multiple tests. <br /><br />Thanks!!

 

[ianyoung77]

Re: 92 Yamaha 40 runs rough, misses

What year is the motor? When you pull the plug leads at idle does it behave the same for all cylinders?

 

[PeteB88]

Re: 92 Yamaha 40 runs rough, misses

Thanks - it's a 1992 and no, I have not tried what you described. What does that test do? I assume it would isolate a bad cylinder but what else can you figure out from this/ <br /><br />thanks again

 

[ianyoung77]

Re: 92 Yamaha 40 runs rough, misses

Yes, I am trying to determine a bad cylinder. You should find it will idle on any one cylinder. If not the carby synch could be out. Unfortunately I have a 50Hp of much the same vintage which behaved exactly the same as yours. The bearings were gone.

 

[PeteB88]

Re: 92 Yamaha 40 runs rough, misses

Thanks - so what you're saying is that the motor should idle on one cylinder? I just got the Sedoc manual and am reading it. I will reference carb synch. What bearings -are you talking about crankshaft bearings? I don't know how that motor can run like a raped ape at WOT with bad bearings. IF you don't mind sharing some more advice I would very much appreciate it. <br /><br />thanks again

 

[ianyoung77]

Re: 92 Yamaha 40 runs rough, misses

Strange but true. Yes I am referring to crankshaft bearings. My compression readings were good as well! The boat would idle and go flat strap but anything in between was intermittent. One minute it would pull me out wakeboarding and the next I could not even get past idle. When pulled apart one cylinder was shredded.<br />Being the same motor as yours it is something to consider. What year is yours? Mine was 1990. How loud is the motor? I hope it's not bearings since it would almost write off your motor

 

[PeteB88]

Re: 92 Yamaha 40 runs rough, misses

The motor is a 1992. Rough running is best described as an occasional miss - - like a cough at low speeds just below planning speed and when on plane, slow plane. It does fine a trolling speed but still seems rough to me. I adjusted idle speed way, way down until it would go very slow then occasionally stall. Then I turned the idle screw up a bit and it trolls great. I do not have accurate speedo or GPS to measure speed. I am going to continue trouble shooting fuel system, carb adjust and timing. I guess we'll see - bearings would be sad for sure. <br /><br />best

 

[PeteB88]

Re: 92 Yamaha 40 runs rough, misses

Forgot one more thing- - spark plugs appear the same but seem oily. It smokes alot at low Rs as well.