4-stroke madness

[shorty_40_4444]

Questions about the 4-stroke 50hp Yamaha motor<br /><br />I have a question regarding a Yamaha; 4 stroke engine. The motor is appox. the first 50hp; 4-stroke of its kind; Yamaha just came out with these motors as the 4-stroke era began. The engine starts and runs fine, but approximately after 2 minutes while in forward gear, at top-speed, the engine RPM fluctuates (speeds up and slows down). This fluctuation occurs all the time. After cool down and restart, the engine behaves the same; runs fine for the first 2 min. then starts to fluctuate again. I just bought new spark plugs, fuel filter, oil filter, and disassembled the Carburetors and cleaned them thoroughly, also cleaned the fuel injectors thoroughly. I replaced the oil filter, fuel filter, and spark plugs w/brand new ones’; so… this eliminates any problems associated with these parts, also electrical problems are eliminated, due to a thorough check using a meter test gauge. Also, checked the air filter and found out that its fine. The gearing and lower Unit and prop are all functioning great. After start, it runs better, but to no avail is the motor in the shape that I’d like it in; still behaves the same. My question that I’ve told myself, is that the thermostat (spring loaded) is malfunctioning, or something along those lines. I’ve asked a friend about the maintenance of this part, and he informed me that I could take this part off and place it in boiling water to free up the gunk-build-up. What do you think about this situation? What else could be wrong with the motor?<br />

 

[DJ]

Re: 4-stroke madness

Shorty,<br /><br />Which does it have, carberetors or fuel injectors? It can't have both. That's going to make a big difference in diagnosis.

 

[shorty_40_4444]

Re: 4-stroke madness

Yeah, sorry... It is carbureted

 

[Dhadley]

Re: 4-stroke madness

What happens if you back off the throttle slightly when it starts doing this? And what rpm does it start doing this?

 

[jim dozier]

Re: 4-stroke madness

Does this engine have a distributor for the ignition or an advance plate under the flywheel. Either way check to see if something is loose or worn and letting the ignition timing wander at high rpm or WOT. Also have someone pump the fuel bulb during these episodes, perhaps your fuel pump is on the edge or you are sucking air through a loose connection and its only apparent at WOT.

 

[shorty_40_4444]

Re: 4-stroke madness

Thanks everyone,<br /><br />Problem solved! I started to pump the bulb during these episodes and amazingly it appeared that the problem was as, "jimd" suggested. Further anlysis concluded that the fuel pump needed to be replaced. Engine runs great! Thanks to all whom replied...