YAMAHA power problem

[wes whitzel]

I have a 1986 yamaha 70 hp I was cruising along as normal and the motor suddenly reduced speed to half throttle I did not hear an alarm I checked the gas and gas line then shut down the motor and checked everything out everything seemed normal I started back up and it ran fine for about 10 minutes then it did the same thing I took it home and did a overheat sensor check as described in my repair manual it checked out O.K. I checked my alarm buzzer and it checked out also I do not have oil injection I have never replace the fuel pump diaphragm or rebuilt the carbs the motor actually has very few hours on it I have ran it for two years with no problems...I replaced the Impeller yesterday just in case and it did the same thing I would appreciate any help thanks Wes Whitzel ....UPDATE!! I replaced the fuel pump it still ran rough I let the engine warm up and then pulled the spark plugs and sprayed in Yamaha carbon solvent and let it sit for 15 minutes replaced plugs and started the motor up and sprayed the solvent into each carb for fifteen seconds each. I shut it back down and cleaned the spark plugs again as per directions on the can.after that it ran great. I am not sure what the problem was but the Yamaha magic solvent spray worked. I greatly appreciate the information obtained on this forum. Thank You...Wes Whitzel

 

[rbu]

Re: YAMAHA power problem

Does it surge when it drops rpm's?<br /><br />My initial guess is fuel starvation. I would go through the fuel system again. If you can get the problem to duplicate again, try squeezing the primer bulb while its running and see if it will pick up the rpm's. It could be as simple as a bad primer bulb.

 

[Yepblaze]

Re: YAMAHA power problem

Had similar problem and it was small bit(s) of debris in float bowl(s) being sucked up into main jet. The debris would settle back out if the motor was shut down or any attmpt was made to diagnose or repair. But sure enough, next time out it would happen again.