9.9 hp Yamaha 4 stroke missing at temp

[gybe98]

I have a 1987 9.9 hp Yamaha 4 stroke pull start that runs great for the first 15-20 mins. After that it begins to run rough. So far I have:<br /><br />1. Replaced fuel pump and strainer<br />2. Rebuild carb<br />3. Replaced spark plugs<br /><br />I tried adjusting the idle screw but that had no effect on the performance. If the engine is shut down for a few minutes and then restared it runs fine for a short period of time (2-3 mins) and then begins to run rough again. <br /><br />Any suggestions? Thanks in advance, this site has been very helpful in the past.

 

[jim dozier]

Re: 9.9 hp Yamaha 4 stroke missing at temp

Do you have any evidence that it is overheating and running hot? Do you have a strong tell tale stream out the back? When was the last time the water pump impeller was changed? When it starts to act up can you lay your hand on the cylinder between the flywheel and the cylinder head for 2-3 seconds without burning yourself?

 

[gybe98]

Re: 9.9 hp Yamaha 4 stroke missing at temp

I bought the engine used so I'm not sure when the last time the impeller was changed. However I do have a strong tell tale stream out the back. I'll have to try putting my hand on it next time.