88 Yam V6 Hi Speed RPM Loss

[drnorust]

Have a 1988 Yamaha Model V6EXCELX-F. Newly rebuilt power head. Motor runs fine at 4600-4800 RPM, then after a period of time (5-30 minutes), the RPMs drop 300-400. The RPMs can be increased by providing more throttle. After a period of time- 5 -10 minutes, the RPMs will increase back to the original level. Sometime they will stay steady for only a few minutes, and other times it will run without a decrease for 20-30 minutes.<br />We have completely drained the fuel tank and swabbed it clean. All hoses, fuel filter, etc.are new. <br />Any thoughs, and if so- how do we test?

 

[rodbolt]

Re: 88 Yam V6 Hi Speed RPM Loss

are you sure the throttle is not creeping back?<br /> sooner or later it has to run out of throttle

 

[drnorust]

Re: 88 Yam V6 Hi Speed RPM Loss

Thanks for the reply.<br /><br />No- not the throttle. If you leave it in the same position, the rpms will kick back up to the original position. We think it may be a coil getting hot, or we have also heard it might be the rectifier. hate to start changing parts on hunches.

 

[Ray Neudecker]

Re: 88 Yam V6 Hi Speed RPM Loss

The throttle position sensor can be a cause. I assume you changed the fuel pumps when going through the fuel system. I have seen also a needle stick intermittently and cause the problem. Many times it is caused by a bowl gasket coming apart or a worn needle.