Yamaha 200 HP RPM acting weird...

[Simong]

Hi all, I'M new here but I know you guys got tons of knowledge here and maybe can help me, so here we go.... twin Yamaha Precision blend
1990 200HP 2 stroke carbs . only at one motor at cruse RPM,(not lower RPM or idle) jump from lets say 3800 to 4100, then to 3700 and back to 4100 this happen very fast and constantly ,and I can hear the motor reving up and down,
is that Spark Plug problem or carbon, what you guys thinking can cause that?
B.T.W carbs rebuilt end of last season and I use Startron on regular basis.
T.I.A Simong.

 

[AQUADJ]

Re: Yamaha 200 HP RPM aucting wierd...

Re: Yamaha 200 HP RPM aucting wierd...

I've got a similiar problem with my twin 250hp efi motors. They normally run fine except sometimes I'll get a fluctuation between 3700 and 4100 rpms, more or less it won't happen.

I've determined it's cavitation. At the right speed, trim angle, rpm, and conditions it will fluctuate. I've changed the toe in - toe out of the motors which helped a little but occcasionally it still happens.

Dave