Yamaha 90 HP Jet Drive low RPM's

[LittleSmoky]

We recently purchased a 93 Yamaha 90 HP with a jet drive. (Model # 90TJRR Ser # - 6H1 L 485331) and are unable to get over 3000 rpm's with it. It seems like there is an rpm limiting device as the engine runs fine at low rpm's. We have double checked all fuel connections and the carbs were just cleaned and set. Any ideas.<br /><br />Thx in advance.<br />

 

[JB]

Re: Yamaha 90 HP Jet Drive low RPM's

Hi, Smokey.<br /><br />Check these things.<br /><br />Are the carb throttle plates opening all the way?<br /><br />Is the throttle linkage advancing the ignition all the way?<br /><br />What is the compression on each cyl?<br /><br />Is it running on all three cyl? You can determine this by disconnecting (very carefully) each spark plug wire while the engine is at idle. A three cyl 2 stroke will seem to be running fine on two cylinders. If you disconnect a good cylinder the rpm will drop or the engine will stop. If you disconnect a cylinder that is not making power there will be no change.<br /><br />Good luck and let us know what you find.

 

[LittleSmoky]

Re: Yamaha 90 HP Jet Drive low RPM's

Thanks JB. After following your reply we found the compression readings to be 98,95,92 from top to bottom. We also found that it was running on only 2 cylinders. Replaced the middle coil and it ran good for a short period of time. Now we are fouling the middle plug consistently. Any reason this might be? Would a hotter plug possibly help? <br /><br />Thanks again.<br /><br />Smoke