carburator/high speed jets yamaha

I would like the forum to weigh in on this mystery. My 1998 Yamaha 150hp 2 stroke outboard seems to have lost some power. This motor is carburated and the fuel is pre-mixed. I have discovered a loss of mph as measured by a GPS is down appx. 10 mph. The engine idles well and seems to perform well until the end of the throttle. The final 1/5 or 1/4 of the throttle is just flat. I mean that there is no increase in RPM. There is also no increase in speed. My Tachometer is not of much use because the needle will fluctuate when the trim button is depressed. The boat used to top out around 42 to 45 mph but now it will only reach 31 to 32 mph. The only thing different that has been done is the engine was de-carbed by me. The engine compression is 120 in all six cylinders. Any advice would be greatly appreciated.

Re: carburator/high speed jets yamaha

May be something as simple as a linkage adjustment of the carbs or throttle cable.Insure that your carb flaps are opening fully horizontal when the controls are set to full throttle.<br /><br />If that checks out there may very well be a restriction in the high speed jets and or porting in carbs.<br /><br />What are the rpms at WOT when the trim button isn't depressed?<br /><br />I'm thinking this topic will get a lot more response in the Suzuki and Yamaha troubles forum.

Re: carburator/high speed jets yamaha

Moving to Suzuki& Yamaha Troubles.

Re: carburator/high speed jets yamaha

What are the rpms at WOT when the trim button isn't depressed?<br /><br />the rpm is around 4800 to 5100. The tach. has had it's own set of mysteries. I recall the the rpms used to be at 5500 for WOT.<br /><br />I should add that the hull is a Cobia Sportdeck 206. The fuel filter was clean and there was no water in the bowl. I have a water seperator filter. There have been no adjustments to the linkage as the factory paint on the adjustment knobs is undisturbed. Engine has around 100 hours. Ring free is used religously too.

Re: carburator/high speed jets yamaha

Broken Boat . . . any luck with this problem?