help with loss of speed at W.O.T.

[brokenboat]

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.

 

[jim dozier]

Re: help with loss of speed at W.O.T.

First you will need a manual.<br /><br />You should probably replace the plugs after a decarb.<br /><br />Check to see if pumping the fuel bulb has any beneficial effect on WOT, if it does suspect the fuel pump.<br />Check to see if partially choking the engine has any beneficial effect on WOT, if it does suspect the fuel lines and or gummy carbs<br />Check for air/fuel leaks on any fuel hoses and connections especially where the fuel line connects to the engine.<br />Check the ignition/carb/throttle linkage. Verify that the timing advance is hitting the max mark, verify that the carb butterflies are completely open at WOT.<br />Check the prop and boat bottom for damage.

 

[brokenboat]

Re: help with loss of speed at W.O.T.

Thanks, somehow I have two posts going. How do I partially choke the engine? By pushing in the ignition key? The hull bottom has no damage. There is some slime on it, but nothing else. It has antifouling paint. There is no damage, that I can see, to the prop. The prop bites the water well and there is good acceleration. There just isn't that oomph at the end of the throttle range.

 

[jim dozier]

Re: help with loss of speed at W.O.T.

Some engines have a manual choke on the engine that you can partially activate with the cowling off (you obviously need someone else to drive) or you can stick a couple of fingers in the carb throat.