40 hp 2 stroke 40plrs runs rough

[mdgpl]

Its been slowing getting worse the last two seasons. New plugs, new coil, cleaned carbs and adj( best I could with condition). It starts easy, even on manual start.

The bottom cylinder is doing all the work. If I remove the plug wire for the top cylinder, it runs at idle and will rev up. If I remove the plug wire to the bottom cylinder (top one connected) it will sputter, but not run smooth. If I remove the link between the carbs and run the top carb throttle, it will speed up but its rough.

It was doing this last fall before I replaced the coil. Last fall, once the RPMs were up to 1/2 throttle, both would fire and I had enough power to get on plane (just a 15' boat), but this weekend with 3 adults on board, it would just plow water.

I was thinking the CDI unit is next, since it does not seem like fuel and the coil is new. But at $150 or more, I just wanted confirmation from someone that has been down this road before.

Thanks

 

[99yam40]

Re: 40 hp 2 stroke 40plrs runs rough

check compression,
check spark with spark tester,
check timing at idle and problem RPM, timing light will also show if it is dropping spark at problem RPM

If all is good then check fuel pressure/vacuum

Clean carbs if nothing is found wrong with other things

Testing the inputs and outputs of the CDI will show if there is a problem with it or other things, but is not needed if there is good spark at all RPMs