75 Johnson 50HP RPM Fluctuation

I have a 75 Johnson 50 horse that just starting having RPM problems. It will start and idle fine. It will also run fine at WOT for about 20 to 30 seconds before it starts to "die down." If I leave the throttle at wide open, the motor RPM will fluctuate smoothly back and forth like someone is playing with with the throttle. If I pull the throttle back between 1/4 and 1/2 throttle, the motor will stop fluctuating. As soon as I go to WOT, it starts acting up again. If I stop for a while, and let the motor cool, I can get it back up to WOT, but the whole process starts over again. So, it's almost like the problem starts when the motor gets warmed up. I have changed the plugs (thought I might have fouled one), cleaned the carbs, and also tried to pump the primer bulb while operating at WOT. None of this has worked. Does anyone have any suggestions?

Re: 75 Johnson 50HP RPM Fluctuation

may be a small amount of debris in a carb<br />a gasket in carb bowl may not be seated properly<br />floats may be out of adjustment <br />may have wrong spark plugs installed

Re: 75 Johnson 50HP RPM Fluctuation

I just cleaned the carbs. I also checked the floats while I was cleaning. The plugs are the correct ones. I have used the same since I have owned the motor. I was thinking it may be electrical since it shows up once the motor gets warmed up. I really have no clue where to start.

Re: 75 Johnson 50HP RPM Fluctuation

Check to see if it will jump a consistent 1/2" gap. You will need a spark checker to do this right. <br /><br />Temporarily try a different gas tank.

Re: 75 Johnson 50HP RPM Fluctuation

while moter is cycling try choking to see if extra fuel helps

Re: 75 Johnson 50HP RPM Fluctuation

Thanks I will try this tomorrow at the lake. Can I get a spark checker at any auto parts store?