engine missing at wide open throttle 1975 9.9 johnson.

[littlelakeden]

O.k. so the new thermostat fixed the cooling issue but the motor still miss fires when at wide open throttle.When you back off the speed it will miss even more then run smooth at half throttle or less.Any ideas or did I buy a lemon.The motor runs great either than that.Starts good idles good.

 

[nwcove]

Re: engine missing at wide open throttle 1975 9.9 johnson.

sounds like its time to do a proper rebuild on the carb ( high speed circuit gummed up?)

 

[Daviet]

Re: engine missing at wide open throttle 1975 9.9 johnson.

I can think of two things that could be causing your problem.
Loosing spark to one cylinder, use a timing light and check each if you are loosing spark on a cylinder while operating at WOT.
A plugged or restricted high speed orifice in the carb, will have to remove the orifice to check it.

 

[jmactacular]

Re: engine missing at wide open throttle 1975 9.9 johnson.

I had a similar problem with my 1979 johsnon 6 hp. It was like it was only running on one cyllinder and then it would kick in every now and again. Turns out the coil was bad. If your motor has never had the coil replaced, chances are it is bad. I think they are only 80.00 and easy to install.

Let me know if this helps!

 

[boobie]

Re: engine missing at wide open throttle 1975 9.9 johnson.

Sounds like an ignition miss. Do the timing light trick as said above.