40hp Evinrude running on one cylinder @ low speeds but both at high speed

[sojo81007]

I don't think it's the exhaust because I turn the flywheel very slowly, inches at a time, and hear air slowly leaking out somewhere--it sounds like on/in the powerhead itself. I'll do a compression test Monday and see if the readings are different from last time.

 

[racerone]

????---When you turn the flywheel slowly air is being compreseed in the cylinder and also the crankcase.----The laws of nature say that this compressed air is going to try and leak to someplace where the pressure is lower.----That would be to the exhaust / to the crankcase or to atmosphere.-----When you understand how things work then the trouble shooting is so much easier !

 

[thdrduck]

Just to toss something else out there... could the seal between the upper and lower crank case be leaking? I know silly to even mention because bad spark is bad spark and the OP is not willing to go back to points because of his good luck with electronic which may or may not be working. I am a bit confused.

 

[pro-crastinator]

Yes - the hissing should be considered normal.
Focus on the spark problem

The video is exactly the same as your 40 - only different

 

[racerone]

The seal between the upper and lower crankcase always leaks at slow speed.----------It is a LABARYNTH seal.------Works when grooves have oil in them.

 

[thdrduck]

Just for my info... if the seal between upper and lower always leaks at slow speed, how does the engine even run. If my understanding of how a 2 stroke works is correct, when the top crankcase has a positive pressure (piston coming down) the bottom crankcase has a negative pressure (piston going up). One pressurizing the fuel air charge for the ignition cycle and the other drawing in a new fuel air mixture, two separate engines sharing the same crank. Wouldn't a leak between the two screw things up? Just wondering, not being a smart ass.

 

[racerone]

????----When you turn the engine by hand air will leak between the crankcases.--------At idle speeds you could calculate how much time there is for air to leak from one to the other side.------------That is why when an engine does not idle properly there could be air leakage / internal problems.------At full throttle there is not much time involved for leakage to occur.-------That is why folks will say ---" runs ok at full throttle "

 

[thdrduck]

Gotcha, thanks.

 

[sojo81007]

Checked compression yesterday and it's hanging tight in the 86-88 range, essentially the same as last time. I ordered 2 Nova II modules to
see if the ignition modules are at fault. Even if they are not, I have another unit I can use them on in time. If that fails I will replace the coils.

 

[pro-crastinator]

Compression numbers low but ok.

 

[joetheis]

Might be missing something, but..
Dual carbs?
dual coils?
If so, maybe low speed jet is clogged........
If 1 carb, ignition......
might be a bad spark plug wire?
dual coils, maybe one is bad.
I agree, go backwards, go back to point see if that does anything different.
Joe
Always remember take any advice I give with a can of beer er 2!