Too much compression?

[Vin P]

I have a 72 Evinrude 50 horse that ran rough at low speeds and great at high speeds. Found an intake manifold gasket leak and fixed it. Now it's better but still not perfect. Maybe this is as good as a 30 year old 2 stroke can run.<br /><br />When we checked the compression, it had 170 - 180 lbs in both cylinders. Does this seem like a lot of compression for a 2 stroke? What are they new?

 

[crazyray2]

Re: Too much compression?

1st try another gauge not sure what it should be but would feel comfortable with it......might have detonation problem in which cause you would have to retard ignition timing a little

 

[JB]

Re: Too much compression?

Howdy, Vin. Your question: how good can a 30 year old 2 stroke run? Better than when new. Modern fuels and oils make them run better,<br /><br />Your compression psi readings are not as important as the comparison among them. They should not differ more than 5-10psi. Readings of 150-200psi are not unusual in OMC in-line loopers in top condition. Unleaded regular is still the proper fuel.<br /><br />Your remaining roughness at low speeds is most likely the low speed circuit in one or both carbs.<br /><br />Good luck.<br /><br />Red sky at night. . . . <br />JB

 

[sony2001]

Re: Too much compression?

J.B. are you sure only a 5-10 psi difference is allowed? Those little Cessna's are allowed up to 20%.

 

[JB]

Re: Too much compression?

Howdy, Sony. Yeah, the book says 15psi difference is acceptable. I'm just being picky about what appears to be a superb old engine.<br /><br />Red sky at night. . . <br />JB

 

[Vin P]

Re: Too much compression?

Thanks for the help.<br /><br />J.B., I've pulled the carbs apart, and they are clean and pretty inside. Looks like they were rebuilt not long ago. (I've only had the motor a couple of months, so I don't know its history. It looks like it was well taken care of). I can't adjust the roughness out with the mixture screws. Any ides?<br /><br />Thanks

 

[corm]

Re: Too much compression?

If your motor has over 5# difference in comp it won't idle good. L77J4C gapped at .035 helps.Also make shure the packing around the idle needles are good as they get sloppy on these motors. Run some engie cleaner through it and get it the best you can. The go and have some fun.<br /><br />------------------<br />...have fun...corm<br /><br />cubic inches = power<br />power = speed<br />speed = FUN<br /><br />Email me if you need to.