compression check for 1958, 5.5hp johnson

[gmarshall43]

what should a typical compression check be for a 1958, 5.5hp johnson? The motor just doesn't run at top speed at full throttle. I checked it and both cylinders are at 40 psi each.

Thanks,
Gary

 

[Paul Moir]

Re: compression check for 1958, 5.5hp johnson

There isn't a published definition since compression testers and procedures vary so much. The only thing is the cylinders should be within 10% of each other.

That said, 40 psi is less than we should hope for. But the fact that you can get it started at all suggests to me that you have a problem with your tester or method rather than the engine. Do you have a screw in type tester? Is it somewhat reliable? Have you tried it on other engines? I find with smaller engines you've got to give it a good 4 or 5 pulls to get a good reading.

Throttle position doesn't matter much on a 2 stroke and don't bother stuffing oil in the cylinders. That doesn't tell us anything.

 

[freddyray21]

Re: compression check for 1958, 5.5hp johnson

40 does seem low. I would try a decarb on it and see if the numbers come up.

 

[F_R]

Re: compression check for 1958, 5.5hp johnson

I agree with Paul. It shouldn't even run with only 40. Suspect a faulty reading. That said, if it does actually have better compression than that, check for spark. It should jump a 1/4" gap with ease. Those motors are notorious for having bad coils, unless they have already been replaced, they are bad.

 

[wbeaton]

Re: compression check for 1958, 5.5hp johnson

I think you need to retest. Those numbers don't sound possible especially if the only problem is a lack of top end power. My 50's Evinrudes test in the 70-90 psi range. I understand they can still run fine into the 60 psi range.

I agree with FR, you should replace the coils (and plug wires). They are still relatively cheap and easy to replace. Also, don't expect much power from these motors. My buddy's '69 Merc will leave mine in its wake - not that either is very fast.