Compression Evinrude 18?

superman111 · Oct 3, 2010

What is the compression on a evinrude 18 supposed to be? The person i got mine from said it ran fine, just needed a pull rope. put one on it and it wont crank for anything, he said the carb just got cleaned out, and that it had a new water pump put on it, I checked both out and that is correct, Idk why he would go through having a shop put this on if the motor wouldnt run in the first place? It has good spark the only other thing i can see is the compression seems awfully low, approximately 65-75, dont think it will ever run this low, But idk what the compression rating is for a engine this old? If it does have to be rebuilt then i have a few more questions. Thanks Caleb

Bifflefan · Oct 3, 2010

Re: Compression Evinrude 18?

As far as I know there is not a "set number" for a 2 stroke. Just that they be with in 10 to 15% of each other. And the fact that it is a rope pull it will be lower than one with a starter.
It looks good like the numbers you have are good.

lindy46 · Oct 3, 2010

Re: Compression Evinrude 18?

If you're sure it has good spark, try spraying some premixed fuel into the carb and see if she fires.

bktheking · Oct 3, 2010

Re: Compression Evinrude 18?

At 65-75 it will run.

jbjennings · Oct 3, 2010

Re: Compression Evinrude 18?

Your gauge is likely inaccurate, but in MY opinion, 65-75psi is just plain awful on an 18 fastwin. If the 65-75 is accurate, that could easily be why it won't crank. Even a worn out fastwin should have higher than that. Mine are usually between 110-125psi. The 125psi motor runs no better than the 115psi motors I have, however. Compression guages are notoriously inaccurate.
However, "good spark" is usually the problem. Does it jump a 1/4" air gap on a spark tester?
Will it run with fuel/oil sprayed in the carb throat?
JBJ

JB · Oct 3, 2010

Re: Compression Evinrude 18?

I agree with bifflefan. This is a rope start outboard, JBJ, and just because his user name is Superman doesn't mean he really is from Krypton.

If the compression really is 65&75 that is a marginal spread but adequate PSI. Once you get her running do a decarb (see the Top Secret File).

Your problem is that the outboard won't start. There is a troubleshooting post in the Top Secret File for finding out why.

Let us know what you find. My guess is no fuel.

bktheking · Oct 3, 2010

Re: Compression Evinrude 18?

Compression testing is one of those things that I always question from the person doing the test and is the gauge accurate, how many times was it pulled over to test?

superman111 · Oct 3, 2010

Re: Compression Evinrude 18?

It was pulled probably 7-10 times on each cylinder, everytime between 65-75 on both, the gauge was accurate because i went and put it on a few other engines and it showed them at what they were rated for. The spark is most definetly good. I sprayed some either in it (I Know that it was stupid) just to see if it would do anything, and absoulty nothing still, is there anything that could be blocking the fuel/air getting to it internally? Just looking for some ideas, I reallly wasnt wanting to spend that money to rebuild the top end if it wasnt neccesary.

bktheking · Oct 3, 2010

Re: Compression Evinrude 18?

I fired up my qd-23 after it was sitting for 15 years in a shed with 50psi in both and I know my gauge is accurate. Before I fired it up I didn't like the compression either so I poured some seafoam in the cylinders and let it sit over night. I haven't remeasured as I know it's up, it starts second pull. Something else is going on with it, what year is the motor?

superman111 · Oct 3, 2010

Re: Compression Evinrude 18?

I couldnt honestly tell you what year it is, but I think it really old, like 50s, I have some sea foam here, I dont know why i didnt think about that, I am going to add some of that tomorrow and see if anything changes, I doubt it will as i believe i have a problem else where also. Thanks for all the help, I really appreciate it. How can I tell what year it is?

bktheking · Oct 3, 2010

Re: Compression Evinrude 18?

Should have a plate on it on the transom clamp saying 15XXX.

In simple terms- Engines need 4 items to start, spark, fuel, compression and timing. You say it has spark, it has compression albeit it semi low, you provided the fuel but who knows if it has timing. If I were you I would pull the flywheel and check things out. Is the key in the crankshaft, who knows, gotta check. Are the spark plug wires on the right plug? Are the coils cracked, points dirty??? Is it good spark or weak spark, sometimes what looks like good spark is not so good spark, are the plug wires good? Lot's o question to answer on a motor that won't fire.

wilde1j · Oct 3, 2010

Re: Compression Evinrude 18?

You can't tell the accuracy of a pressure gauge by using it on a different motor. The only way to verify it is with a known pressure (as measured by a test gauge, which is a specially calibrated and certified gauge) and comparing against the reading from your gauge. This is usually done done at the gauge's scale quarter points. There are other, more involved methods (using dead weight testers, etc.), but that's a subject in itself. I know this stuff BC I worked over 30 years in the industrial process controls area.

superman111 · Oct 3, 2010

Re: Compression Evinrude 18?

I can assure you the guage is pretty accurate, It is practically brand new and we use it almost everyday in our shop. It may not be perfect but it is very close. I will try it with another guage just to be 100%. I am most definetly trying to keep an open mind. Thanks

samo_ott · Oct 3, 2010

Re: Compression Evinrude 18?

It should run with that compression. How did you test the spark? By holding it against the block or with a spark tester? You need to use a spark tester.

I dunno where JBJ gets his great compression 18's as most of mine are 80-90 psi and run great. Albeit not as fast as the odd one I have that has over 100psi.

superman111 · Oct 3, 2010

Re: Compression Evinrude 18?

I did it against the block, I guess I probably do need to test it with a spark tester. I will have to get one.

jbjennings · Oct 3, 2010

Re: Compression Evinrude 18?

jbjennings said:
Your gauge is likely inaccurate, but in MY opinion, 65-75psi is just plain awful on an 18 fastwin. If the 65-75 is accurate, that could easily be why it won't crank. Even a worn out fastwin should have higher than that. Mine are usually between 110-125psi. The 125psi motor runs no better than the 115psi motors I have, however. Compression guages are notoriously inaccurate.
However, "good spark" is usually the problem. Does it jump a 1/4" air gap on a spark tester?
Will it run with fuel/oil sprayed in the carb throat?
JBJ

Dang Steve,
Your 18's are junkers!!

All my 18's have about 110 or 115 psi with my one exception that has about 125psi. My gauge shows 125psi on my '99 30hp as well, so I feel like it is reasonably accurate. My '50s QD's have around 85 psi, and my '56 15hp has 105psi. Of course, these are my measurements........ My only 18 that I feel like is "low" hours has about 115psi and runs like new. My 125psi 18 from 1957 looks WELL used and I suspect the high compression may just be carbon build-up in the cylinder head. I believe the last time I checked it was after a long run (recently) and it was warm and down to 120psi. I get my 18's from up north where the boating seasons are shorter.

I don't notice more than 5psi difference, when any, when testing my motors with the starter rope, or with the electric start. Then again, not all of us can be high class electric/rope start outboard owners like me....

That said, my starting arm is a highly experienced, well-oiled outboard starting machine!

JBJ

Compression Evinrude 18?

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