still bad compression after rebuild

mrcrw009

Seaman
Joined
Dec 26, 2010
Messages
73
I have a 1976 35 hp evinrude model 35602S and have just rebuilt the powerhead by boring the cylinders .030 over and new oversized pistons and rings. Along with all new gaskets and seals. It had horrible compression before in the 30's to 40's, cause it sat for a really long time and the cylinders got severely scored. Well i got it all back together and the compression is in the 40's and 50's. Wow really good huh!! I'm pretty disappointed in it. The only thing is that I can think of is that when i checked the ring gap they were at the max limit on the top ring, and the bottom ring was a little better. I guess my question is how important is the ring gap in these engines, will a thousandth or two over max totally destroy the compression? they did a good job boring the cylinders and my other question is there any bigger rings out there for this engine like .035 or .040 over.
 

mrcrw009

Seaman
Joined
Dec 26, 2010
Messages
73
Re: still bad compression after rebuild

oh and also the plugs get completely soaked with fuel after just a few pulls. you can look into the cylinders and see standing fuel in the bottom. is there a chance the carb could need some work like maybe it is just dumping to much fuel in there? the carb has not been rebuilt in 20 years.
 

Rick.

Captain
Joined
Jul 30, 2006
Messages
3,740
Re: still bad compression after rebuild

This must be very disappointing for you. Is there any chance you should have rebuilt the compression gauge and not the motor? Just wondering. Best of luck. Rick.
 

Haffiman

Commander
Joined
Dec 17, 2009
Messages
2,454
Re: still bad compression after rebuild

Might just be your cranking speed that is a bit low, or your compression gauge. No big worry.
The sign that the cylinders gets filled up with fuel is both a good sign and a bad.
The good part is that you have crank case compression, sucking fuel in at lower rpm, which promises good!
The bad part is that you perhaps should tear down and rebuild your carb as well, might be flooding, or you might just have been choking too much??
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: still bad compression after rebuild

Try a different compression gauge.
 

mrcrw009

Seaman
Joined
Dec 26, 2010
Messages
73
Re: still bad compression after rebuild

ya i will try a different compression gauge and i'm also going to rebuild the carb for sure. honestly if the carb makes it run good then i'm not to concerned about the compression. but the more i think about t the i am pointing to the carb more and more. thanks for the advice.
 

cajuncook1

Chief Petty Officer
Joined
Sep 3, 2009
Messages
559
Re: still bad compression after rebuild

I would definitely get a another compression gauge and recheck the compression. You can borrow one from Auto Zone with a refundable deposit. If you have an electric start then use the starter to turn over the motor. I know recommendation are to have the compression values within 10% of each other, but for a 35hp year 1976, it should be up in the 120's psi, especially after a rebore, ring rebuild.

The only reason, I am poking my nose in is that I would not want you to waste money on a carburetor kit if your compression is true in the 40 and 50psi.

Remember

Basically(not all inclusive) for an engine to run and run well. There are 3 important thing to evaluate and resolve.

You will see a lot of experience guys ask about compression, spark and fuel. (They are using a systematic approach to diagnosing an engine problem or problems....in this order)

C= compression ( if your compression is low are uneven between cylinders >10% then others checks don't matter until the that is resolved) Low compression will cause your motor not run or run very poorly.

S= Spark (if your have problems with your ignition then you can have the cleanest carb and great fuel delivery but it will run bad or not at all) If you have bad coils (cracked) or degraded points or coils, replace. An iffy ignition system leave you stranded in the middle of the lake or give you constant grief.

F= Fuel (if your carbs are not clean and fast/slow idle needle are not set right then it will run but like crap. If you have it set too lean then you run the risk of poor fuel/oil delivery and damage your pistons and cylinders then you run into the problem of C (poor compression from damage).

For your motor to run and run well (this a general statement) all three of above categories need to be fulfilled.


The above is just a general statement.


Just my 2 cents
 
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