I just got the heads redone and the valves seated. I went from 102 for compression up to 126-129 Is this normal I just thought I would really gain like get back into the 150-160 after the head work I know it was really low just wondering if maybe their is still somthing I need to check out. Thanks like i said no complaints on how it runs I just thought I would see more I have carbs heads and exhaust and still only turning 4150rpms???

First of all how did you do the compression test?

Kill switch pulled?
Spark plugs pulled?
Cold engine?
Valve lash adjusted correctly?
Choke in off position?
Throttle held WFO?
Engine cranked 3-5 full revolutions?



4150 under load or in the driveway?

You should be in the 180-190 range.

Thats kinda what I thought. Would it be the pistons and rings. I may try to do another leak down test and see what I come up with. The engine runs great well alot better than it was running before. It handles a load and gets up on plane but I thought the compression was low. I tried it twice and also tried another tester both were netween like 126-129 both sides are really close to the same

Craig, first let me say that you know a lot more about this than I do, and I have yet to see you post something wrong..

But isn't 180-190 a little high?

I thought anywhere in the 115-130 range was good and the main goal was that they were equal or close to it (10%)

Would be interested in hearing your thoughts...

Is it because he had work done on his heads? or the type motor he has?

I had a 40hp merc and it was 126 and ran great and on my 32 briggs I am at 115 and it seems to be fine..


Just wondering.... and wanting to make sure I am ok on my briggs...

A good moderate engine will have 140 to 150 out the factory. That is not always the case but after checking multiple stock engines thats what ive seen. After decking the heads and getting a good valve job done with stock pistons and rods you should see between 165 to 185 depending on how much you took off the heads, how good your rings are etc..........
Now, if you dont get a good valve job done I dont care how much you deck the heads you will still have low compression till you get the seats cleaned up and valve matched. Now, just talking from my expreince every engine I drove that had under 130psi compression on the cylinders was a terd with no low end power when compared to one with say 170psi on each cylinder. So I would call 120's low.

Also you cant compare a 40merc to a briggs 35hp. A merc is a two stroke engine, if you 180psi on a 40merc I dont see it lasting long. 126 on a 40 merc is very good but you dont see compression much higher than that on two stroke outboards. 4 strokes on the other hand need higher compression to get things moving thats one of the ways they make there torque since they dont turn 6,000 rpms at the drop of a hat, at least not a briggs anyway. High performance 4 strokes will turn pretty high rpms but that not the case here.

Ok thanks guys... was just curious..

You think I have a problem on my 32 briggs with 115?

Seems to run fine....

All right guys.

thanks for the input.

Didn't mean to jack this thread, but guess I kind of did..

Besides having the heads decked to .055 he also did a triple angle valve job and shimmmed out the springs. Im going to run a leak down test on it later today to determine if I still have a problem or my motor just has low compression. I know their are alot of variables as to how the compression tester reads so I would like to try the leak down test. Iam happy with how the motor runs I guess im just checking for a little peace of mind to know that nothing else is wrong. Might be time to sell it otherwise

Hey Fowl,

Not to be insulting, but did you do the compression check the right way.

I just rechecked mine and did it right this time (pulled kill switch and turned the throttle wide open) and mine is actually at 149 and 151. Right where it should be. As stated, I thought I was at 115.

That makes a lot more sense as the motor was running strong and fine....

Like I said, not to insult you, but I figured I would at least tell you what a dumbazz I was, and see if you maybe you had the same issue.

I thought everyone here new how to comp check correctly?

I know I'm still learning how to do stuff. Just realized how to properly do a comp check today.