Is 35 psi compression too low?

[John Lapic]

I keep reading posts that suggest actual compression is not as important as the variation between cylinders. My 71 Johnson 9.5 measures 35 psi in both cylinders. This seems way low to me, but I don't know what it should measure. Anyone have the specs on this motor?

I did a decarb before I ran compression. Just to be sure I was getting accurate readings, I remeasured each cylinder three times with the same number of pulls on the starter rope. I can get no more compression. The compression gage came from my local automotive parts store as a loaner. I believe I am the first to use it. I can only assume it is rairly accurate.

 

[evinice66]

Re: Is 35 psi compression too low?

is the motor running wierd? if not idk if id worry about it too much.... as long as their close and your not loosing top end speed...

 

[John Lapic]

Re: Is 35 psi compression too low?

You hit the nail on the head evinice66. My top end isn't. I originally thought it may need a rejet since I took it from 1300 ft to 6000 ft. I have no power at WOT. My local dealer suggested changing prop pitch rather than a rejet. With this compression, I am now thinking compression, or lack of, is more of the problem. It runs fine, just will not get up and go.

 

[wilde1j]

Re: Is 35 psi compression too low?

If the motor has fixed high and low speed jets, it needs to be re-jetted. Are you dealing with a dealer @ 6000 ft? The motor is running WAY rich at 6000 ft and down ~ 15 to 20% in HP just due to altitude, forgetting about the mixture issue. Find another dealer.

If the motor has high and low speed mixture adjustments, they must be adjusted to compensate for altitude.

 

[John Lapic]

Re: Is 35 psi compression too low?

I have a fixed high speed jet and an adjustable low speed. The dealer is a local (6000 ft) dealer. I have not called others.

 

[tashasdaddy]

Re: Is 35 psi compression too low?

try your compression with another guage. but i don't know if altitude would make any difference in compression. if the compression is correct it is low. i would want to see 90 atleast

 

[wilde1j]

Re: Is 35 psi compression too low?

Air is less dense @ 6000 ft., but 35 psig is pretty low. Most high altitude applications need both re-jetting and at least one pitch increment lower pitch for planing hulls. In your case, with a small motor, I wouldn't bother with the prop.

Most pull start motors, it's hard to get much over 75 psig, but 35 psig suggests stuck or worn rings. I would start by getting the motor re-jetted, then repeat the decarb and recheck compression. You may need new rings or more (bore, oversize pistons and new rings).

 

[tmcalavy]

Re: Is 35 psi compression too low?

That's very low compression in my book, which leads me to believe you need to check it again with another compression gauge. It's good that the readings are consistent. If another reading/gauge comes up the same, you might try running it with a lot of Seafoam in the gasil mix...that might help free up a stuck ring, or two.

 

[John Lapic]

Re: Is 35 psi compression too low?

I ran a can of Seafoam through it last weekend. Put a good shot in each cylinder, squirted 1/4 can through the carb and mixed the other 3/4 can in 1 gallon of gas/oil mix, then ran it a WOT. It seemed to help a bit, but I will run another can through it and check it again with another compression gage. Thanks for your advice, particularly regards to prop replacement. I'll check other dealers and see if I can get rejetting info

 

[JB]

Re: Is 35 psi compression too low?

The only thing I can think of that would hurt both cylinders equally is a leaky head gasket.

 

[Tacklewasher]

Re: Is 35 psi compression too low?

Did the gauge screw into the spark lug hole, or did you just hold it in tight? There are two kinds of gauges and the one you just hold in are next to useless.

But I'd try it with a different gauge regardless.

 

[jbjennings]

Re: Is 35 psi compression too low?

I check all my small motors with a pull cord. On 50's 7.5's I get about 80. 10hp's about 85 or 90, 50's 18hp's I get about 110, and my '54 bigtwin 25 I get about 110. '53 johnson 25hp has about 110 as well. Of course, my tugging arm has 24inch biceps.
from my brief experience messing with outboards I find that compression problems show up most obviously on idle as much as wide open. Also, compression gauges are notorious for giving screwball readings. Take it with a different gauge or test the compression on an engine of known compression, like a car with less than 30,000 miles on it that you can look up the compression specs on to see if your gauge is accurate. Better yet, borrow a neighbors gauge. Also, with just 35lbs of compression and the spark plugs in, it should be obviously weak just by pulling it through by hand. Heck, I SERIOUSLY doubt you could possibly get that motor to run AT ALL with just 35 lbs. of compression. I'd bet a steak dinner that your gauge is wrong. You've got some other problem.
Good luck,
JBJ

 

[jay_merrill]

Re: Is 35 psi compression too low?

If the compression is really 35 psi, yes its too low, whether it is even or not.

 

[tashasdaddy]

Re: Is 35 psi compression too low?

when you do the test, keep pulling, until the guage quits going up. 3 pulls is not enough on a rope start.