TOPIC: compression ?

Anyone know what compression I should have on my 1960 Evinrude
starflite 75 hp.
I checked and have between 68 and 73

from my readings here on the forum, i'd say your numbers will be most accurate if you have wet rings vs. dry rings. i think a few squirts of WD-40 will do the trick, but don't quote me on that.

as far as what the compression should be, i'd expect 100+psi for a solid, reliable runner. some folks get by with less, some don't. here on the forum i see most folks start to raise concerns when motors start producing compression ratings down under 100. most of the time, when people are happy with their compression, i see numbers in the 115-140+ range. i'm not sure that this applies directly to your particular motor, but i would imagine this is decent general idea. also, the general rule of thumb is that there can be a 10% (maximum) difference between the high and low cylinders.

make sure your compression gauge is accurate, the relief valve is not leaking, there's no damage to the hose/fittings, and that there's no leaks around the spark plug hole where you screw it in. a minor flaw in any one of these things can produce terribly skewed results.

keep in mind, i have 0 experience with compression testing on boat motors. i've pulled this info from prior posts i've read here on the forum.

Best to test the comp dry. What you have is low,. The motor should start, however it may not idle too well . Has the motor been sitting up a while?

If so running it may loosen the rings up.

I can only say I had a 69 9.5 HP that had 60 psi in each and ran beautifully. I have a 73 9.5 now that I am restoring (40 psi each)
These 9.5's were low to begin with though.

My 73' 65HP 3 cyl. was 100/100/80 before I got it running.
I am on my 4th season now with it.
It is now 96/95/97.
Your numbers low wouldn't worry me, it is mostly the difference between the two.
How does the gauge you have measure other engines like that 30 HP you posted about?

If the motor has been sitting for a year or more the rings can be sticking a little. One way to losen the rings up is running plus put a can of seafome in a 6 gal tank of mixed gas and run it though it then check the compression. A dry cold motor will show lower reading then a motor thats up to temp.
Mike aka the pirate

As mentioned - correct compression readings should be done on a Dry cylinder. Dry does not mean bone dry - though. If the engine has been sitting for a million years, it is best to squirt a little 30 wt into the cylinders and slowly pulling the engine over a few times. But be aware that an oily cylinder will give falsely high pressure readings. Correct readings require the engine to be run until warm - then shut down. All spark plugs removed & the ignition disabled to prevent damage to it. A full charged battery or strong arms to pull the engine over quickly. 5 compression strokes usually is adequate to "read" each cylinder. 4 cycle e3ngines should have the throttle held part way open to allow adequate air to get into the engine. Most 2 cycles will pull more than enough air through the carburator without having to open the throttle up.

Most old OMC V 4s will read 90 to 100 lbs per cylinder when new. It is common to see them down in the 70s and 80s when they get old. The top rings are almost always stuck at the exhust side of the piston. Fixing this require a tear down and overhaul. Some times the numbers will improve with using SeaFoam as Mike mentioned. Your engine will likely run fine as it is 90 cubic inches and is as basic as an engine can get. If it runs OK, run some top oil through it and use it.

Omc Looper engines should have compression readings over 120 per cylinder. Some new engines push 140 per cylinder.

The old Big Twin motors seem to keep good compression forever. A good number on a big twin cylinder is between 100 & 110lbs compression.

heh... you all fell for my evil plan! all i had to do was give a little bad information and the knowledgeable folks quickly revealed themselves... muwahaha! now you can all expect lots of PM's full of technical questions, whereas i will get none!

yeah... my bad. seems my desire to be helpful overrode my knowledge of my lack of knowledge. just ignore me and you'll be just fine!