Compression check

[artman12us]

Has anyone done a compression check on a newly rebuilt 185? If so what did you get? Thanks Art

[lazair3ca]

Art,

We have been trying to determin why one of George's engines is limited to 5200 to 5400 RPM. So we did a compression check on 4 engines. With a normal compression tester. It accumulates the air. We had a reading of 95 psi after 3 pull throughs. My engines had a reading of 110 psi but they have the L rings in them. With the practised wrist flip we could get the other engines up to 110 psi as well. Geroge prefers to use the starter cord to pull the engine through with results in the same range.

Cheers,
Bill

[huronflyer]

Bill,
Does this mean I can use a standard comprssion gauge to determine the condition of my rings?
drh

[lazair3ca]

Dave,

It will give you a good indication if they need to be cleaned or not.

I don't know at what PSI they need to be replaced. The pressure is very dependant upon the turning (leakage) rate.

Cheers,
bill

[art]

We had a reading of 95 psi after 3 pull throughs

Why three pull thru's" Isn't the first pull the compression reading?
Art

[ozzie]

are you doing this with open or closed throttle?

[art]

Closed. Art

[sparky]

why closed art?? all compression test should be done Wide Open Throttle, as a basic rule of thumb if you have less then 100 psi your taking the top end off

[Chappy]

Art,

For compression checks, wide open throttle, pull the engine through enough times to give you a maximum reading. Don't expect a very high number, either, on a 2 stroke engine, certainly no where near as high as a four stroke engine. It would be interesting to check a bunch of 185s, but there seems to be a fair amount of variation between compression testers for some reason, so I'm not sure we could trust all the results for comparison.

Actually, compression tests can be rather misleading. I just purchased an old Mopar 340 CID engine for a replacement for the totally ragged out and neglected 318 in a '66 Dodge Coronet that I'm trying to get back on the road after it's sat for the last few years. I just did a compression test on it, with all the cylinders but one reading in the 175 -185 psi range. The odd man out showed only 150. This could be caused by something as simple as carbon buildup on the back of a valve. The engine doesn't run too bad either!

To learn more, I did a leak down test. Basically, air is feed into the cylinder under 100 pounds pressure through a metered orifice, and the output depends on the amount, or percentage, of air that leaks out of the cylinder. The cool thing is that by listening carefully, you can tell if it's a valve or rings (intake, exhaust pipe or crankcase). On this engine, I measured less that 10% leakage on the cylinders that read the highest during the compression test. This showed a well used engine with plenty of life left in it.

The low cylinder, however, showed 75% leakage! Massive amounts of air are leaking into the crankcase and out the exhaust. It is now obvious that this engine has to come apart for repair, if not total rebuild.

Although a leak down test is usually done with the piston at top dead center, it can also be done with the piston at different points too. This can give you info on possible cylinder damage or wear.

Of course, on our little engines, it's easy to just pull them apart and look/measure, but a leak down test is still a great diagnostic tool.

Chappy

[art]

Thanks a lot. I'm new to this . My engines have been bored And have new oversized pistons and haven't been run yet. I put a little oil in the cyclinder. I'm trying to get a reading before a and after break-in. I thought the data would be helpful to the group. Is there anything more I should know about compression check.
Thanks again Art