Compression: How high is to high?

[Aaron B]

Hi guys,

My little A-4 didn't want to start the other day, so I ran through the steps and found that my plugs were pretty evenly fouled in a sooty way.
I don't know how many hours are on those plugs, but it could not have been more than 20 or so. Any thoughts? I have been giving her fairly frequent MMO treatments in the cylinders, and I haven't noticed any oil loss.
I changed the plugs (Champion RJ12C) and she is starting right up again and seemingly running about as well as ever.

Anyways, back to my original question: As I went through the engine, I checked the carburetor (Good) battery's (Good) and the compression. My compression numbers were 1: 95 2: 115 3:120 4: 150
Is it possible to have compression that is to high? After searching the forum, I haven't been able to find any mention of a compression value above 130, and I also have not been able to find anything that mentions compression that is too high, and I am curious if there is anything that I should read into that.

It's entirely possible that I'm just being paranoid, so thank you in advance for your help!

Aaron

Thanks in advance!

[sailbristol]

high

sounds to me like you have an exhaust blockage your #4 cylinder is probably loaded with a carbon buildup causing the high #s

[Will Jacocks]

Maybe running a tad too rich to cause carbon buildup?

[hanleyclifford]

Your paranoia is well founded. Time to pull the head with a view to a good old fashioned cleanup. The 150 compression, if correct, will lead to trouble down the road. Head it off (pun OK?).

[Al Schober]

Before you go to that trouble, check it with a different gauge. 150 psi compression is hard to believe.

Al

[Will Jacocks]

Make sure you don't have back pressure preventing the pressure from releasing better.

[Aaron B]

Thanks!

Hanley, I'm taking your advice and will be removing the head and cleaning her out.
To better my own understanding, what would have been the reprecussions to overlooking a high compression number like this? Bad rings?

Thanks as always,

Aaron

[hanleyclifford]

The most likely consequence would have been a failed head gasket. A warped head is also possible

[rigspelt]

Don Moyer FAQ: "Based on the original design compression ratio of 6.3 to 1, the compression would be 95 PSI. In fact, original test data recorded compression values very close to this value. However, most in-service engines will have average compression readings of 100 to 115 PSI. There are several reasons why compression is usually higher than the design specification: 1) carbon buildup in the cylinders, 2) heads being milled, or the use of only one head gasket during overhaul, and 3) variations in combustion chamber depths within cylinder heads. NOTE: While Universal never officially changed the specification for the depth of combustion chambers in cylinder heads, many of the later engines (circa late 70's and early 80's) have considerably shallower combustion chambers than earlier heads. Fortunately, new heads sold today (both our aftermarket heads and the more expensive heads sold by Westerbeke) have combustion chambers of the original depth. It's important to point out that since the Atomic 4 is a low compression engine by design, performance does not seem to improve by artificially increasing the compression ratio above the design specification of 6.3 to 1. - Updated: December 6, 2003."