1975 230, what should compression be??

[IDTag]

Still don't have my MB manual and Chilton's doesn't list the spec. References to "compression specification" all over the book but no spec anywhere.

It's the M115 4 cylinder gas motor. Thanks.

[t walgamuth]

i don't have my books here but i am guessing 8.5 to 9.0.

tom w

[IDTag]

I do have the compression ratio but not the actual pounds of pressure. I'd guess it should be around 200 pounds.

[t walgamuth]

that is pretty high.

more in the range of 130 to 140 i would think.

what is the ratio?

and how high above sea level are you?

tom w

[IDTag]

We're at 2800 feet. The compression ratio is 8:1.

Starting off the readings were between 110 and 130. With a little oil in the cylinders it jumped to 170 and one was at 190.

You know that does seem high. Just looking at specs on other motors seems high. Wonder if my gauge is messed up.

[t walgamuth]

ok the ratio of 8 to one to start with you multiply 8 x 14.7 lb per sq ft at sea level. that is about 120. so i guess there must be a multiplier due to heat. but the one that is 190 i am betting has a lot of carbon build up inside the cylinder head and or on top of the piston.

your readings dont sound too bad.

they want to be within 10% of each other, so yours look good. i suspect some carbon build up though. this will cause pinging so it is a good idea to try to burn it out.

my great uncle used to run a gallon of diesel in each tank of gas because he said it kept the cylinders clean. i cannot verify this. there are many ways to clean it out. with any luck we can start another thread similar to the famous oil threads with everyones home remedy for cleaning carbon out.

tom w

[IDTag]

The only one that didn't look too good was #3. It's running weak and pressure didn't come up when I put oil in the cylinder. I'm guessing that I've got a valve issue there.

[Mark DiSilvestro]

Originally Posted by IDTag

The only one that didn't look too good was #3. It's running weak and pressure didn't come up when I put oil in the cylinder. I'm guessing that I've got a valve issue there.

Suggest checking valve adjustment next. If one's too tight it can affect compression.

Happy Motoring, Mark

[IDTag]

Did the valve adjustment first. That's not an easy job! Not a lot of room in there to work.

I'm going to get another compression tester today and see how it reads.