What compression reading should I get?

Charlie, Thanks.

I'll give retorque-ing the head a try. I'll also loosen up the compression release.

I was getting 90-100 psi before I rebuilt it.

I didn't check the compression before I started it (a few hail Marys and gave it a kick) but it's certain it was getting more than 75 or it wouldn't have started (Would it? -- I mean sometimes a shot engine will start, but this started right up twice.)

My thinking now is that the new rings might be the wrong size (?). (I know I should have checked the gap.) A bit of roughness was then smoothed off the new bore or maybe once it started getting oil, the rings couldn't hold and it lost compression.

The woman nextdoor, a lawyer's wife out cutting back her petunias, was kind enough to run behind me and push me down a hill several times, but no soap. (A few Yankees have Southern hearts buried deep, deep down inside.) There's enough compression so that at 10 mph the back tire locks up and drags before the engine goes through a compression stroke.

Does anbody know if there's some temporary fix I could put in the spark-plug hole, say a squirt of 90-weight oil, to bring up the compression enough to diagnose it as the rings?

How can I tell if the head gasket is leaking?

Suppose Carl is laughing at my claim that an SR is a seat-of-the-pants bike that doesn't require a pro to maintain. Come on up to Minnesota and help me sort this out Carl and I'll do right by you.

For me, if I were to get a variety of readings like you have, regardless of what they were, I would assume that something is wrong and start tearing it down to find out. Whatever is causing the low/erratic readings, you can be sure you will have to tear into it to find what is causing it for sure and correct it.

Unfortunately, I can not think of a thing that would correct the problem without tearing it back down. Are you testing the comp. with the throttle open or at rest? Try testing again with the throttle open if not.

You mentioned squirting some oil into the cylinder throught the plug hole. When I was taught to do compression checks, and I have done this ever since, I do my first test (with the engine warmed up), and then after that put a couple of squirts from an oil can into the cylinder and repeat the compression test. If the rings are worn, squirting some oil into the cylinder will temperarily seal the rings and the reading on the compression tester should rise. If the pressure reading on the tester does not rise, it is normally an indication that the compression loss is due to a leaking valve.

I would sujest giving it a go, if the pressure rises significantly, the its the rings. If the pressure remains the same. them it is probably something like a valve not seating/sealing correctly or a leaky head gasket.

CJ

Thanks everyone.

Now at least I've got lots of things to try before pulling the engine apart again.

Hey, I didn't know you had to hold the throttle open to do a compression check... You mean I might have had more compression than I thought and didn't need to rebuild it in the first place? Ain't life a comedy.

I've been flogging it for several days now. What a workout program. As soon as I get some feeling back in the right side of body, I'll hit at it again.

I've designed some special motor mounts for those of us who don't get it right the first time. Speeds up the re-do.

Hey just a thought, its had a re-bore and hasn't really run so the rings are not sealing , the compression will probably be erratic until its had a few hours running-in time, if the engine is flooding slightly the petrol will wash any oil out of the cylinder so after a few kicks all the oil is gone so the compression will be low.The motor should start even with very low compression so check everything like spark and fuel it should start. Just aside my motor was rebuilt as was my mates and we just finished a 10,000km trip the bikes were garaged after the trip for a month and when we went to start them they had almost no compression could push it over by hand !!! I kicked it over it started on the second kick then i stopped it and checked and the compression was back to normal , think the rings were stuck in the ring groves and once it fired it loosened up. Good luck
Mark

Okay, I got it running a few days ago.

I went through the steps everyone above recommended (all except solo2racr's, which would have come next if nothing else worked.) Thanks again.

My thinking now is that the exhaust valve was sticking and not always closing completely or seating properly.

(I'll take credit for inventing the "Cold Break-in Method" which may solve after-build problems like this. If you kick the starter consistently for several days, say 5,000 times, that's like running it cold at 1,000 rpm for 5 minutes. Eventually you could probably break-in the engine that way and get in great physical shape at the same time.)

The engine always had compression after sitting overnight, but lost it during starting attempts. I checked the valves cold, and they were right. After trying to start it, and the loss of compression, the exhaust valve was loose. I adjusted it, and in the morning it was too tight.

Maybe, given time, the force of the spring was pulling it closed, but then after a few kicks it would hang up again. (?)

Mornings, before going to work, I'd give it a few kicks before it lost compression and on one of those it started. I let it run for a few minutes, shut it down and it started right up again.

It's running good now, starts easy, sounds good and the valves are holding adjustment. It's a great feeling to be able to drink a beer and think about other things besides tearing the engine down again.

Everything's stock, but it seems to run and sound better than it did new. The Wiseco piston?