Compression

This is similar to Maverick's thread, but I didn't want to hijack it. I checked the compression today, and I got the following readings: #1 132 PSI #2 108 PSI #3 99 PSI #4 120 PSI. I squirted oil into the #2 and #3 spark plug wells and got the follwing readings: #2 150 PSI #3 110 PSI.

I did not hold the throttle wide open while checking any of the cylinders - should I re-do the test? I am assuming that I have some sort of valve issue on #2, and a piston ring issue on #3. I plan on tearing down the engine and checking everything. Should I go ahead and replace all piston rings, or is it okay to replace just one set?

I have never done anything like this before, so I will probably be doing a lot of searching and posting.

Thanks in advance for all replies.

Kev,
Yes, you should redo the test with the throttle wide open. Are you trying to test the engine warm? The engine should be as close to operating temp as possible.

As for the rings you should do them as a set, if only because you're there and should get it done. By "as a set" I mean all the pistons.

Good Luck
Bill

Post edited by: KZCSI, at: 2006/01/29 20:59

definately redo test and hold throttle open.is motor smoking or using oil?check your valve clearances to make sure they are close,a valve hanging open could also cause compression issues especially if motor is not a smoker and its a lot easier to check than doing a ring job!goodluck,happy wrenching!:whistle:

I ran the bike for about a half hour before doing the compression test. I think the engine was warmed up sufficiently. The engine is not a smoker. I put about 1,200 miles on it last year, and added about 1/2 quart all year. The oil seeps up the cylinder studs and comes out from the bottom of the nuts on top of the fins. I think that is where the majority of oil loss is from. Thanks for the replies. I guess I get to hook the exhaust back on and redo the compression test!

Post edited by: KevZ, at: 2006/01/30 06:40

Conventional wisdom says the engine must be warm when doing a compression check, one day I decided to see if that was true or not, so I tested 4 bikes, two KZ650s, one 750 twin, and one GPz750. I checked each one cold then warmed them up (one at a time obviously) and checked them again. On each bike there was an increase of ~10 psi when tested warm.
Since you are looking for differences between cylinders, as long as all the cylinders are at the same temp it makes no difference whether the motor is cold or hot.

Closed throttle however can make a large difference between one cylinder and another, so the throttle must be wide open when checking compression.

Some of the studs run in oil journals. These are the journals that carry oil to the cam shafts. If leaking on top the head, remove the stud nuts and re-anneal the COPPER WASHERS. That is, heat them till they are red-hot and then drop them in cold water. This will expand the copper and allow the copper to recrush when the nuts are torqued down. You can also put some sealer under the nuts... I would apply it to the bottom of the washers. I have used Hondabond HT for this purpose. Some will squish out; clean it up before it dries.

I also agree with those who instructed to hold the throttle open. With the throttle closed, you are basically wasting your time checking compression. You can also remove your carburetors if you find that easier.

I would say you have a valve problem on #3 and a ring problem on #2 as #3 did not respond to oil down the cylinder as #2 did.

So what are the readings with the throttle opened all the way, and the engine at operating temp?