TOPIC:

According to Clymer, my SR should have 170 PSI of compression. It's 105-110 PSI per cylinder, though, but all of the "low compression" posts in the archives give me the impression that usually bad rings will cause uneven compression.

Is it likely that my rings are bad, or are there other things I should check first?

Im going to say check valve timing and while your in there check the clearances

I'm sure I won't be the last to suggest it, but I would definitely check the valve clearances. They are normally very small and it doesn't take much for them to tighten up. Checking them is easy enough, adjusting them can be kind of a pain in the butt though.

How many miles are on the motor?
Another possable could be the valve seats, if not properlly seated they will get blow by.
this can be checked with a leak down tester.

with the compresion you listed, I would say its a combo of both.
If your going to do the rings, reseatting the valves is a must do.

Squit some oil in the cyliders and retest the compresion. If they improve it would sugest rings Easy to do first.
Sal

Your carbs were at WOT, right?

GC - Before going any further with chasing rabbit trails of tech issues, please explain exactly how you checked your compression.

11,400 miles on the bike, but it did sit for a few years, and probably wasn't decently prepped for storage. I wasn't sure if improper valve clearance could kill compression.

KLACKO wrote:

Your carbs were at WOT, right?

D'oh! I tried some oil, which didn't bump compression up more than a few PSI, but I won't believe I was giving it throttle while cranking. I do remember reading something about doing compression tests at WOT, but it must not have sunk in enough to keep me from doing something stupid.

I'll have to try it again this weekend, when I'll be dropping in on a friend with a compression tester (and hopefully a newly purchased engine for his 90... can't WAIT to ride that thing!).

I'll pick up a set of feeler gauges before next weekend. Time to track down a timing light...

Thanks for the responses so far.

AHA! Check compression with throttle all the way open and report back in. ALSO... your oil poured into the cylinders and no significant jump in compression doesn't mean much until you check with throttle open. Repeat the oil thing AFTER you check with throttle open. Also, the valves hanging ajar just a shade WILL keep your compression low... First thing you should do on an unknown engine is check valve clearances. Setting the proper clearance doesn't necessarily mean you will get great compression but if the valves are seating you will get as good as you are going to get. Check a manual for tune up steps and the second step before messing with ignition timing is adjustment of your cam chain tension... don't forget this step! Good luck and let us know how the new tests come out.

Yeah, I'm a bit slow at times, which is why I figured I'd post here before trying to order rings and have the jugs machined. It's easy to put the cart before the horse!

Thanks for the feedback. I'll report back sometime next weekend -- hopefully I can set aside the time to ride to a friend's house (well-lit garages are NICE!), pull the valve covers, and check clearances. If I can track down a timing light, I may try to do some actual work on it. If not, I'll ride the 90 around the yard until the owner demands it back.

who says I don't have a timing light?!

How can you have a big, well-lit garage, and no timing light? I wouldn't dream of it. Why we didn't test compression at WOT? I was pretty sure that the carbs were setting on the bench when we tested compression. Of course, I was under the influence...

Oh, and you are gonna have to fight me for the 90cc. I haven't ridden that thing since 1995 or 1994, so I am gonna be just as happy to see it run as you!

EDIT: I got feeler guages too.

Post edited by: john68, at: 2005/10/18 16:29

That's settled, then. I hope I wasn't presumptuous about dropping in over the weekend, but if that 90 engine does get there in time I'd sort of figured you'd be clearing some floor space.

I guess it's good to go in some sort of order, and take notes while drinking.