Trouble with compression on 82 gs850g

[Nick63]

So I did a top-end rebuild replacing the gaskets. Base gasket head gasket and valve cover gaskets. After setting the timing on the cams, I put the engine together enough to crank it to check compression. It came up to 50-60lbs! Not good, I followed the manual for the timing procedure and am confident in it as. long as the service manual is right. What else should I check? Will a valve tappet adjustment cure everything?

[patrino]

A couple thoughts:
1. confirming that you're doing the compression check with the throttle WFO, correct?
2. Yes, if your valves are tight, you'll have low compression, since they won't seal

[patrino]

Also, did you do a compression check before the top-end rebuild? If so, what #s did you get?

[Nessism]

So I did a top-end rebuild replacing the gaskets. Base gasket head gasket and valve cover gaskets. After setting the timing on the cams, I put the engine together enough to crank it to check compression. It came up to 50-60lbs! Not good, I followed the manual for the timing procedure and am confident in it as. long as the service manual is right. What else should I check? Will a valve tappet adjustment cure everything?

The valves won't adjust themselves so you better get busy.

[Grimly]

Did this top end job include re-lapping the valves?

[Nick63]

I didn't think to do a compression test before, because it ran fine, other than oil leak on valve cover gasket. I'm waiting on the tool I need to get the shims out properly.. Would a lap job be absolutely necessary. I know the gaps are below. 0015" that was the smallest feeler gauge I had. So I'm really hoping this is my problem. Will let you know what happens in a couple days

[patrino]

Thanks for the additional info, @Nick63, that helps. No, lapping (or regrinding etc.) the valves wouldn't be necessary with the head off.

Were you holding the throttle at wide-open when you did the compression test? If you did not hold it open at WFO (the whole time you were cranking the engine) then your readings might be lower than you think. So your compression might be fine, but your compression measurements might be low.

Since the bike ran fine (oil leak aside) before you replaced the gaskets, your low compression readings are probably something pretty straightforward; always start with the simple stuff:
0. confirm you were holding the throttle WFO while checking compression
1. valves too tight
2. cam chain off a tooth (or more). I know you followed the manual when timing the cams, but I've done that too, and I don't always get it right.

[Maddevill]

You should always lap the valves when you have the head off. And re setting the valve clearances is almost mandatory. I'd do a breakdown test. It will show you exactly where the problem is.

Mad

[bwringer]

Set valve clearances correctly, verify the timing is correct, and fire it up.

Don't worry about lapping; it's already assembled, and there's no reason to take it apart again.


Honestly, this is far from the first time a post-rebuild compression check has caused undue anxiety. I think the only way to get a remotely useful reading is to warm up the engine, remove the carbs and spark plugs, and make sure you're using a battery fresh off the charger.

Compression readings on a cold engine, especially with fresh rings, with CV carbs installed (even if you hold them WFO, there's a LOT of restriction) just don't really tell you anything useful. The 850 isn't a high compression engine in the first place.

If the valves are good and the fit and seal against the valve seat are good, and all the valves are in their original places, there's usually no burning need to lap the valves. But it is always something to check. If you didn't check or lap, it's certainly not worth disassembling or feeling guilty; unless there was some other problem, it's probably just fine. As you stated, you started the project to fix an oil leak, so if it was running fine before, it'll run fine now.

[Nick63]

So I rented a compression gauge from orielys,just to double check myself. Sure enough the guage read up to 137lbs all cylinders close to that number. So I was sweating something that was nothing. Still did the shim adjustment, putting it back together now.