low compression

A couple years ago I rebuilt the top end on my 77 1000. I had a local shop do a valve job. They usually do v-8 heads for cars. (I now know this was a big mistake) however the valves seem to seat and seal ok. I checked them by fliping the head over and pouring in alcohol. waited overnight and none leaked so i assumed the head was ok. Also put in a wiseco 10.25:1 1015cc piston/Ring kit. The bike runs but not real well. I get about 90psi compression across all cylinders. So I'm wondering if there is a way to isolate the problem before I tear it down again. wondering if the head is the problem or maybe the rings didn't seat .

Have you checked your valve clearances and cam timing?

90psi is very low...

Valve clearences are good . As far as cam timing I aligned the marks on both cam gears as the service manual says to. But I don't have a degree wheel. If thats what you mean.

Could do a leakdown test to determine where compression is escaping from the combustion chamber (which valve, piston/rings, and/or head gasket).

The alcohol test indicates valve are seating okay.
Am guessing new head gasket also sealing okay.

Perhaps rings haven't yet seated.

If satisfied that camshafts/camchain assembly is correct and engine turns over freely and runs without any unusual noise, could put a few hundred miles on it, and then recheck compression again, all before tearing it down.

Good Luck!

Hard to believe rings not seated could do it. My 750 was cranking about 160 psi like ten miles after I put the top back on.

The interesting thing is all four showing about 90psi? Makes me wonder if the valve timing is off.

yes it sounds like your cam timing is out if the engine is not smoking it doesnt sound like ring seal problem the fact it isn't running right and probably doesn't make any power is pretty much a dead give away i would be too freaked out just go thru all steps in your manual to verify cam timing and you will probably find the answer to your problem

A couple of reasons you could be experiencing the reduced cranking pressure.

First let's assume you have the throttle wide open when your checking the cranking pressure and the spark plugs out of all of the cylinders.

Are you running stock cams and stock cam sprockets? If not then that could be your problem.

When you installed the 1015 kit did you check the deck height? This is the distance the top of outer flat portion of the piston is from the top of the sleeve. If the pistons are too far down in the sleeves you'll have less cranking pressure (lower compression).

If the cyclinders are cranking the same pressure across the cylinder I doubt this is ring seat problem.

If your using stock cam sprockets then cam time is not the issue.. unless you didn't install the cam correctly or miss counted the cam chain links during installation. IF..the cams were not installed correctly then you could have bent a valve or two..in which case a leak down test will tell you right away.

Anyway..to answer your question.
1-leak down test.
2-Check cam timing

3--Last resort..remove head and check deck height.

Good luck,
Buzz

I'm at a loss, even with worn cyls, sealing heads usually render 120-140 lbs comp. stock. a fresh bore w/wiscos's at@ 10-1 I'd expect 160-180. 90lbs comp. is barely enough to start. there's a remote posibilty of mixing crank/cyl/head that ratio is compromised. changes in crank/cam gearing also possible. leakdown test is in order. if less than # 6 % and cams are in time, would look to head.luck G

I may be off base here, but wondering if someplace along the way a J head was put on.

other than that, I would agree with the cam timing problem.maybe being off by one tooth

Sounds like I need to do some testing first before tear down. I suspect the head. since after the guy I had do the valves almost took me completly out of shim range. (bare minimum shims) That has always bugged me.
I bought an original '77 head with 14k miles on it to put on it. The plan is to swap heads. Also noticed some seepage of oil between the cylinder and head on the left side.
So what does everyone think of using a copper head gasket next time? I understand you need to o-ring around the top of the sleeves . True or False.?

Sorry , No J heads here.

Was just wondering about the deck height. What should It measure With these pistons? I can't find anything on it.