Uh Oh Low Compression

So I was simply changing a valve cover gasket on the GPz750 when I decided, while I was at it, to check my valves. THe valves were off by quite a bit but I was not sure if I was doing it right so I decided to do a compression check.
My cylinders were 90-0-0-90.
Pretty awful. The valves in cylinders 2 and 3 have no clearance so I am sure an adjustmenmt will bring those up but they may end up at 90 too.
So... The bike has about 22,000 miles on it. It had been running rough and seemed way down on power. Does it make sens to adjust the valves, put it all back together and see how it runs or should I just go ahead and tear it down and put rings in it? I guess the third option is splitting the cases and doing a complete rebuild.
Thoughts?
What about an 810 kit? Is it worth it?

Set the valve clearances and you'll be amazed at the difference.
Make sure the throttle is wide open, very important.

Thanks Steell, I might as well try that first.

Could also add a teaspoon of motor oil through the spark plug hole into each of the 90psi cylinders, and repeat the compression test. Sometimes referred to as the "wet" test.

A significantly higher psi with the wet test would indicate suffering from "blow-by" past worn piston/rings/bore.

Roughly equivalent dry-psi and wet-psi would indicate somewhat better condition of piston/rings/bore.

Or better yet, perform a leak-down test, which will show where compression is being lost.

Good Fortune!

ThatGPzGuy wrote: So I was simply changing a valve cover gasket on the GPz750 when I decided, while I was at it, to check my valves. THe valves were off by quite a bit but I was not sure if I was doing it right so I decided to do a compression check.
My cylinders were 90-0-0-90.
Pretty awful. The valves in cylinders 2 and 3 have no clearance so I am sure an adjustmenmt will bring those up but they may end up at 90 too.
So... The bike has about 22,000 miles on it.

If you adjust clearances and the compression is still low, it might just need a valve job. Doubt the rings are toast that soon. The "oil add" compression test would tell about the rings.

Ok Thanks Everyone. I am going to redo the compression check just to make sure. I will also do the wet test to see what that tells me and post the results. At this point I will plan on just the valve adjustment (unless the wet test tells me otherwise) and see where that gets me.

I have a few questions about doing the valve adjustment but I'll post that seperately. I know there is a lot of info on the board already for that topic.
Thanks,
Jim

Here are some results:
I redid the compression check making sure the throttle was wide open. I got the same results which was 90-0-0-90. I did a wet test and got slightly better results. 120-35-35-140.
I also did Patton's poor mans leakdown. #1 was fine, #2 leaks thru the intake due to zero clearance. #3 leaks for the same reason. #4 is Ok.
Compression still seems low to me. Is it time for rings?

ThatGPzGuy wrote: Here are some results:
I redid the compression check making sure the throttle was wide open. I got the same results which was 90-0-0-90. I did a wet test and got slightly better results. 120-35-35-140.
I also did Patton's poor mans leakdown. #1 was fine, #2 leaks thru the intake due to zero clearance. #3 leaks for the same reason. #4 is Ok.
Compression still seems low to me. Is it time for rings?

zero valve clearance equals crappy compression.

bountyhunter wrote: zero valve clearance equals crappy compression.

Got it. I understand 2 and 3 need to be adjusted but I am not getting very good results with 1 and 4. I am afraid I'll go thru all the work of adjusting the valves just to have to tear into it again to do the rings.

ThatGPzGuy wrote:

bountyhunter wrote: zero valve clearance equals crappy compression.

Got it. I understand 2 and 3 need to be adjusted but I am not getting very good results with 1 and 4. I am afraid I'll go thru all the work of adjusting the valves just to have to tear into it again to do the rings.

With only 22,000 miles, I would concur with Steell's advice, adjust the valves to specs, and ride.

During the leakdown test, was any air heard escaping from the crankcase vent?

Good Fortune!

ThatGPzGuy wrote: Here are some results:
I redid the compression check making sure the throttle was wide open. I got the same results which was 90-0-0-90. I did a wet test and got slightly better results. 120-35-35-140.
I also did Patton's poor mans leakdown. #1 was fine, #2 leaks thru the intake due to zero clearance. #3 leaks for the same reason. #4 is Ok.
Compression still seems low to me. Is it time for rings?

You don't know what you're doing, so I'll try to be kind.

Oil seals the pistons in the bore so no air can get by. 90-0-0-90 with air escaping by the pistons, 120-35-35-140 with oil seal, that means you have 30 escaping by #1, 35 escaping by #2 and #3, and 50 getting by #4.

Stock compression on that bike in excellent condition should be 175-180. Subtract the oil test numbers from 180, and the remainder will be the amount of air escaping somewhere else, and that's the valves 60-150-150-40 (I think #4 has a stuck ring). That means you need to do what I said, "adjust the valves THEN check compression".

Do the steps in the correct order or you'll drive yourself crazy.

Did anybody else notice the numbers jumped between 30-50 psi by dumping a little oil in the cylinders or is it just me again? :laugh: