Help me interperet compression numbers plz

Cold compression numbers:
#1 130
#2 130
#3 121
#4 150

Bike is a 79 KZ1000 shaft. 'Supposedly' has new top end but I suspect I was the victim of bullshit. If it was actually rebuilt, it hasn't been broken in yet. #3 won't run. Spark is decent. I haven't completely rebuilt the carbs yet. Suspect an intake leak. Just thought I'd test compression first. Don't know about valve clearance yet. Have a gasket, plan on pulling cover and checking that next.

I was using a cheap harbor freight comp tester. Haven't tested warm yet because #3 won't run.

I'm fairly handy but inexperienced with these engines.

Assuming the guage is inaccurate but consistent- what's your read on these numbers.
Do they warrant tearing into the upper end and checking things out?
Is number 3 low enough that I should pour in some oil and check if its a valve problem?
Is number 4 too high?

Keep the shiney side up. Thanks.

Mortarmanmike wrote:

Cold compression numbers:
#1 130
#2 130
#3 121
#4 150

Bike is a 79 KZ1000 shaft. 'Supposedly' has new top end but I suspect I was the victim of bullshit. If it was actually rebuilt, it hasn't been broken in yet. #3 won't run. Spark is decent. I haven't completely rebuilt the carbs yet. Suspect an intake leak. Just thought I'd test compression first. Don't know about valve clearance yet. Have a gasket, plan on pulling cover and checking that next.

I was using a cheap harbor freight comp tester. Haven't tested warm yet because #3 won't run.

I'm fairly handy but inexperienced with these engines.

Assuming the guage is inaccurate but consistent- what's your read on these numbers.
Do they warrant tearing into the upper end and checking things out?
Is number 3 low enough that I should pour in some oil and check if its a valve problem?
Is number 4 too high?

Keep the shiney side up. Thanks.

Perhaps the compression figures are not so unusual for a freshly rebuilt engine where new rings have not yet gotten seated.

Given decent spark and 121 psi compression, combustion failure in #3 cylinder would seem carb related.

I would change the oil and filter, install a brand new sparkplug in #3, and then test ride to determine whether #3 cylinder regains function.

Would also proceed with assuring correct valve clearances. But I wouldn't tear into the engine until first trying to gain combustion in #3, and further not until performing a leak-down test on #3 cylinder.

Good Luck!

Thanks. The more I thought about it during lunch - thats kinda what I was thinkin.

Mortarmanmike wrote:

Cold compression numbers:
#1 130
#2 130
#3 121
#4 150

Bike is a 79 KZ1000 shaft. 'Supposedly' has new top end

"Top end" usually means a valve job. The engine may have that, but still have rings that are causing these comp readings.

the bike will run with those numbers no prob the one cylinder maybe low due improper valve clearances

Mortar, agree w/prev.posts. personal experiance is that @ 90 lbs compression will make bang.little things, tight valves,poor carberation,vaccum leaks,leaking plug caps,raw fuel leaking past rings,rings not sealed could cause low cyl. make it run on all four if possible before launching into tear down. my 2 cents g

Thanks guys. You're a bigger help then you know confirming what I've suspected. I checked the valve clearance and that appears to be the problem.

I have a couple questions about shimming so time to do a little research.

Thanks all. I consider this thread closed.