High Compression Numbers?

Hey guys,

Been working to diagnose some slight issues with the KZ650 see this post and got a compression tester and ran some tests.

I did them several times - following the procedures mentioned here and in the Clymer. Once with all the plugs removed, once one cylinder at a time. Bike was run for 5-10 minutes then turned over until the pressure gauge stopped increasing.

I got the same results each time and they are the following:

#1 - 220 psi
#2 - 210 psi
#3 - 210 psi
#4 - 220 psi

The Clymer gives a spec of 180 +/- 15 psi as the "good" range. I can't come up with something off the top of my head that would cause the readings the be higher than "normal".

The bike has just under 27,000 miles on it...

Any ideas

Just for the heck of it...try a different gauge(or 2)and see what readings You get from them,because YA...those are pretty high readings for a (stock???) 650!:ohmy:

turn it over so it bumps the needle 4 times.

Double check with another gauge. If they both read the same then its possible you have carbon build up on the piston domes. Do a search and there's topics on here how to take care of it without removing the topend.

Later, Doug

I got the same results each time and they are the following:

#1 - 220 psi
#2 - 210 psi
#3 - 210 psi
#4 - 220 psi

The Clymer gives a spec of 180 +/- 15 psi as the "good" range. I can't come up with something off the top of my head that would cause the readings the be higher than "normal".

The bike has just under 27,000 miles on it...

WOW are you sure that the clymer said 180 psi. I think my clymer says it is supose to be 140psi. I havent looked at the manual for a while but I'm pretty sure it should be around 140.
A couple of things will give you high comp.:
advanced cam
carbon build up
too much gap in the valve clearence will allow the valve to close sooner too. creating more comp.

You ain't gonna get 180 PSI on a 650 I think... 150 is more like it in a good engine. I think you are reading the gauge wrong... I think you have 153 PSI showing in the pic... just a theory but look at the pic. Could the PSI / KG type scales be printed on the gauge in a misleading fashion? I am guessing they might. Last, when checking compression, pull ALL FOUR plugs after warming the engine. Turn engine over using the starter button if possible. Hold throttle open or remove your carburetor assembly. Crank till the PSI peaks and record number. I haven't seen a Clymers actually give instructions on how to check compression... they usually say to follow the guage manufacturer's directions but your Clymers might... If they didn't say something like what I noted, then I would do it the way I described as that is about the only way to get real good readings.

Post edited by: wiredgeorge, at: 2007/08/22 09:58

wiredgeorge, here is a pic from my clymers 900 & 1000 fours 73-80 that has a "how to compression check"

Your clymer also says lack of compression is least likely cause of starting problems.
I dont know abouot you guys but if you dont hae compresion I dont believe that the engine will start right up:P

my Kz1000's compression right now is 80 and it fires and runs!


Part 2 of the above

Hey ya never know

Someone may have gone and shaved the S*** out of the head.

I'm probably going to get flamed for this. But who cares! Get yourself good spark (clean out advancer, new plugs), good fuel (lines + gas tank), good air (good air cleaner and air-tight carb boots), and clean well adjusted carburetors.

I spent a lot of time looking at compression testers when I was younger. They never told me anything I didn't already know.

I bet your bike is fine, stock, and the gauge is adjusted wrong.

Keep in mind compression testers tell you more about engine trends (when readings are taken over time), problematic cylinders (when readings are taken over all four cylinders and one comes out particularly high/low) --- not the real absolute state of the engine as the clymers manual suggests.

220 psi / 14.7 (atmospheric pressure at sea level) gives a 14.9, almost 15:1 compression ratio.

I'd use another gauge.

The bikes are listed at 9.5:1.

KaZooCruiser wrote:

220 psi / 14.7 (atmospheric pressure at sea level) gives a 14.9, almost 15:1 compression ratio.

I'd use another gauge.

The bikes are listed at 9.5:1.

Only the GPz750 is 9.5:1, the others (650 and 750 fours) are 9.0:1.

And you keep forgetting to factor in heat to that equation, air does get hot when it is compressed, and when it gets hot it expands.

150 psi would be a 650 with really good ring and valve seal, as in freshly broken in. The low milage 650's I have checked averaged 135 PSI hot.