1983 GPZ 750 FRESH REBUILD 110PSI COMPRESSION????

...110 PSI...IS 150PSI THE TARGET?....

Note from Patton:

My apologizes to Scott for unintentionally deleting this post. :blush:

If available, please send content, or similar content, to me by PM, or just re-post it in this thread, and it will be restored.

Thanks, Patton

This might be better posted in the engine section...

Check the ring gaps are not aligned - they have to be staggered...

everything checks ok, should run just how well!!

A good target for a completely new bore/piston match after break-in, would be 160-180, I think 110 is seriously low, sorry.

When you measure everything very carefully and bore it snug and straight, you get awesome compression and power

Were the jugs bored directly to the correct size per piston measurment, or just bored. Rings don't usualy seat and raise the compression by huge numbers, but I have seen stranger things I will admit.

Compression test should be done hot after a decent run, and always with throttle open wide (as you described), and with plugs/wires grounded. All plugs out too (to eliminate blown head gasket).

It might come up to 125-130 after the rings seat and she's hot - or it could come up to 145? who knows! If there is a big difference between cylinders (more than 10% after break-in), then there is deffinately some questionable work there!

Good luck, and try a different/known good compression guage!



Dont forget to calculate your compression ratio! You can usually shave a little off the head to improve things.

thanks for your help, dude your nut house video seriously cracks me up, its great!! the bore was done correctly using the piston for sizing. The ring end gap was perfect per Wisecos feeler measurement and consistent through bore. So I think it is a good bore.

The 110 psi so far was cold and not ran yet. I hope after it runs and for a while it will seat and bring up the compression. I should check another compression guage. All the shimms are .005 intake and .007 exhaust. within the .003-.007 spec. the cylinder has good crosshatch. it will run tonight and i will post how it turned out,

thanks again

If not already done, might be interesting to do a leakdown test now, and another leakdown test after a few hundred ring-seating miles, for purposes of comparison.

Standard compression psi should be provided in FSM.

Here's link to downloadable FSM:
Click here > kz.mbsween.com/manual.html

Good Fortune!

skemp101 wrote:

thanks for your help, dude your nut house video seriously cracks me up, its great!! the bore was done correctly using the piston for sizing. The ring end gap was perfect per Wisecos feeler measurement and consistent through bore. So I think it is a good bore.

The 110 psi so far was cold and not ran yet. I hope after it runs and for a while it will seat and bring up the compression. I should check another compression guage. All the shimms are .005 intake and .007 exhaust. within the .003-.007 spec. the cylinder has good crosshatch. it will run tonight and i will post how it turned out,

thanks again

Throttle wide open when cranking the compression test?

Did you coat the pistons, rings and walls with STP/oil or some kind of assembly lube?

BTW, a 750 twin will hold about 155 - 160 psi after rebuild. Mine did.

Did you double check that your cams are installed correctly ?

DFIGPZ wrote:

Did you double check that your cams are installed correctly ?

Seconded. Common and easily corrected cause of low pressure after reassembly. Don't ask how I know....

CAMS SHOULD BE CORRECT, I USED ASSEMBLY LUBE PUTTING BACK TOGETHER. I GOT IT RUNNING LAST NIGHT. AT FIRST RAN LIKE CRAP. REVIEWING THE FIRING ORDER I HAD BOTH COILS REVERSED. IT RUNS A LOT BETTER. ANY IDEAS ON THE AIR FLOW SCREW SETTINGS. IT HAS FACTORY AIR BOX, UN OILED FILTER, 4-1 KERKER K2S. THE IDLE IS ALL OVER THE PLACE, I HAVE A SYNC STICK. I THINK IT IS CLOSE. I HAD IT RUNNING FOR 10 MINUTES OR SO. I CHECKED THE COMPRESSION AND HAD ALMOST 120. WILL IT GET BETTER AS I RIDE IT? THE CAMS SHOULD BE RIGHT

get another gauge and compare the numbers that may be your problem. if that's not it double check your cam timing if it's out a tooth it will have a huge effect on your compression

timebomb33 wrote:

get another gauge and compare the numbers that may be your problem. if that's not it double check your cam timing if it's out a tooth it will have a huge effect on your compression

Agreed - this was the cam installation that I was referring, not so much about whether you used lube or not... One tooth off can cause a considerable drop in cylinder pressure.