1980 KZ1000 G rebuild 1075cc low compression

Greetings to all from Ireland -First post - Looking for some help. Just doing a first rebuild on a kz1000 motor. New 1075 wiseco kit fitted and all new valves/seals/springs etc. Measured the shims clearances and fitted new shims. Doing first compression test cold (engine was not running when i got it). Bare engine - no inlet/exhaust connected.. Getting 75psi on cylinders 1/2/4 and 50psi on 3 with teaspoon oil down the spark hole of all 4 cylinders. Connected compressed air and do think i am getting some audible leak into crankcase when pistons are TDC. Could be leaking past rings?. Read somewhere that i may not get true values until engine has run in for a while? - or should i just strip engine down again and recheck piston rings and gaskets now?

Bad cam timing, bent valves? Valve clearance near Zero? Missing Piston rings???
50 psi is really a way to low on a new rebuild engine!!!

Timing seems right going by clymer manual and web searches. Measured for shims with feeler gauges and can spin buckets/shims when lobe facing away. All new valves and engine has not been ran so not bent valves. Comitech full length head gasket so left out the rubber cam chain tunnel rubber and used rtv. Is 75psi considered ok and 50psi not ok?. I’m guessing myself I will need to pull apart and review piston rings

Recheck the cam timing.
Steve

Have to agree with Steve on this. Re-check your cam timing and make sure you are spot on. It's my understanding, but not personal experience, that you can bend a valve just manually turning the engine over without having it running or using the electric starter. It isn't all that difficult to be off on cam chain timing especially if you are new to this motor. Anyway, that's where I would start.

Rick H.

Also pull the plugs and look through the holes with a flashlight. Should be able to see any significant cylinder wall damage.

Check you cam timing and your valve clearance, but I would do a proper break-in before you tear it all down.
Read up on doing a proper break-in. Your oil choice will be important. They even make oil just for breaking in the ring/cylinder surfaces.
And in many situations synthetic oil is not reccomended for breaking in as it's too slick.
That will help your cylinder sealing tremendously.





and an argument against break-in oils


research, research, research

Hello All, many thanks for the quick feedback and guidance. Made a bit of progress this evening. Checked valve timing and looks spot on.
TDC mark for 1/4 - Exhaust mark on cam sprocket (not the rubber mark but the metal mark) and 28 pins to intake sprocket 28 mark, all line up.
(1) Read Manual again and saw i never subtracted 0.15mm from my shim calcs - recalculated and moved shims around as close as i could to optimal
(2) Recharged battery overnight to get fastest turnover for compression test
(3) Put some more oil down each spark hole
(4) Cylinder 4 - 125psi cold with correct shims Ex 235 In 230
Cylinder 3 - 100psi cold with incorrect shims Ex 230 should be 220 / In 235 should be 225 (10 out on each)
Cylinder 2 - 80 psi cold with incorrect shims Ex 200 should be 205 / In 220 should be 215 (5 out on each)
Cylinder 1 - 80 psi cold with incorrect shims Ex 220 should be 225 / In 230 should be 215 (5 on Ex and 15 on In)

Still a big difference across between each.
Hoping the shim changes will improve and bring each closer and that having to add oil to get good readings is just an indication that the new pistons/rings/rebore are not broken in yet.

I guess after fitting correct shims and retesting compression, the next step would be as was suggested to "break in" the engine for short periods and changing the oil.
Not semi-synthetic oil. This will hopefully give the desired 175psi on all cylinders when hot after breaking in.

Regards and many thanks from a hopeful Irishman for a firing engine in 2020.
Also rebuilding a set of carbs Mikuni VM28's i think with accelerator pump, to replace the fuel injection that was originally on this bike - but that's another step.

Very good!!
Keep us up to date on the engine's progress.

Getting closer. Disassembled back down to inspect piston rings. All ok. Top two compression rings correct. N facing up, smaller shiny ring on top and larger dark ring second. Ring gaps measure ok. Not too tight. Readjusted 180 out of phase. Checked valves, no sign of mechanical damage. All held petrol, no leak by. Reassembled everything “again” Compression test 115,100,95,95. Think I will try some run in tests and hope that all the new rings/cylinders/valves just need to seat. Will keep updated.

What kind of compression tester are you using? I've been warned you need to make sure it has a valve in the tip or else it will throw the compression numbers off. Reason being the engine is now compressing the volume of the gauge as well as the cylinder. Also throttle fully open or no carbs mounted?

Hi Matt. I was looking into that alright as my tester is a cheapish one, but it does have the schrader valve about 8 inches from gauge, and I use a solid screw in quick release to spark hole. Measurements appear consistent. Might try and borrow a second tester to be sure. Mine is made by Silverline.