Impossible compression issue GPZ 750 1983

Hi guys! 
I am working on a Kawasaki GPZ750 Unitrack from 1983 and I am struggeling for over a year now with a compression issue. I already have found several topics about a low compression issue on GPZ's but I did not found a solution so far. I hope someone here recognizes the problem and can help me out, I really dont know what to do anymore. 
 
I bought the bike about 2 years ago and the previous owner also never rode the bike, so it's a bit of a guess what exactly have been done to it in the past. The bike did run though and before I disassembled everything I rode a couple of meters on it.  
 
The problem: 
Every cilinder measures about 60psi. 
No mather what I do, I cannot get much more out of it. 
I already checked and verified loads of things, but nothing really change the 60psi; 
 
Cylinder: 
No damages or other deviations found. 
New base gasket and cylinderhead gasket (and all other gaskets). 
 
Pistons: 
There are .50 oversized pistons installed by the previous owner. 
No damage detected on the pistons or piston rings.. 
Pistons and rings have been checken by a local expert and confirmed they didnt see anything distrubing. 
 
Valves: 
In-and-outlet valves have been "grinded" again (not sure if it is the right English word for it). 
Valves have been shimmed according to Kawasaki manual. 
 
Timing: 
I have adjusted the timing about 10 times now. All according to the Kawasaki Manual, but nothing changed. 
 
Additionally: 
- I performed a pressure test, with the in-and-outlet valves closed I only hear a very very ight escape of air coming out from the (removed) "oilpan". I guessed this is the air escaping between the gaps of the pistons rings (it is barely audible). 
- New compresion gauge. 
 
I am for 99% sure it is a timing issue. 
There are a couple of things I am thinking about that can cause or contribute to the low pressure. 
 
1) Due to the already oversized pistons I wouldn't be surprised if the head has also been remilled. I checked this and it doesn't look like it. If the head would be milled, would this also mean a different timing then advices in the Kawasaki  Manual? 
 
2) The Kawasaki Manuals tells to first install the cams, tighten the cam bearing blocks and then install the chain tensioner. When I first install the cams and bearing blocks, the alignment of the cams are spot on. The marks on the in-and-outlet cams are exactly aliged with the top of the surrounding surface. Also the amount of chainlinks are spot on. When I then install the chain tensioner and rotate the crankshaft a couple of times, the timing is completly off.  
Is this the issue. Or is the insteuction from the manual correct? 
 
3) Oversized pistons. 
Could it be there have been wrong pistons installed. Pistons that are shorter in size and creates less volume in the combustion chamber? 
 
Any sugestions or advice is greatly appreciated. I am almost thinking of throwing the whole engine in the bin and buy a different one. 
English is not my main language, so if any typo's are in it ; please ignore them.

I have fixed your pictures (before you submit the post click "insert all") once timed if you rotate the engine 2 full turns back to TDC (camshafts turn at half engine speed) the timing should not have moved !
are you tensioning the chain and rotating the engine with the cam cover removed ? I,m not sure about the 750 but the 1100 has a chain guide in the cam cover so you can't rotate the engine with the cover removed and tensioning the chain with the cover off will lead to it being overtightened

You have checked piston and cylinder condition but have you checked piston to bore clearance and ring end gaps ?

Going by the information and pics provided eveything appears correct BUT the chain tensioner must be fitted after the cam cover NOT before otherwise the the camchain will be dangerously tight.
After timing the cams, the cover ( which contains a slipper block ) should be fitted first and then the tensioner.
That 60 psi is very low and if the compression remains the same despite everything you do are you sure the gauge you are using is working correctly ?








 

Hopefully you are not rotating it w/out the 4 camshafts center caps.

Everybody thanks for the tips and advice!
Yesterday and today I gave it another try and I've got good and bad news:
Good news:
I have found additional PSI's!
Bad news:
It are only 30 PSI's.

I now measure around the 90 PSI's on all cilinders.

The cause: a bad compression gauge (even though that damn thing was brand new). Thanks zed1015!

@wookie58: thank you for the tips on posting the photos.
The 750 also has a chain guide inside the cover. I have tested the compression with and without cover on it. I also tried to first tension the chain and then put the cover on it (as described in the manual), but I also tried to first put the cover on it and then the chain tensioner. Both had the same outcome.
Also the pistons and the bore clearence have been checked, same as ring end gaps. 

After confirmation from a local expert (he owns a shop restauring engines), I am sure that it cannot be the piston/cylinders. But nonetheless good to point it out!

@Hugo: No, it was just for the photo. I do test with all cam centercaps mounted.

Does anybody have any other suggestions, tips, advice? I am still thinking it must be a timing thing, but it looks I have the timing exactly as in the manual. I don't understand how it can be that low.

Would 90 PSI be driveable? I understand it is much too low and will definetly be noticeable in lack of power, but if it is driveable, I will put the block back in and look for a replacement one. But I atleast can drive on it.

Does the timing now show correct after 2 full rotations of the crank with the cam cover on and chain tensioned correctly, if the timing is correct the pressure can only escape mechanically - pistons/valves head gasket - the only other thing is if the battery is poor then low cranking speed would also result in lower readings. It will run at 90 psi just won't perform well

PS: if you pressure tested on the bench how did you stop the air pressure rotating the engine (I wouldn't expect to hear air escaping in the crankcase as you described, if it can't hold 100 psi of air pressure then it won't build the kind of compression you are looking for)

90psi is deemed the minimum for combustion to occur.
If the manual is stating to tension the chain before fitting the cover it is WRONG.
You must fit the cover before tensioning the chain.
You should NOT spin the engine without the cam cover fitted as the cam timing will be incorrect ( retarded inlet cam ) .
I suspect the compression will increase once the engine is run if the rebore is fresh as the rings will need to bed in.. 
 

If you remove the tensioner completely I think the camchain can jump on the crankshaft when the cams are installed.
As you mentioned the timing is not correct after a couple of revolutions.
I just remove the cap and spring from the tensioner when I remove the cams.

Are you sure TDC mark is correct with piston TDC
Maybe previous owner used an impact wrench on the 17mm instead of the 13mm at the ignition and broke/bent the guide pin?
 

Everybody, thanks again for your help!
The problem has been found. Finally!

So last week when I measured the 90 PSI, I installed the chain tensioner after I put on the cam head. The result stayed at 90 PSI.
Because I always questioned the timing and the amount of links from the inlet tot the outlet camshaft, I rotated the inlet shaft with one chain-link: Result 120 PSI!

And even though I always tested with a fully loaded motorcycle battery, I've hooked up my car battery: 150 PSI!
After that I tweeked around for bit, added some fresh oil through the spark plug holes, rotated the cylinders by hand and give it another try with the car battery. Now I am measuring somewhere between 180 - 200 PSI. 
I guess the oil, that I put in it through the sparkplug holes, must have left the combustion chamber by now (after the amount of full rotations when testing).

Next step is to install the carbs and exhaust and fine tune everything. Once that's done, I will test the compression again.