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TOPIC: GPz 750 compression problem

GPz 750 compression problem 03 Jun 2020 11:31 #827218

  • F64
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I'm almost afraid to ask...Are you holding the throttle fully open when you are doing the compression test?
And choke plate open.
81-KZ440-D2. Louis Dudzik's GM HEI ignition module converted 2015--no issues, Motogadget m-unit blue.

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Last edit: by F64.

GPz 750 compression problem 11 Jun 2020 07:30 #827866

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Ok think i know where you are at now !
I received all the parts needed, the valves arrived, i lapped them in and thoroughly washed in petrol and assembled the head, honed the barrels, new rings to the pistons and put everything back together . Timed the cams and measured the valve clearances , cylinders 1,2 and 4 wouldnt accept the feeler gauges at all, number 3 had clearance. With these measurements i knew the only cylinder likely to have compression would be 3, connected battery and spun it over , sure enough only number 3 had compression and blew my finger away so have taken out the shims and done the maths then ordered the shim sizes to correct the clearances.
Think this is where you are going wrong !
If the bores , pistons and rings are good then nothing can be wrong here, if good valves and lapped in properly then can only lead to valve clearance and them being open just in the slightest and not even visible.
In what i have just learnt by being on the spanners is that the valve clearances must be checked exactly how the manual states and only at the exact positions, i noticed that as the cam moved around from the back side it gave on some a false reading and maybe this is whats happening to you, no clearance means the cam is pushing the follower slightly, even if its not visible.
When the new shims arrive i will let you know the results !
Check those clearances mate.
talk soon

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GPz 750 compression problem 11 Jun 2020 09:20 #827881

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Splendid news about yours. I took a step back from mine for a few days, took the tatty clutch and alternator covers off and rattlecanned 'em while I was waiting for a new toy to arrive.

Just picked up a compression leak meter, the type with two gauges on, so I can give it a definitive test. Only a cheap one, but it'll do the job well enough. I've decided to pull the head and barrels again so I can get the front and back bolt holes helicoiled as I'm not confident torqueing them up and it's better to be safe than sorry.

The plan is to leak test it with the cams in at tdc etc. then to repeat the tests with the cams removed, that way I'll get a definitive answer as to whether it's bores, valves or other. When the head and barrels come off I'm going to triple check that they're not warped (pretty sure they're not, but while I'm there it's worth it as I can get them skimmed at the same time as the helicoiling).

Since the cam cover's off already I'll give your advice a whirl - I've got a copy of the factory manual and a Haynes to refer to. I must admit, I only did a visual check when in place and measured them when I reseated the valves to make sure they were within tolerance.

Good luck with the shims. How far were they out?
Why does everybody stand and point?

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GPz 750 compression problem 11 Jun 2020 10:51 #827894

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Hi pal
Exhaust clearance should be between .08-.18, i had .04- shim adjustment needed, .08-ok, .10- ok, .08- ok
Inlet clearance should be between .13-.23, i had 0, 0, .08 and .0 so all need adjustment
It can be the only answer to your problem also if all other things check out.
Follow the procedure, bet its the cure !

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GPz 750 compression problem 11 Jun 2020 12:03 #827907

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Only a tiny amount i know but keeps the valves open just enough to cause a fail !

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GPz 750 compression problem 14 Jun 2020 10:17 #828154

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Just glanced through everything, and only caught a couple of notes on valve clearance. One sounded like after significant work to the head no shims needed replacement? That doesn't happen. Using OEM cam positions for adjustment, what are the clearances?

Maybe I missed something?
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Last edit: by VTEC.

GPz 750 compression problem 14 Jun 2020 12:04 #828161

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Jimski wrote: Since the cam cover's off already I'll give your advice a whirl - I've got a copy of the factory manual and a Haynes to refer to. I must admit, I only did a visual check when in place and measured them when I reseated the valves to make sure they were within tolerance.


Just noticed this one. Measuring the shims doesn't accomplish anything if you don't know what the clearance is. And you cannot check the clearance visually. I'm worried you're not familiar with the valve adjustment procedure and all valves may be partially open at TDC due to negative clearance.
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Last edit: by VTEC.

GPz 750 compression problem 15 Jun 2020 01:42 #828213

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Ta much for the advice.

I was working on the assumption that the bike was running when it came off the road (original owner passed away) and only did a size check to see they weren't over worn, but that might be an assumption too far.

So it's back to basics, check the clearances from scratch, see what happens. Fingers crossed.
Why does everybody stand and point?

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GPz 750 compression problem 15 Jun 2020 06:41 #828223

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The GPz manual looks a bit confusing on the adjustment.

Maybe this will help.

From the right side turn the crankshaft clockwise and line up the 1-4 T mark. Then look at the the cam lobe positions at cylinder #1 (far left cylinder). The top of both lobes should point directly away from each other. If they point towards each other, you're at TDC of cylinder #4 -- and have to rotate the crank another 360 degrees clockwise to 1-4 T mark and you'll be at #1.

-- At TDC #1 check exhaust valves #1 and #2
-- Then rotate crank 180 degrees to 2-3 T mark which is TDC #2, and measure intake valves #1 and #2
-- Rotate 180 to 1-4 T (TDC #4) and measure exhaust valves #3 and #4.
-- Rotate 180 to 2-3 T (TDC #3) and measure intake valves #3 and #4.
*firing order 1-2-4-3

Get a good set of feeler blades that go down to .001". You're looking for about .005" clearance at each measurement. Slide the blade between the top of the lifter and the bottom of the cam lobe working your way up in size. When you get to the one that won't slide in, the clearance is the previous size that did.

If you can't get a .001" blade in, and cannot spin the lifter freely by hand, you have negative clearance and the valve is partially open at TDC.

You're engine is shim under, so you would have to pull the cams, measure shims and go several sizes thinner, reassemble and re-measure clearance.

Let me know what you get....
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Last edit: by VTEC.

GPz 750 compression problem 09 Jul 2020 14:27 #830220

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Hi been a while i know but just wanted to update you on progress with the zx turbo, all valves shimmed to spec clearances, fitted the coils,connected the battery and fitted a temporary fuel supply, turned the key and she burst into life right on the button. Had to tickle the throttle at first to keep it going then it settled down and ticked over nicely. Was no silencer on so lots of noise, old carbon deposits shooting out of the pipe and fumes filled the garage as oil and old paint from the previous owners attempts burnt off, run it for about five minutes then let it cool off before pulling the cam cover just to make sure oil was being pumped around , all was good so put it back together again, starts on the button every time since with only a joint to the oil cooler needing tightening ! Really pleased as when i purchased the project i was told it hadnt run for thirty years, on the second start i filmed it so when i have the chance i will post it on the forum . Can get on with other jobs now and crack on with the resto.
Have you made any progress? your problem must be negative valve clearance also !

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GPz 750 compression problem 25 Jul 2020 07:02 #831392

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Kawasaki turbo 2nd start in 30+ years (please can you let me know if the video is attached)

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Last edit: by just wont start. Reason: video attachement

GPz 750 compression problem 27 Jul 2020 12:57 #831618

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No video attached...
1976 KZ900A4 (1105 Project)
1976 KZ900A4 (Stock Project)
1978 KZ1000A2 (Completed Project)
1983 ZX750 A1 aka GPz 750 (Completed Project)
1983 ZX750 A1 (Almost Road-Ready)
1973 CL350K4 (Completed Project)

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