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TOPIC: GPz 750 compression problem

GPz 750 compression problem 22 Sep 2019 11:54 #811173

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ayeckley wrote:
Pretty sure the intake cam should not line up as in your picture, just the exhaust. There are supposed to be a certain number of links between the marks instead (can’t recall off the top of my head). This is because with the cam cover off the rub block is not pushing down the chain. From the looks of the picture you are not factoring in the cam cover. Perhaps this is the issue?



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GPz 750 compression problem 22 Sep 2019 12:40 #811174

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I stand corrected. I guess I’ve always paid attention to the link count rather than the alignment mark.
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GPz 750 compression problem 23 Sep 2019 02:25 #811183

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Thanks again for your replies. The cam alignment is the thing that niggles me - I've been setting the exhaust in line with the rocker cover then installing the inlet at 45/46 on the chain (the bike hasn't got enough mileage for the chain to have stretched enough to put the cam timing out). As can be seen, both seem to line up perfectly. The 'IN' looks a little below the edge in the 4th pic but this is just because it's taken on a cheap camera phone.

I can only see 2 things to do:

1) Try and find someone who'll lend me a compressor for a couple of hours and do a leak down test on the cylinders at TD, middle and BDC to see if the valves or rings are leaking badly (a fluid test on the head didn't show up any issues, and the head isn't cracked around the plug, just scratched).

2) Pull the cam caps and turn the cams one tooth counterclockwise . The crank timing mark is a degree or so off TDC in the pic according to my protractor and screen. According to what I've read, the cam cover chain guide pulls the slack between the cams in by 4 degrees, but this shouldn't affect the setup - why have alignment marks that line up when you can't see them because the cover's on? But with some tension in the chain it may pull the exhaust cam forward and this would put the timing for both out.

In my hunt for an answer I've read some horror stories. People who have bought bikes and found the inlet set up between 44 & 45, but still runners! It looks like the stock timing is 105 degrees but I've read some of the mod guys who have run their motors anywhere between 100 and 110 degrees so I'm going to concentrate on the exhaust cam timing + 45/46 pins once I've rechecked the valve clearances. Then it's the leakdown test - maximum loss 8% for a good engine I'm told.

*Sigh* Never had a problem like this on my old Z thous. That's the problem with trying to resurrect an oldie I suppose. Still, it'll be worth it.
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Last edit: by Jimski.

GPz 750 compression problem 05 May 2020 14:09 #825065

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Hi im new to the forum, just wondering if you got to the bottom of the problem, i have a gpz turbo that i bought a couple of years ago as a non runner and only just starting to work on it. same problem as you, wanted a chat before a top end strip incase you found a simple answer ?
Noticed that the first cylinder cam lobes were not facing inwards at tdc so the ignition timing is 180 out but compression should still be there,
Hope to talk soon :-)

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GPz 750 compression problem 20 May 2020 04:02 #826129

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Apologies - just seen the reply.

I tried timing and retiming it over and again thinking it was the cam timing but even when I look through the spark plug port and see the valves are closed on compression there's no pressure whatsoever.

Barrels are excellent, pistons aren't worn, I've had the valves out, cleaned them up (they weren't bad, just a bit of carbon and gunk but I fitted new stem seals at the same time), leak tested them, but it's made no difference. This leads me to the only thing left that I can think of - rings.

When I took the barrels off I noticed a couple of the rings were stuck so I freed them all up before reconstructing (with the notion that if it was running before, they should be relatively ok). I wonder if this was an assumption too far? I wonder if they're distorted? The bike sat for over 15 years without running.

I've had a new set of rings delivered - well, for £100 it's worth it for the peace of mind - and I've just got to get out there and fit them. If it's not them then it's back to cam timing because there's nowhere else to go. I must admit, it's one of the jobs I just don't like doing so I'm building myself up to a day in the sunshine tinkering. Will keep you posted....
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GPz 750 compression problem 22 May 2020 14:11 #826349

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Hi again, i stripped the top end, all the piston rings were stuck tight in their grooves, one of the barrels has some pitting, luckily near the bottom so honed it, all the valves except two were bent, put them in a drill and watched them wobble, some more than others but bent none the less bent so have ordered a good second hand set from america. I have new rings waiting, even one of the valve buckets was damaged.
Please keep me informed how you get on and i will do the same. cant be your valve timing as you had it dead on !
Talk soon

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GPz 750 compression problem 25 May 2020 04:53 #826478

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Well, I was hoping to post a positive update here today, but I had a minor setback.

I stripped the head and barrels off and changed the rings. This turned out to be far easier and stress free than I was expecting, and the new rings look better quality and are a nice, snug fit. There was a bit of gunk in the grooves but it they cleaned well.

Got the barrels over the pistons, jiggled the camchain tensioner to get it into the tunnel then disaster! One of the square rubber grommets popped off the arm and fell inside the engine! So I’ve drained the oil and have to drop the sump to locate it before I can progress. Talk about red faced. I’m fuming with myself. Luckily, I bought a complete gasket set before starting so it’s just a slight delay, but....
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Last edit: by Jimski.

GPz 750 compression problem 25 May 2020 09:41 #826487

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just wont start wrote: Hi again, i stripped the top end, all the piston rings were stuck tight in their grooves, one of the barrels has some pitting, luckily near the bottom so honed it, all the valves except two were bent, put them in a drill and watched them wobble, some more than others but bent none the less bent so have ordered a good second hand set from america. I have new rings waiting, even one of the valve buckets was damaged.
Please keep me informed how you get on and i will do the same. cant be your valve timing as you had it dead on !
Talk soon


Make sure you use the turbo supplement manual when you set the timing www.750turbo.com/forum/viewtopic.php?f=24&t=13322
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GPz 750 compression problem 25 May 2020 15:43 #826506

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It happens to the best of us mate, hope you have better luck when you go back to it. I cant do much it the moment till the valves arrive then it lap them and give everything a good wash down before assembly. so looking forward to it firing up !

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GPz 750 compression problem 25 May 2020 15:45 #826508

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Cheers my friend, are those your bikes in the photo ?

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GPz 750 compression problem 26 May 2020 09:47 #826564

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Well, I despair. After my recent disaster with the camchain cush dropping into the engine (it lodged in the primary drive chain sprocket) I dropped the sump, had a nose about, fitted new gaskets, filter and magnetic plug, everything looked fine.

Replaced barrels, head, cams, timing is spot on. No compression. I mean NO compression. Barely registers. 20-30 psi (carbs and exhaust are off, plugs are out, newish battery, fully charged).

Valves have been dropped, checked, cleaned, resealed and reseated. No sign of piston or barrel wear. The head gasket looks like it's clamped down fine. The rings went easy over the pistons (could swear I measured then before I ordered the rings) so I'm confident they're standard.

I'm at a total loss, and barely containing myself from taking a rather large hammer to the thing, such is my frustration.

One more strip, that's it. Gonna measure the cylinder diameter in case someone's been stupid enough to bore them but keep standard pistons (surely not?). Gonna get the forward and back threads for the shanked bolts helicoiled as they feel like they're on their way out and barely torqued up, which won't help.

I can't see what the problem could be. Even if the barrels had been bored, surely there's be some compression? The valves can't be out on every cylinder. Again, the cam timing is spot on (it's idiot proof, I can't have messed up 5 times and I can see the cams operating at the right time through the plug holes.

What I really need is a blow down test, try and find where the pressure's going, but I just haven't got the gear to do that.

Right, I'm off to vent my frustration. Good luck!
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GPz 750 compression problem 26 May 2020 10:41 #826568

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O no, i know you would of checked but are the valve bucket-follower shims correct and another maybe silly question is the compression gauge working, is their still nothing with a finger over the plug hole ? crazier things happen sometimes !
Good luck

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