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TOPIC: GPz 750 compression problem

GPz 750 compression problem 20 Sep 2019 12:23 #811116

  • Jimski
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Could you folks put my mind at rest that I'm not doing anything completely stupid please?

I'm a reasonably able home mechanic but not rebuilt an engine in years (I've previously had z1000s, 750, 250s, xs11, various Triumphs).

I picked up an ealy unitrack GPz750 a couple of years back as a project. Previous owner had died in 2002 so it hadn't run since then.

I had to strip the head for a helicoil, rebuilt it but had no compression, all 4 cylinders. It went in the shed for later.

I just had some spare time so I restripped the head, checked valve leakage (fine), bores looked fine, rebuilt it - no compression again.

So I pulled it again, barrels as well, rings were sticky so I freed them, bores look fine. Head doesn't look warped (checked in case helicoil had warped it).

Since I had a full gasket set I pulled all the valves, checked they're straight, seat well, cleaned the carbon, replaced the stem oil seals etc. Then I rebuilt the engine with new gaskets all round, turned it over - no compression. I mean *none*. Can't read any on my compression gauge, not even finger over plug hole (engine spun with new battery, exhaust and carbs removed).

I'm pulling my hair out! I've got the right cams in place, set the sprockets as per the manual (Kawasaki and Haynes), cam operation looks correct.

I've got to try and beg a compressor from somewhere for a leak down test but until then I'm stumped.

Cam sprockets are set to z7ex exhaust, IN inlet, 45/46 pins apart flush with head (as near as possible). The notches on the ends of the cams are 180 degrees opposite at tdc. Even with ring wear there should be *some* compression.

I read on this forum that one manual says use the zx6 marks for exhaust but I really don't want to jump in and just try it for obv reasons but the compression loss is so big across all 4 cylinders and the components so unworn I keep thinking valve timing.

Short of another pulldown and a ring check (may as well rebore anyway since I'm there anyway....) any ideas?

Will post pics if req.
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GPz 750 compression problem 20 Sep 2019 12:43 #811117

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Sounds like a cam timing issue. I suggest pulling all plugs and dropping a long screwdriver down the #4 hole. Rotate the crank while watching for TDC on the screwdriver and the 1-4 T mark on the timing plate. Be very careful you are aligning to the proper crank position. After that follow the FSM on how to install the exhaust cam. The Z7EX line will point slight above the gasket surface because of chain stretch.
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GPz 750 compression problem 20 Sep 2019 15:43 #811122

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Ta muchly for the speedy response. You're thinking exactly the same as me - done it, looks right, scratching my head still. Even had a biker neighbour do exactly the same. TDC on crank plate is spot on. Even with shot rings, there should be something.

Will post a pic of the cam settings later (when light). There's always a little question in my head when setting cams after a rebuild because of chain slack but I trust myself enough after a couple of resets to think I should be right. Never been wrong before.

But zero compression? Never had it before, even with worn bores, at least ones not showing definite, serious wear.

I'm doing something stupid. Something so obvious I'm not seeing it. Bores fine (look perfect, no glazing or scratching), rings tight (by sight and fit), no obv piston wear (crowns polished, not bad anyway), valves straight, cleaned, seated, head and barrels refitted with new gaskets, retorqued (several times) shims all fine but still 4 cylinders no compression. I should be getting my finger blown off the plug hole on the starter even with bores like a metro tunnel but nothing but an asthmatic puff.

The only variable is valve timing, and it just looks right. Would one pin out be that bad? Absolutely zero compression across all four?
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GPz 750 compression problem 20 Sep 2019 15:52 #811123

  • baldy110
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How 's the valve clearances?

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GPz 750 compression problem 20 Sep 2019 16:14 #811126

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I am with baldy110. If you drop in a diffrent set of cams with a greater base circle you can lost your valve clearance and your valves stay open all the time.

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GPz 750 compression problem 20 Sep 2019 18:53 #811132

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Scirocco wrote: I am with baldy110. If you drop in a diffrent set of cams with a greater base circle you can lost your valve clearance and your valves stay open all the time.


I think that’s the key!
The Unitrac zx750’s exhaust cam has a smaller base circle than the intake cam. If the cams are not the original ones or if the intake and exhaust cams or sprockets got reversed, that would cause the lack of valve lash and the valves to be constantly held open.

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GPz 750 compression problem 21 Sep 2019 09:03 #811145

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Thanks for the responses and ideas. The cam with the blank for the tacho is definitely in the exhaust position. I even had a quick check of the diameters in case the previous owner had fitted different ones but they checked out (35.5 & 36.5 mm from memory).

Going to check the valve clearances now it's together again in case I boobed first time but I didn't see any probs. Shims looked fine, all within tolerance, all nearly identical, all in the correct buckets, which don't stick.

Gotta be rings or valve timing to my mind, and the compression is so low it's like there's no rings at all.

Still, I like a challenge. If the clearances are fine then I'm gonna have to consider moving the cams one tooth counter clockwise and turn it over *very* carefully by hand. Think I'd prefer to get slotted sprockets and just shift be a degree or two.

Any more thoughts on the z6ex cam mark that someone said on this forum was in the Clymer manual? Seems a damgerous suggestion to me and I can't see anything in the K service manual or Haynes.
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GPz 750 compression problem 21 Sep 2019 14:06 #811150

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There was someone here a few years ago had a bike and the camshaft(s) were attached to the sprockets using the wrong bolt holes.
See pic below from the Kawasaki manual

Probably a long shot but should be easy the check if you have the valve cover off

1980 kz750E1, Delkevic exhaust
Attachments:

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GPz 750 compression problem 22 Sep 2019 04:41 #811162

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Spot on, I'd say, even though the camchain is slackened off. Even the notches in the cams line up.
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GPz 750 compression problem 22 Sep 2019 05:34 #811163

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>>> "Spot on, I'd say, even though the cam chain is slackened off. Even the notches in the cams line up. "
With engine at tdc, valves closed suck on intake manifold to check intake leaks.
If intake is good remove plug, insert compressor fitting, check exhaust for leaks.

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GPz 750 compression problem 22 Sep 2019 06:32 #811164

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Jimski wrote: Spot on, I'd say, even though the camchain is slackened off. Even the notches in the cams line up.


WHAT IS THAT CRACK AROUND THE SPARK PLUG LAND?
Z1b1000 1975 Z1b
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kzrider.com/forum/11-projects/598262-kz-...-will-it-live#672882
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GPz 750 compression problem 22 Sep 2019 09:13 #811170

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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?
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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