GPz 750 compression problem

Ta for the reply,

Yeah good ideas but I checked the shims when I dropped the valves. All well within tolerances, didn’t have to replace any.

The compression tester is a brand new screw in type bought specifically for this (before that I send a push in one, which even though I didn’t trust it, we tested on a neighbour’s tyre compressor and it worked fine).

The finger test was the big give away before. I’d expect my finger to be clearly blown out the plug hole, but it’s not. The 2 outers barely lift my finger, inners not really at all.

I’m still dubious about those shanked bolts. I bought some spares because I managed to snap one in my old z750E - it just let go without warning, within the torque setting. I know the thread’s a bit dodgy, but I wonder if they’re stretched? They look like typical use once stretch bolts but I’ve never seen anything to say replace them after every use. I just assumed they were shanked so if you snap them you can still get the head off.

Like I said, there’s always something else to try, but I can’t justify throwing any more cash at it. Strip, Helicoil the two dodgy bolt holes, hunt for a compressor to do a blow down (or out) test, measure the bores, recheck the valves are seating, new bolts, rebuild, test. I suppose a man’s gotta have a hobby (and a sense of humour).

Goo£ luck, and tell us how you get on. It’d be nice to hear something positive.

Just to confirm your cam timing, please upload some cam position pics with piston #1-4 and #2-3 @ TDC.

Will do mate, might be another week or so before those valves arrive from across the pond though. Just wanting to get the thing running again, from what i was told its been dormant for about thirty years !

Have you had the assembled head upside down on the bench and poured petrol into the combustion chamber to see if any escapes passed the valves?

Hiya, here's the pics of the crank & cams at 1-4 tdc. I set the crank, then the inlet, counted 45/46 for the exhaust (backwards, I know, but I was more interested in testing compression. I realised after that the cams might shift slightly, but it wouldn't result in catastrophic compression failure. Will reset before running). The cam end notches both face inwards as expected. The timing marks and notches look about a tooth advanced (?) after turning it over but I haven't set the tensioner since fitting which may back it up again slightly . When I fitted them, the IN was perfectly horizontal and the Z7EX mark was pointing to a pin flush with the head. The IN line pointed between the 45 and 46 pin.

I did the valve leak test before and after reseating. Was a tiny bit before but none now.

I know I fitted the inlet first, that was just quick to get the compression tested. I can't see anything so fundamentally wrong that would cause total compression failure. If I've done something really, really stupid please tell me, I'd actually be happy to hear it!

Best wishes and thanks.

If you click "insert all" when posting photos they will show up in high resolution instead of tiny thumbnails.

Regarding your cams, they are advanced a tooth as you mentioned but also as you mention that won't cause no compression.

Just for shits and giggles you might want to double check through the spark plug hole to verify you are at 1-4 TDC by looking at the piston top while the cams are being positioned. Also double check to be sure the cam sprockets are properly positioned on the cams and you have the right cam on the right side (intake and exhaust.)

Ta for another reply. I’ll bear the thumbnail thing in mind. Some forums prefer clickable attachments, some inserts.

Just noticed the 750E note on your profile, so I had a quick peak. I’ve got one in my shed waiting for resto (I actually bought the GPz as a runaround). I slid off mine in traffic at the Blackwall tunnel in London is n a thunderstorm around 1997, put it to one side while I round my Bandit 12 and others around. Got a Boyer kit for it as the timing side had sheared off,, started to fit it but had to work abroad, so I carefully labelled all the wires with masking tape. Came back to it a couple of years ago - all the tape fell off! Got about 12k on the clock, Harris exhaust (slightly rusting, but sound) and some other useful exchange parts. Just need to sit down with US spec wiring diagram and work out what the hell each wire is!

Anyway, I digress....

Good suggestions - keep ‘em coming, all welcome, nothing too minor or obvious ignored.

Yep, the piston’s at tdc looking down the bores. The valves look like they operate fully when the engine’s being cranked and appear to reseat, or at least spring back, timely.

Regarding cam identification - I’ve always worked on the thought that the exhaust cam has a blank journal just off centre where Zeds used to have a mechanical tachometer (mental note - will double check that’s what I’ve done).

I know that you can’t install the cam sprocket upside down as the holes aren’t 180 degrees apart. I have a vague memory of someone I know getting the cams swapped and getting piston/valve contact so I’m wary of trying them the other way round.

It just yells out “cam timing”, but I can’t see where. In my time I’ve rebuilt zen thous, elevens, 650s, gpz11s, gpx7s and all sorts of air cooled stuff from Bonnies to Superdreams (and some of these really were worn out dogs, but they still ran) and I’ve never had this problem.

Ta to all again.

The exhaust cam timing mark will be above the gasket surface due to cam chain elongation with mileage so yours are one or maybe too teeth out of phase. Something seems more drastically wrong here though, almost as if like the cams are 180 degrees out but I'm not sure how to explain that.

You should be able to determine if your air leak is the valves or rings by setting each cylinder on TDC of the compression stroke and blowing some compressed air through the spark plug hole into the combustion chamber. It is easier of course to hear an air leak past the valves with the carbs and exhaust removed. I grafted an air chuck and a regulator to a spark plug with the porcelain broken out for this purpose.

Just to see if there's any pressure, have you try compression test without the cam? Just don't forget to take all the plugs out.

Good ideas, which I’m very grateful for.

I’ve been trying to locate a portable compressor to do a blowdown test, but no success so far. Seems like the best first option.

I hadn’t thought of turning it over with no cam(s). That’s a really good idea, though I’ll have to work out how to keep the tension on the cam chain to stop it catching in the bottom sprocket and locking the crank. Wish I hadn’t put the sump back on now - could have suspended it clear, or even removed and changed it altogether. 20/20 hindsight.

Will keep you posted. Catch you on the flip side....

your timing is a little out, the exhaust needs to go back 1 tooth & so does the inlet, however you should still have comp unless you've bent the valves (inlet maybe)?
because these motors rely on that block of rubber in the cam box keeping the chain under tension it can be a ball ache when turning the motor over !! hope you get sorted mate