NO COMPRESSION

After dissasembling the top end for a full cylinder and covers gasket change, I´ve assembled things back but my bike -´84 GPz550, running good and strong before this- never started again.

After blaming it on carburetion issues I finally took her to the mechanic shop, where the guy told me -after doing some testing- that the engine had almost no compression. I´ve never touched the rings except for aligning them at the proper angles before sliding he pistons back into the bores, and cam timing was assebled as indicated in my Clymer manual, engine rotated manually before attempting to start it, just to be sure that there was no binding.

Can an engine lack all of its compression when assembling things back without honing and re-ringing? The mechanic told me that probably there were two causes:

1.- Timing is too far off, or

2.- Because weared cylinders tend to get off rounded to an oval shape, rings weren´t sealing well thus lowering compression to very, very low readings (20 to 40 psi)

What do you think guys? Is this man being honest or should I take the bike to another shop?

If I only took her to the shop (another one) before, the gasket change would have been around 100 bucks for the labor plus spares, now I´m facing a 200+ bucks labor charge and god knows what else...

Now that the top end is gonna be opened again, what do you think about re-boring and some new pistons and rings? That´ll be another 300 bucks at least, but, what the hell!

Please let me know what do you think, cheers.

If the shims are not the right size then the valves can be stuck open. Head gasket could also be blown out and causing the lack of compression. Did he sau what the compression was? Was he checking it with the trottle wide open as it should be? You can just clean up the piston tops and get new rings and a nice cylinder hone if all is within spec.

Post edited by: kawtoy, at: 2006/12/20 19:53

I would check valve clearances before spending money on anything in the engine. While you have the valve cover off, check cam chain timing. I suspect something along those lines and not the pistons/rings. Compression loss from wear is a very slow and gradual process and won't just happen overnight. Is there any possibility that the cam chain jumped a tooth or more on the spockets? This could be from being worn/stretched...

Post edited by: wiredgeorge, at: 2006/12/20 20:03

Kawtoy, the shims were put back in each corresponding buckets as they were before disassembling. If the bike was running well I don´t think that it´s a shim issue.

All gaskets are new, so a blown gasket is unlikely, I think. Compression testing was done with a gauge and throttle wide-open, and everything is gonna be measured again to see if we can just go with a hone and new rings or, in the worst case, rebore and put fresh pistons as well.

Thanks for your concern, cheers.

Hi Sergio, last time we discussed this Duck suggested you "eye up the cams" to confirm valve timing. Have you done this yet?

wiredgeorge wrote:

I would check valve clearances before spending money on anything in the engine. While you have the valve cover off, check cam chain timing. I suspect something along those lines and not the pistons/rings. Compression loss from wear is a very slow and gradual process and won't just happen overnight. Is there any possibility that the cam chain jumped a tooth or more on the spockets? This could be from being worn/stretched...<br><br>Post edited by: wiredgeorge, at: 2006/12/20 20:03

This guy (wiredgeorge)is almost always right , and I agree with this .

My 2 cents worth.
Check what his post says .
Then if you dont find the problem,
Leakdown test.
Just make sure you are there when it is done to hear where it is leaking from.

kawadruida wrote:

After dissasembling the top end for a full cylinder and covers gasket change, I´ve assembled things back but my bike -´84 GPz550, running good and strong before this- never started again.

After blaming it on carburetion issues I finally took her to the mechanic shop, where the guy told me -after doing some testing- that the engine had almost no compression. I´ve never touched the rings except for aligning them at the proper angles before sliding he pistons back into the bores, and cam timing was assebled as indicated in my Clymer manual, engine rotated manually before attempting to start it, just to be sure that there was no binding.

Can an engine lack all of its compression when assembling things back without honing and re-ringing? The mechanic told me that probably there were two causes:

One does not need to disassmeble the engine to diagnose low compression resulting from poor ring sealing.

1.- Timing is too far off, or

If rings are sealing and compression is low, then valves are probably leaking. Could be timing, could be ent, could be wrong clearance. Timing and clearance are easy to check.

2.- Because weared cylinders tend to get off rounded to an oval shape, rings weren´t sealing well thus lowering compression to very, very low readings (20 to 40 psi)

This is possible but do not assume. Do the compression test to see if it is the rings leaking.

What do you think guys? Is this man being honest or should I take the bike to another shop?

The mechanic should charge you to diagnose the problem. He should not perform work based suspicions or guesswork.

If I only took her to the shop (another one) before, the gasket change would have been around 100 bucks for the labor plus spares, now I´m facing a 200+ bucks labor charge and god knows what else...

Hell, if you get the right mechanic, he might let you pay for a vacation to Barbados.

Now that the top end is gonna be opened again, what do you think about re-boring and some new pistons and rings? That´ll be another 300 bucks at least, but, what the hell!

Why open the top end up BEFORE you know what is wrong. A mechanic who does this is not too bright. A mechanic who does normal diagnosis, fixes a minor problem and tells you he did this is a crook. The mechanic you want is the guy who DIAGNOSES the problem before starting to work. Now it's possible that he may find the timing to be off, fix that, and then find that some shims need replacing, and then find that yo uhave a bent valve. This guy is doing his job.

Please let me know what do you think, cheers.

Did your mechanic try putting a little oil in the cylinders to see if the compression jumps up? I highly doubt that the rings went bad all at once. I was just suggesting that you get new rings if the cylinders were coming off anyway. You can bet money that the problem is elsewhere in the top end, not the pistons. Bad rings would give you low compression but at least you would have 30 or 40 PSI, and that is woth really bad rings. Even a holed piston will give you a little compression, a few PSI or so. I would have your mechanic check it again while you watch and verify the results just to rule out a missed step or something. Put a little oil in the cylinders and if the compression climbs way up then the rings are likely bad. I going to bet that it won't make a difference. I would then take the cams out again. Make sure the chain is not bound up on the crank causing everything to be a tooth or so off. If it all looks good put the cams back in and triple check everything even if you are sure it is right.

sounds like the intake cam is retarded one notch.

Thanks guys for all your comments! I´m going tomorrow (saturday) to the shop to hang around and do some re-checking with the mechanic. I really hope that it´s a top-end issue, that´ll lower the bill considerably!

Cheers and Merry Xmas to you all.

Take your manual with you, and both of you check the cam timing first. I agree with what's stated above about timing.


What gaskets did you have replaced?

Post edited by: tellietubbie, at: 2006/12/22 12:24

Post edited by: tellietubbie, at: 2006/12/22 17:27