[84kawgpz750]

I have a 1984 Gpz750 non turbo. Put a 810cc big bore kit in it. There is less than 100 miles on rebuild. Once the bike warmed up it would smoke excessively. Other than that in ran excellent. The other day it didn't want to run unless the choke was partially on, so I decided to do a compression check. All four cylinders are less than 70 psi. I'm not sure how it is even running.
1: I did not replace or do anything with the valves other than check and make sure the clearances were within specified limits. The reason I didn't is because it was running perfect before the rebuild. The reason I rebuilt it is because two of the cylinders were low on compression. One at 90 and one at 120.
2: I put Castrol GTX 10W-40 Synthetic blend in it. This might have been part of my smoking problem.
3: It has the stock timing chain adjuster. I did replace the internal parts of the adjuster.
4: The compression was good in all 4 cylinders after it was rebuilt. Around 150psi.
What could have gone wrong? is it possible the timing chain tension all of a sudden changed and is causing the valves to stick partially open? Or I just had some kind of catastrophic failure in all 4 cylinders at once? The part that boggles my mind is how it is running with virtually no compression.
Please Help! Thank You!

 

[ninjanut]

Since the only part that was worked on was the bores, I would say that is the issue. If you did an over bore, pistons and rings, that is where I would start.
Were the bores done correctly? Ring end gaps set correctly? Rings installed properly?
Your timing cannot get that far out without collisions occurring internally.

 

[Streetfighterkz]

It’s possible that using the synthetic blend oil caused the rings to not seat properly causing the loss of compression? I’m with Ninjanut though; it’s something with the pistons, rings, and/or bores.

Later, Doug

 

[ninjanut]

To me, it sounds like something was missed in the rebuild.
If the boring was done improperly, nothing will seal up very well. If the rings are incorrectly installed and/or gapped it will not seal up.
Not sure about the semi synthetic oil causing the failure but it is possible. I always use mineral oil to break a motor in for the first 1000km.

 

[92Ninjas]

To me, it sounds like something was missed in the rebuild.
If the boring was done improperly, nothing will seal up very well. If the rings are incorrectly installed and/or gapped it will not seal up.
Not sure about the semi synthetic oil causing the failure but it is possible. I always use mineral oil to break a motor in for the first 1000km.

mineral oil as in conventional oil?

 

[92Ninjas]

To me, it sounds like something was missed in the rebuild.
If the boring was done improperly, nothing will seal up very well. If the rings are incorrectly installed and/or gapped it will not seal up.
Not sure about the semi synthetic oil causing the failure but it is possible. I always use mineral oil to break a motor in for the first 1000km.

Thank you for the confirmation

 

[zxr400L]

compression test w/throttle's open? synth blend should be OK as long as the manuf. claimed it worked for m/cycle wet clutches(Maxima?) if comp. check still shows 70psi w/throttles open you maybe able to deglaze the cylinders & try again.be sure the rings are installed correct side up & end gaps are to spec.there is a break in oil available.after 30 seconds change to reg. oil.

 

[Chevy Fan Attic]

Was that a turbo bike and the turbo was removed? Reason I ask is that because that 150 psi is on the low side, Should be more like 170 ish. Your looking at pulling the motor apart no matter what but, Sounds like the oil rings were installed upside down. Are you sure your compression tester is working right or is it all fudged up with oil? There was a guy in here with some low numbers and found out his gauge was not reading correctly. You could rent one from a auto parts store for free.
To make you feel more sure about pulling the motor apart I would squirt a liberal amount of 90 wt gear oil in the cylinders and do compression test again. Also take the air valve out of the hose that threads into the spark plug hole and instead of hooking it to the gauge hook it to a compressor and listen where the air is escaping from. Basically a leak down test.