Compression values

[Freebird]

Hi

I am planning to remove my engine soon for a re-paint and thought I would do a compression test while it's still in the frame. It vibrates more than I think it should between 3500 and 4000 and then again from 6000 upwards so one thing I was suspecting may be contributing to this it was poor compression on 1 or more pots.

The bike goes fine I think, though I have no point of reference having never ridden one before. It's certainly not as responsive as my 955i Speed Triple, but I wouldn't expect it to be.

The compression readings I got were reasonably close together, but look on the low side to me at 120 to 130 PSI. This is with a hot engine and wide open throttle. I know that compression testers are not particularly accurate, but this compares to 150 to 160 using the same gauge on my XJ6 Jag and Triumph Vitesse.

Any advise would be most welcome.

Cheers

Glen

[cbxchris]

The compressions on mine were all 125 psi, with a hot engine and throttle wide open, i am now in the process of doing an engine refresh ie new piston rings , bore hone, and head dismanlte , but waiting for valve spring compressor so to take valves out , and de coke , so watch this space !

[cbxchris]

Finished engine re build , de coke , valve and seat cut , head skim , new piston rings and hone, re shim,its a different bike to ride now, lots more low down grunt and drive, but have not opened up fully because of running in ! ( Oh, new timing chains and main primary below as well ). Conclusion ,the above has increased power, but also found the cam timing was one tooth adrift on strip down, so that obviously has improved things as well , but power is well up and i am a happy bunny, the only other nagging problem is , it is still suffering from a hanging throttle, been through the carbs completely but have not solved it , but hey ho, it still is a smashing bike !

[NobleHops]

Result!

Well done Chris!

If you had nailed the carb rebuild on your first attempt I believe you would have been the first in the galaxy.

N.

[steve murdoch icoa #5322]

Congrats on the rebuild, Chris.
What is a hanging throttle? Revs are slow to return to idle?
If so it could be improperly routed throttle cables, a sketchy sleeve at the throttle or possibly an air leak.
Check around the carb connections.

[cbxchris]

Congrats on the rebuild, Chris.
What is a hanging throttle? Revs are slow to return to idle?
If so it could be improperly routed throttle cables, a sketchy sleeve at the throttle or possibly an air leak.
Check around the carb connections.

Yes Steve , a hanging throttle means the revs are slow to return to idle, very slow in my bikes case , i have to bring them down with the clutch , anyway thanks for your input !

[daves79x]

Congrats on the successful rebuild. Your carb problem is very likely a sync issue, likely one or more throttle plates are slightly cocked on the throat, not allowing the whole bank to return to idle. Or they are just plain way off on the normal sync. A still-plugged idle circuit can cause this, by making the normal idle position too high.

Dave

[Mouse]

Result!

Well done Chris!

If you had nailed the carb rebuild on your first attempt I believe you would have been the first in the galaxy.

N.

[Freebird]

A bit of an update from the original poster.

I now have my engine in bits and found the reason for the poor compression. I clamped the head vertically in a vice and filled the exhaust ports with petrol and all but 2 of the exhaust valves leaked.

I then repeated the test on the inlet and 2 of the inlet valves leaked too. All had the correct clearances by the way. The bores are good, but it needs new rings as the gaps are over spec.

So now I have stripped the head down and will be lapping the valves in over Christmas once it's been vapour blasted.

Glen.

[Mouse]

Ah, I had thought your original post got a bit hijacked there Glen.
Thanks for the update.
And;
Merry Christmas!