Compression?

[Eddie]

Does anybody know what the compression should be on a 74 gt750? I have been having problems with carbs and now I'm thinking it's either compression or timing, I set the timing with a dial gauge so I don't think it's that, but my plugs are all all looking different when I check them one is lean and one is rich, and I have checked everything in the carbs, anyways my compression is 115. 110 and 100 I had read a guy had like 140 on his and it was original so just wondering if I have low compression..

[GT750Battleship]

Hi,Don't forget the 2 outer cylinders plug holes are angled so you can get the wrong top dead centre readings wrong with a dial gauge, because of this..best method is to remove cylinder head to see exactly where the pistons are..I did read on "here" that a figure of 120psi is good,but don't quote me...!
Cheers,
GT750Battleship.

[Craig380]

Those compression numbers are on the low side, ideally you should have 120+ psi with no more than 10psi difference between the cylinders.

However, did you do the compression test with the engine at normal running temperature, with the throttle held fully open? You also need to kick the engine over hard 7 or 8 times in quick succession, until the gauge's needle reaches its highest position (a 2 or 3 second spin on the electric start would also do the trick - although I'd remove all 3 spark plugs just to be safe!).

[Eddie]

I checked the compression like the book said, I had all three plugs out and throttle wide open, the book I have said to check while motor was cold so that's what I did, I guess my timing could also be slightly of because of the outer cylinders,I will check that out, I'm thinking of just rebuilding the motor but I hate to if it doesn't need it, but the far right cylinder looked to be scared , i had the exhaust header off and it looked like it had seized at some point in the past, I keep playing with the carbs and now I think it's either timing or low compression, or both, or a bad crank seal,the far left cylinder always smokes more than the others and the exhaust baffle is always wet, and only on that cylinder.