77 gs750 top end rebuild questions.....

[my77gs750]

just got a 77 gs750,had a hard time starting,did a compression test.when on the bike the left cyl only had 30lbs and was dropping slowly,the 2 middle ones had 90lbs and the right on had 80 lbs.I toor the motor apart and the cyl walls are in great shape and the rings all looked good to me,the valve's were seated properly.what would have made the compression low in that cylinder?

[hampshirehog]

Did you do the test on a hot engine with the throttle wide open?

[my77gs750]

the engine was cold and never touched the throttle

[tkent02]

the engine was cold and never touched the throttle

Throttle closed will make them read very low, that was your problem. Yesterday I tried this on my GS1000 just to see, compression went from about 120 to about 60 by not opening the throttle.

Depending on throttle synchronization some cylinders could be a lot lower than others with the throttle closed.

Did you check the valve clearances first? This could explain the one cylinder being so low.

The engine being cold can effect the readings somewhat too, and if it's sat a while.

[my77gs750]

I didnt check the valves,whats the correct setting on them?how do I shim them up if need be?
how would the throttle beeing open effect compression? although before I did the test I tried to get it running for about 20 min with no luck.there might have been gas in there,would that have effected the compression?

[tkent02]

I didnt check the valves,whats the correct setting on them?how do I shim them up if need be?
how would the throttle beeing open effect compression? although before I did the test I tried to get it running for about 20 min with no luck.there might have been gas in there,would that have effected the compression?

I would check them now with the head off, should be .03 - .08 mm on all of them. Too tight makes for hard starting.

No air can get in to get compressed with the throttle closed, so no compression.

Gas could have removed a little bit of oil from the cylinders, lowering compression readings even more. Just a guess.

[tkent02]

You must be bummed right now, after having taken apart a perfectly good engine.
Might I suggest a set of 850 pistons and cylinders to go back on?
They just bolt right on, no extra effort required at all.
About 15% more displacement, very cheap to do.
Got mine for $2 on Ebay.

[my77gs750]

how do I check the valve clearence with the head off?
what years 850 pistons will fit?whats the difference between them and the 750's?.also the motor was never touched and it has almost 33000 miles on it and had very little carbon on the tops of the pistons

[tkent02]

Bolt the cams back on, turn them by the sprocket.

The 850 has the same crank stroke and larger bore, ups the displacement from 748cc to 844cc.
If you use a cylinder from a 1979 850 it slips right into the 750 cases. Cylinders from later 850s apparently need to have some grinding done to the cases, the part of the sleeve that sticks out is a larger diameter and just barely won't fit.