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Compression too high

  • Thread starter Thread starter Dod9878
  • Start date Start date
D

Dod9878

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Hi there please don't shoot me down as I'm a complete newbie to forums I've done a search in forums on subject but to no avail.

Any way I've got a 79 Gs 750 which hasn't turned a wheel in over seven years been in a shed all them years. Requires a lot of work but started off with a compression test.

Have done a compression test and number 1 is above what the manual says. It's over 200psi when 2,3 and 4 are all between 160Psi and 180psi which is ok I think. Carried out test the next day thinking it was the tester that was faulty but its nearly brand new and compression on number 1 very high. Can't think there wold be that much carbon build up and looks ok when looking down inlet and into spark plug hole.

Any pointers would be appreciated. But please don't shoot me down if I seem to be asking stupid questions.
 
Was it cold as it sounds like it isn't running... These numbers sound healthy (too healthy for a bike that hasn't run in a while).

I'd do the other work (including checking the valve clearances which can also affect the compression numbers) & then try again when it's been run in.

Unless it has a million miles on it or has been left outside or with the carbs/exhaust off the likelihood is it will be fine. These bikes are tough old things! :)
 
Hasn't turned in anger for many years. Just did a compression test. in the middle cleaning out carbs at moment having problems with float pins, not the easiest to strip. Cv mikunis.
 
I would suspect that the tester is reading about 90-100 psi high. See if you can hook it up to an air compressor or something else to "validate" it.
 
A 79 GS750 came with VM mechanical slide carbs, not CV's.
 
Bad tester.

Bad tester.

I'd buy the faulty tester theory or at least faulty testing as your psi numbers are so far apart. My faulty tester helped me buy two bikes cheap as it was reading too low before I figured out it was faulty. Now I take it on every trip to go look at a motorcycle. Anyways, if your tester was good you'd need an overhaul as you should never have that much variance between cylinders.

Get the thing running, run the smoke and cob webs out of it then check it again.
 
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Been away for the last two weeks but got round to removing rocker cover, measured the valve clearances some were tight some slack but the exhaust valve on number 4 was way out. On close inspection when turning engine over I noticed that a gap was appearing between head and cap A so went to remove bolts one was snapped and the other three had aluminium springs on them. Stripped threads Also on rocker cover there were marks where bolt heads had been hitting the rocker cover. So thinking seeing that the valve timing out that it may have something to do with it. Still have to test gauge for accuracy. Presume a sticking valve caused this damage or maybe previous installation.
 
Those cam cap bolts don't take much torque and people tend to not follow the instructions, so stripped bolts are common
 
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