G80 compression.

crawsue · Post by **crawsue** » Wed Oct 31, 2007 11:41 am

After a first summer season on my G80 it is now time to "fettle"! On removing the rocker cover to investigate a minor oil weep I discovered that one of the exhaust valve springs was broken.While waiting for the replacement parts I have been lent a "compression test guage" which is easier to apply to the sparking plug hole now that the tank is off.The guage is calibrated in p.s.i. and also "bar".Can someone tell me what would be an acceptable reading? Another issue I have had this summer has been a "grabby" front brake.An engineering pal offered to get the drum "skimmed" but suggested that I had it measured beforehand.The measurement showed only 2/3 thou of difference in circumferance.The inside of the drum is a bit corroded but will "emery up" OK,and it could do with re-shoeing.I dont want to disassemble(?) the wheel if this will do the trick----any thoughts

itma · Post by **itma** » Wed Oct 31, 2007 11:52 am

file a chamfer on the leading edges of the linings and see if that helps the grab. clean out all the grot as well.
an old trick that sometimes helped was to put a shallow hacksaw cut diagonally across the linings just above this chamfer, theory being it collects all the grot.
No need to take the wheel apart to skim the drum, there are people who do this service for the complete wheel. Never really had any problem with AMC hubs gong out of round myself.
As for the compression check, I have no figures, if you can stand on the kickstart on compression stroke its OK by me; my attitude to gadjets like this is, 'seek and ye shall find. then ye shall worry and spend ye shekels'Edited by - itma on 31 Oct 2007 11:53:15 AM

crawsue · Post by **crawsue** » Wed Oct 31, 2007 1:17 pm

Thanks for quick response,the brake linings are on their last legs so will need replaced anyway,so will clean up drum and see how it works.I have suspected poor compression for a while,but the only comparison I have is with 2 competition engined machines belonging to a friend--so not much use.I can push it through compression without resorting to the lifter ,with a bit of effort,but "no way" would it support my full weight.The engine starts OK,does not appear to be burning oil,but just doesn't have the "oomph" I had expected from a 500.I thought that using the compression tester would be a simple way of guageing if this was in fact an issue.It could ,of course improve when the exhaust valve spring is replaced.

paul knapp · Post by **paul knapp** » Wed Oct 31, 2007 1:39 pm

Remove and grease the cam (expander) spindle and smear some dry lube on the cam faces. Remove the round adjuster 'buttons' from the shoe ends and shim out so to remove excess rotation of the cam when operated. Make sure the cable is oiled and operates smoothly with no tight bends.
Check that the engine tappet clearance is correct.

Pablo
in warm sunny OZ.

Circlip · Post by **Circlip** » Wed Oct 31, 2007 2:24 pm

If the shoes are worn then this will be your problem.

You will no doubt notice that the shoe material is not in the centre of the metal shoe . This is to encourage an anomaly called 'self servo action'.

This is basically encouraging bite , the more the show wears the worse this can get.

Along with many other reasons this is why we went to discs.

crawsue · Post by **crawsue** » Wed Oct 31, 2007 2:32 pm

Hi,Paul,brake was pretty well set up with everything working well,cams free and shimmed and fitted with new cable ,so operation was OK,but pretty violent juddering suggested out of true drum -- but not so! I will shine up drum and renew liners and reset it and see what the result is,fortunately rear brake is up to the job.Compression testing is on going with rocker box off so valve clearance not an issue----still very keen to find out what an acceptable reading should be,any ideas or clues as to where this info is available gratefully received.I'm sure our trans-atlantic cousins are very likely to have this info,they seem to have a more "measured" approach (no pun intended) to this kind of thing,whereas one local "guru" here suggested "sticking your thumb over spark plug hole and kicking it over" Gawd knows what that tells you!CR

crawsue · Post by **crawsue** » Wed Oct 31, 2007 2:35 pm

Thanks ,Graham,that sounds a very reasonable explanation as the shoes are almost "done".Will report back when re-assembled.Still awaiting info on pressure test!

itma · Post by **itma** » Wed Oct 31, 2007 4:01 pm

Have you actually used the gauge and got a reading?
what is it?
I think Don has some guidance on these figures you`ll have to wait for him to wake up.
As a rule of thumb, [pun intended,] there is not a lot wrong with the thumb on plug hole assessment. if you can keep your thumb on, you need a rebore!!!
Is your performance down?would you have worried about cylinder pressure if you did not have the gauge?

Its also important to centalise the brake using the little counter sunk allen screw you will find on the brakeplate.
Edited by - itma on 31 Oct 2007 4:03:02 PM

Charles · Post by **Charles** » Wed Oct 31, 2007 4:39 pm

ITMA, would you expand on the centralising please as that element is new to me? I have used the clamp drive side, pump forks with the brake on and the offside slider clamp just finger tight to get the forks/spindle set right but this aspect is new to me. I too had a very juddery brake and found that allen screw to be loose once upon a time....... Are you saying the screw affects this as I believed it just held on the cotton reel type fixed brake pivot on t'other side of the brake plate. Is the cottton reel centre offset from the screw then?

regards
Charles

bjork · Post by **bjork** » Wed Oct 31, 2007 4:57 pm

Same as already said, in the case of these engines, a figure is not really that important. I wouldn't expect a touring single to hold a riders weight on the kickstart for very long. As you say, the valve spring may help a bit! It's surprising the difference replacing soft old springs can make!
If you really must measure things and get scientific sounding results, multiply the compression ratio by 1 atmos to get a guide figure (no-this is not particularly scientific either!) Warm the engine to operating temp. Then take your measurment with throttle wide open and keep spinning the engine till the needle stops rising. Fuel off of course and no stray sparks. If you get anything from 75psi upwards, then be happy. You will of course need to be sure what compression ratio your engine has. There are ways of checking this too.........but seriously folks, if you want to check the health of your cylinders a cylinder leak tester will tell you more.