Low Compression

[Miles]

Ran a compression test on the bike and found it to be out of range on two cylinders and just within on the other two.

So I thought that maybe some mud from the river had stuck the piston rings onto the piston's or maybe the head gasket was gone (No coolant in the oil though). Anyway, stripped her down and to my surprise no mud on the rings, head gasket looked fine. So cleaned up the piston's, cleaned the rings, new head and base gasket's and re-built.

So ran another compression test last night and unfortuneatly the compression was still out.

Anyone any thoughts? Going to try and get ahold of another compression tester in-case mine is bust. Bought a new sealey one though

[jackdim]

Worn rings, incorrectly installed tester or test carried out in the wrong manner, blown gasket.

Most likely worn rings though, where did you buy your tester from?

[RedexRobB]

valve clearances correct?

[Miles]

Brand new head gasket and base gasket we're installed

It's a gunson compression tester fom halfords

Test was carried out as stated in the manual

All the valves and shims we're checked and done before I binned it.

Rings looked ok to me but not sure what it is I'm looking for so I just cleaned up the rings and installed them as said in the manual

[RedexRobB]

must be the rings then, either that or your bores have excessive wear. im sure theres measurements to take for the rings in the manual.

[Miles]

Right guys still stuck with this problem of low compression. Here's the story so far:

1. Fitted new head and base gasket
2. All the shims we're done and checked before strip down
3. I had a engineer company check the bore's and all we're within the 57.000-57.012mm
4. Left paraffin on the valves over-night to check the seats and the paraffin was still there in the morning

I guess it's going to either be the rings or the head is warped?

[RedexRobB]

Looks liek youll be getting it apart then, no other way of knowing. Head will need to be skimmed and rings checked / replaced.

[Miles]

New rings and had head skimmed.

Still low compression.

Have now given up and handed it to my mechanic, see what he finds.

[RedexRobB]

Are the spark plug threads ok? A long shot, but maybe someone has cross threaded the plugs but theres still enough there for them to bite and not get spat out. Did you measure the cylinder diameters? Maybe too large and beyond the service limit?

[Miles]

Threads looked ok but he'll check them again.

As you turn the crank by hand without the plugs in, once you get 180 you get a tight spot in the turn and then it goes. Is this the piston pushing up against the valves?

just a thought

[cargo]

Prob not

As you turn the crank one cylinder will be on the squeeze stroke or possibly the bang stroke without the bang..................

For valves to be hitting pistons you would have had to make a reall mess of setting the camchafts

[zimm]

sounds like its cooked rings..

however,

regarding the tight spot when hand cranking... it could be that the cam timing is off.. they will run 1 tooth out (yep, been there.. ) but like this they have poor compression... so it could be ... its def worth double checking.... may be a very worn camchain retarding the cams... check the marks line up exactly, if the cam marks are retarded when comapred to the crank mark, then the chain is shot.

[Miles]

FIXED IT!!!!

Turns out that both my compression tester and my mechcanics was only catching on the top threads of the spark plug hole. Therefore leaving a gap!! Believe this too be caused by the slight taper in the spark-plug barrel. So after some machining work we made an adator that fitted.

So my bike recived a full set of new rings, a new head gasket, new base gasket, a head skim and several hours of love and care. It now has a compression range of 110-130psi.

I still however have my sticky rev problem. Have balanced and cleaned my carb's (thought butterflies we're sticking). I also have replaced all the diaphram slide springs. Oiled my throttle cable, it now snap's back as soon as I let go. I have also liquid gasket between the inlet rubbers and the manifold. For good measure I put grease on the bottom of the carb's to try and add a seal.

It only sticks at 2,000rpm no-where else.

[extonyg]

hanging revs can also be air leaks from the airbox to the carbs (the airbox on mine was warped so used foam tape to fill the gaps) also hanging revs can be cured be messing with the mixture screws.

[RedexRobB]

I was almost right, wasnt i?

Are the spark plug threads ok? A long shot, but maybe someone has cross threaded the plugs but theres still enough there for them to bite and not get spat out.