Very low compression; burnt valve/valve seat recut needed?

[droog]

Hello folks,

I just did a long overdue valve clearance check on a 2001 cbr600 after the bike started stalling – all valves tight and compression 50% out of spec on cylinder #2*

To cut a long story short I would be interested to know if anyone has had a bike with 50% compression on one cylinder due to tight EX/IN valves and who has successfully got the compression back to factory spec via a simple re-shim – or is it the case that 45/50% compression is unlikely to be fixed with a shim change and I am looking at valve replacement/seat re-cutting to get compression back to factory specs?

*I got slightly higher than spec compression figures for cylinders #1/3/4 even though all the valves are tight and out of spec (possibly due to carbon build up) but compression on cylinder #2 was 50% lower than spec at 87 psi/6 bar (factory spec is 178 psi/ 12.5 bar).

Thanks in advance for any advice.

[Riejufixing]

You'll have to have a look at it. There's a vague possibility of some problem other than reduced valve clearance. I suppose you could try putting in a thinner shim and see what happens, it's less trouble than getting the lid off. Whether you need to re-cut the seat or not or replace the valve (most likely exhaust valve) "depends".

[davebike]

I seen similar a few times on customers bikes
Most times open up clearance to upper limit and giving it a good run sorts
If not it megga bill time either used engine (in my view best option ) or head off 16 valve out clean and re seat ! each valve can take 30 minutes ! It may be an engine in frame job but possible engine out anyway !
____________________
Dave
NC750Xdct + others at work !
davebike1@gmail.com

[droog]

Thanks for the replies Riejufixing, davebike.

Yeah - I'll try re-shimming to the upper limit and see if I can get compression back to spec.

If not I will drop the engine (if necessary), remove the head and take the head to an engineering shop for a recut/valve replacement.