No 2 cylinder down on compression

[Max4music]

I thought I try the collective expertise on here in case there is something obvious i have missed. During an end of season race my bike suddenly started running on 3. I nursed it home to the end but thought it was probably a carb issue. Over the winter Ive checked and swapped carbs and discovered that the No 2 cylinder was running much cooler than the others. I did a compression check and its well down at about 90psi. I found the ex valves were leaking fractionally so Ive lapped them in. No change Swapped the head gasket, no change, replaced the rings on said cylinder, no change. I cant see any probs with barrels or piston. I swapped CDI, Coils, plugs etc.

To me it must be the low compression that is causing that cylinder to not fire, but cant think what else other than a problem with the head (hairline crack perhaps?) It went suddenly, so something must have rapidly happened. It was a wet race and if anything the bike was probably running a little cool rather than too hot, but it was well into the race so all was up to temp. Could I stone have got in via the RAm air and caused a prob?

Thoughts appreciated. I have the engine in and out and apart 3 times. I'm ready to douse the f'ing thing in petrol and send it up in smoke..... Perhaps I remove the Maxton shock though first.

[Mori Man]

Blimey that is a head fook !

Without reading your post I thought the shims were needing done - but I guess you have done them three times now.

My only experience with this sort of issue was a bit of debris lodged in the valve seat stopping it sealing proper - I chased that gremlin for months and like you was near on torching the fooker :smt013

Check all valaves are seating right, flush the port out with them removed with a parts cleaner , check the diagonal run off on the head with a straight edge, how do the other three cylinder read ?

Good luck & don't give up just yest

[wes]

I guess you did flood the head with fuel to ensure no leaks when the head was off?