2005 800sxr - compression issue

[Superjetron]

We recently test the compression on a sxr and we found the rear was reading 144 and the front 160, on removing the head (JSU with 28cc domes)
we noticed the rear piston was burnt on the top edges but the front piston was fine. Removed the barrell and that was not marked and the rings were ok. We then decided to check the domes and found the rear one was pitted round the edges.

Any clues to the problem, alot of people are saying detonation but why would this only do one piston ?

running super unleaded fuel and 2 stroke oil, has factory dry pipe and riva carbs.

Could the carbs be lean (would this not mark the bores ?

Any help would be appreciated, pictures are to follow.

Thanks

 

[w-nut]

detnation shouldnt mark your cylinder , from the decription it does sound like you were getting some detnation on your back cylinder . are both carbs turned the same (jet size, and screw settings)

 

[john zigler]

a picture would help.

it sounds like detonation. it was probably lean on the back carb for some reason.

zig

 

[Superjetron]

Please find attached pics

Thanks for all your help so far much appreciated.

Alot of people are saying detonation and it should be running at least 15% race fuel. Currently only using Super Unleaded (in UK about 98 Octane)

 

[ANT]

leaned out your back cylinder. what was the compression when it was stock? 160 is about what stock is so you may have had low compression to begin with then put on the head which bumped up the compression on an old top-end. Definetly do a top end if you aren't already, make sure your piston to wall clearances are ok or if it needs to be bored. Then do a compression check on the new top end and I think you'll find your compression shoot way up in the 200 range, because even 160 sounds low for a 28cc dome. You will probably have to get bigger domes.

 

[Big Kahuna]

definately detonation............ Go with what Ant said........ something up.