barkgbjc

I have been out of skis for some time and now my boys are ready to ride. I bought a 91 SN SJ with the following set up. 6m6 cases, 62t cylinders, 64x pistons and head. It has dual 38's on it. It is showing 110psi even so I tore it down and can not find a big issue. Question is: before I rebuild the top end I need to know if the cases can work for this configuration., or should I get some new cases. ie 61x etc. Does this configuration require cylinder decking to get the compression up? any help would greatly appriciated

RossThaBoss11

you should definitly get bigger cases, 62t would be best but 61x work to. your not getting enough air with that motor set up you have now cause the 6m6 reads are to small. and the compression should definitly be higher

Group K

We have constructed many 6M6 cased motors with 701 (81mm) top ends that ran "very" strong. We have even done a few with 84mm pistons in a 61x cylinder (requires minor case boring). While the smaller 6M6 reed cages are not the ideal choice for a high rpm setup, the single carb 81mm and 84mm setups we built had a fantastic low speed characteristic that worked great for free-riding. If your duals carburete well, I'd keep them.
As far as compression goes, when you big bore the cylinders (6M6 or 61X) you get a big compression ratio increase that is generally right where you want to be. Additional cylinder decking is a judgement call ... but not required.

Hope that helps.

Harry Klemm

__________________
Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.

www.groupk.com

barkgbjc

Thanks for the help. So I should not be overly concerned about the 110 psi. It is what you would expect from this set up?