What would be the pro's and con's of having a longer intake between the carb and crankcase on a offroad engine ???

In theory a little more low rpm torque at the cost of a little high rpm.

Research intake harmonics for more detailed info.

Superflow has some really good info on intake harmonics.

BK

All about 2-stroke tuning:
http://edj.net/2stroke/jennings/

A venturi in the intake will give more performance, if it will be wider and longer I suspect it will be better. If it is only longer I think the performance as the reaction will slow down.

But read the book, it is very interesting stuff.