Yep! Everything is different. The 91 is a fuelie and the 90 is carbed. There IS a way to install the short bolck from the 90 into the 91 and use everything else from the 91.. But it is rather involved..
If all you need is an engine you will need a 91 up to directly swap.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />


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