I think my engine is getting weak due to hard starting and was wondering if any of the NA 2nd gen engines would work work with my 91. Thanks for the help.

 

Stick with a Series 5 engine, a Series 4 engine would be a big step backwards.

 

What is a series 5 engine??????

 

series 5 = 89 - 91

series4 = 86 - 88

series 5 NA engines have

lighter rotors
higher compression
higher redline
and a completely different intake system

definitely not worth it to go back to a series 4 NA engine..

peace