honda 300 keeps shutting off

i baught 1987 honda 300 4×4 and its runs good only thing is second you put into gear it stalls right away buddy that i got it from seems to think its a spacer or something on the left side of the engine said his buddy had the same thing and changed that piece i have no clue what he was talking about any body got any info on it or what esle could be the problem