Is your starter up to snuff? Good cables? How many amps is your battery?

We would replace the ground cable with a heavier one that ran down to the starter and bolted right to one of the mounting bolts for the starter, giving a better connection. The old cables can get corrosion inside and gain resistance.

Still, it is possible to have everything in shape, and the 6 volt can be a little less than desirable, especially on a new motor. My 88 cranks just like you tell, but it also has high compression M &amp; W pistons in it. But every once and again it will spin just fine, that one has me baffled.

I'd start by checking cable connections. New paint may need to be scraped off to get a good contact. It should get better once the engine has run some.