I put a new head on my 8v, torqued it all up, set the timing marks. Checked all the hoses/lines and even turned the intermediate shaft over to prime the head.

Went to start it, started fine, ran for about 2-3 seconds, then a crack and knocking sounds. Killed the motor. Checked timing marks again- all good.

Started it again, same thing, 3 seconds then knock knock. Killed it.

What could this be and do I need to take the head off again to see or what?

It was a brand new reco head.

Any help/ideas appreciated.