I’m about to pull my 340 out after about 25 hours for a head swap that @yellowrose did the porting job on along with a cam. We were having some timing issues, at 3k rpm all in at 36* we were getting about 8* retard bouncing a around in the light. We tested the MSD box and dizzy and that came back with no issues. So I bet that the Cloyes double true roller timing chain is stretched and causing timing issues...