Just got motor back from its second rebuild in 200 miles. First time at 100 miles rod bolt came lose.
Just got the motor back again and no bolts came loose yet. But,

under a load the motor runs fine. When its not under a load it makes a puffing sound and feels like it missing.
Backfires once or twice when I let off the gas . Then I give it some gas and it goes away.

New plug wires, plugs,carb,cam,lifters,springs.
I will check the timing but I don't think that is it.
. It was under warrenty so go from there.
Not a mech but wonder if the preload on the valves could be the problem? The valves were redone on the first rebuild. Only about 300 miles on valvejob. ?????