have an open animal that every time I rev it up it back fires and pops back through the header, is that normal? it did not do that before it was rebuilt. it revs up good and fast but just wondering about the back fire.
If it has an exhaust leak it can suck air into the pipe and cause the very rich mixture to re-ignite and cause a backfire.
Check your header gasket, exhaust pipe flange and bolts for tightness.
What is the timing set at and how did you set it?
Is there a key in the flywheel?
Is it backfiring when you are up in rpm then let off the throttle? That is very normal. (Over rich for a moment when the air (butterfly) is closed down but piston velocity/vacuum is still ihgh enough to continue drawing fuel into the cylinder.)
If it's backfiring at full throttle, it could be extremely lean. Also check your valve lash -- maybe you've got something awry in the valve train...the EX valve is hanging open or is bent. More often it's an ignition timing issue, or as Jim suggested - an exhaust leak.