[Healeys] Not Healey related: exhaust popping noise

[Allyn Richardson]

NOT HALEY RELATED but: I have a 1988 Jeep Grandwagoneer I am restoring. I
recently changed out the old muffler for a new one, (allegedly OEM). when
the engine runs there is a distinct pop, pop, pinging noise coming from the
exhaust system. Not a back fire thru the carb. But seems to be in the
exhaust system. No restriction in the catalytic converter and I checked the
valves, none are sticking. This happens when idling and it's a dog when
driving. I'm stumped. To small of a muffler causing back pressure? Any
suggestions and enlightenment would be greatly appreciated. 
 
Allyn