[A Cobra guy]

Thanks for the ideas.
It seems to have built up gradualy over the last few weeks.
I use this as a street car so I'm pretty tame with the gas pedal. ( I doubt I've abused the engine it that regard) It had a miss fire before this started to happen, A loose plug wire I believe.

Madmax: The bad plug could be a possibility. I replaced the plugs about a month ago so a bad one is possible. I'll put the old ones in and see if things change.

Olddog: a cap and rotor are cheap. I'll try that one first. Regarding a lifter and backfire, If the exhaust valve is not fully closed during the intake stroke I'd get raw fuel into the exhaust. That would "pop off" (backfire)at some point wouldn't it?

FWB: It is a single coil system off a 91 mustang. Wouldn't the backfire move around or at least happen on both sides of the motor? It is definatly only on the left side.

Gaz64: I may not have done the compression test correctly. I pulled 1 plug at a time and cranked it with the coil wire off. Each cyl would "pump up" to about 110psi. I only did this for the left side, assuming the right is OK.
A leaking injector is pretty high on my list too. About 10,000 miles ago I put on a new BBK intake and while I was at it I put in the newer 4 jet injectors (still the stock 19lbs) They were a set of re-conditioned ones that I got on-line. The nozels are very small so I run a can of injector cleaner through once or twice a season.

NEW SYMTEM: This morning the car started fine. After 30 seconds it began making a loud taping which cleared up after another 15 -20 seconds. I drove enough beaters in my youth to know what a rod bearing or wrist pin sound like. This was not evan close to that much noise, but loud enough to hear from the drivers seat with the hood down.