Could still be fuel. You could have a piece of dirt in one of the needle vlaves in the carb. Backfiring in the exhaust could be a sign of a too rich condition. Did you completely drain the fuel system over the winter or at least put a ton of fuel stabilizer in it? The fuel of today is junk!

Is it a consistant condition where it backfires every firing cycle?

What about your timing??

Checked timing- it is set @ 10 degrees advanced. Starts right up, runs good- just the backfire at idle. It is not consistant- or it doesn't seem to be. Cleaned the carb last night( thought it might be a vacuum leak, or plugged jet, no go.) Switched carb from other engine, ruled that out. I am going to buy a cap and rotor today, I will let you guys know what happens.

Thanks for all the input-

Scott

Is that the mark IV motor or the gen 6 motor If it the mark IV then you might need to re-adjust the valves.

Sounds like you have a valve problem. I had the same symptoms when I was breaking rocker studs and also when I had a lifter gain lash and didn't pump up. Pull spark plug wires one at a time until it stops popping to narrow it down to which cylinder bank is having the problem. You might just need to redo the rocker arm adjustment. I forget if you have hydraulic flat tappet or hydraulic roller. If not broke in properly with high moly content additives, I have seen flat tappets wipe out a cam lobe.
Jason

Well- putting on the cap and rotor helped quite a bit- but it still does it very occasionally at a low idle.(800rpm) I guess it could be a valve but I have hydraulic rollers- and it runs really good at everything above 800 rpm. If I set the idle at 1000 it never pops- kind of weird. I'm kind of leaning towards 502's thinking, because the gas is old- so I am going to mix in some race gas when I fill it up. One more question- should you use throttle return springs, and if you do, is there a trick setup out there I should look at? Thanks guys for all your input-

The Gen 5- its been so long I can't remember!!!

Sounds more like u have a flat spot on the cam ,,and the intake valve gets hung up @ idle and at higher RPM u can't feel it because it turns to fast !!!!

But if that was the case- wouldn't it happen all the time? I mean it pops once every 3 or 4 minutes-

Thats it...I thought you had a real problem

I have return springs on mine...Nothing fancy...just in case.