Skip Burney

I have built many many engines and have never had this problem. I built a 383 for my 73. I built it up with a Comp Cams 270 magnum cam with roller rockers, an Edelbrock Performer intake and 600 Holly and Pertronix ignition module in place of points. The car has a 4 speed transmission. The engine starts and idles just fine holds good oil pressure and dosent run hot. When I take it out and get down on it the engine will backfire the first few seconds and then seem like it gets it`s breath and takes off like it should. Going down the road I can feel the engine missing really slightly every now and then and it will softly backfire and it happens on both sides at random times. I have checked everything from timing to rocker arm settings, carb adjustments and everything and I can`t find it Does anyone have any ideas?

toobroketoretire

Any time an engine backfires it means the spark is occurring too early or an intake valve isn't seating well enough to seal combustion pressure or an exhaust valve isn't opening (such as when a push rod breaks). Its very possible you have an egg-shaped intake valve guide that is allowing the valve to seat sometimes and not seat sometimes and is the cause of the intermittent backfiring. Its also possible you have a broken intake valve spring that is barely enough to hold the valve against the seat. I suggest you take a compression test and suspect any cylinder that reads exceptionally low.

Skip Burney

That isn't it I had the heads redone with new guides, s/s seats and new comp springs, retainers, locks. Compression is 5% between cylinders. It`s almost like it`s to rich as the backfire is out of the exhaust if it were lean it would be back through the carb. I am stumped on this one because it will only backfire for the first 3 or 4 seconds when I get on it then it spins on up like it should. I had thought about going to a smaller center squirter but it has a soft random backfire cruising down the road. If it were a broken spring or bad guide it would backfire on the same side but it will do it at random times on different sides.