North Coast Cab

Finally got out to run my car. It is popping and backfiring a little on decel. It is a little more pronounced from higher RPM's.
It has new plugs, new ignitions wires, & a rebuilt dizzy. I checked all the wiring.

Ideas?

Thanks

geolab

In my opinion, if it is a very slight backfire, check the alternator and fan belts tensions as a first step

Crimson Nape Racing

do you have cats? That's normal behavior for a non-cat car. I love the pop-pop-pop as I am slowing down.

North Coast Cab

Belts are new and i will check.
I have no cat, Fabspeed bypass. Sounds reasonable, and yes I like the sound too.

Crimson Nape Racing

the backfiring is the unburnt fuel igniting when it hits the atmosphere. the cats block the fuel, so a car with cats won't backfire.

2Many Cars

Correct about the cause but a properly set up fuel injection system should NOT backfire. It was very common with carbs because when you lift off the throttle suddenly the intake butterflies snap shut creating a temporary rich condition. Early emission cars had gulp valves that added extra air to burn the extra gas so you wouldn't get a backfire. Fuel injection on a street car kills the gas almost instantly so there should be no backfire unless something is amiss (sorry for the pun). You'll hear race cars backfire because they tend to have much more aggressive fueling maps since unlike street cars they don't have to worry about any emission issues so can be tuned accordingly. By the way cars with cats can backfire, the cat merely makes it harder to hear. Backfiring in cats is bad news because it can crack the ceramic substrate and cause early cat failure.

Are you sure when you are talking about backfiring you don't mean the neat little burble you can get on rapid decel? Backfiring makes a "pop" explosion sound, the burble is, well, a burble and normal.

Benton

Have you driven it before with this exhaust setup and it *didn't* pop? Every 993 I've heard with muffler bypasses or no cats will pop and gurgle when lifting off the throttle and engine braking. It's not a real backfire (fuel igniting in the intake or whatever like carbed motors); it is what Bob said it is.

SWEET car by the way.