1990 K2500 with 289,000 miles.
350 crate engine with about 75000 miles. It runs great.
Just had a complete Flowmaster ehaust system installed. The catalytic convertor was removed due to the condition of it & not replaced.
Every now & then when it decelerates going down a steep grade it will backfire one time & sounds like a cannon going off. Any ideas?

 

It’s probably because there’s no converter, and if you deleted other emissions control, that could also be contributing.

The unburned fuel that exits the combustion chamber would normally be consumed/burned by the original emissions system to prevent liquid fuel from making it to the converter.

Depending on the original factory set up, that would include a converter, EGR, and/or an A.I.R. System (smog pump). Removing those components will cause a rich air-fuel mixture, resulting in excess unburned fuel in the exhaust.

 

Short of replacing the converter, what are my options of fixing this?

 

You said this is a crate motor - did you keep the TBI or convert to a carburetor?

Options are going to be limited either way - even the latest model vehicles don’t fully burn all the fuel in the combustion chamber, which one reason why emissions control is utilized.

If you kept the TBI, tuners are available for OBD1 systems but they require chips to accept the tuner programming, but even then I don’t expect you’ll eliminate the popping.

If it’s a carb, a converter is likely the only solution.

With either set up, keeping the ignition system in good condition will assist. Make sure the rotor, cap and plugs are clean, plug wires are tight at both ends and routed correctly (not touching each other or the block). You can try advancing the timing a few degrees but going too far can result in detonation/spark knock.

I’d recommend researching racing solutions - because of the high demand driving, racing engines run very rich - I don’t know what’s available specifically for racing but it seems the most likely place to look.

 

Thanks for the replies,
It still has the TBI. New plugs, new rotor, new distbutor cap. Timing is set at spec requiremnts. This thing has only done it three or four times it was always after some hard accelleration going up a mountain & letting off the accerator going down the other side. I still have the cheap Fram air filter. Would a higher flow air cleaner be of any benifit?

 

To the best of my knowledge, no.

It doesn’t sound like you’re having issues with the engine running too rich when it shouldn’t be.

Although a quality air filter should be used and replaced as necessary, backfiring is only going to be resolved by means of consuming the unburned fuel in the exhaust. This pretty much narrows it down to installing a converter.

There may be other options but none that I’m personally aware of which is why I suggested looking into racing solutions.

 

It has only did this a couple of times so I guess I'll live with it. Really don't want to buy a converter & since the Flow Master muffler is pretty wide open I hope it doesn't blow it apart.