Ok, here is the story. I just started noticing a back firing and stumbling when you accelerate from idle. The engine was just rebuilt and the carb was also restored and rebuilt. Prior to yesterday, the car ran great better than it has ever run before. The vehicle is a 70 El Camino SS 396 with a Q-jet carb. The timing was set to 8 degrees before TDC with the vacuum hose to the advance disconnected and blocked. The dwell was set to 30 while the vacuum advance was disconnected. The engine still stumbles and backfires through the carb. I called the carb place that rebuilt it and they said it must be an electrical or iginition problem as a backfiring problem is 99% electircal. Also when the engine is floored with the accelerator pedal, the engine dies unless you let up right away on the gas.

Any ideas what could be the problem? I need to get this fixed as quickly as possible as the elky will be leaving for the AACA museum Muscle Car display on May 6th and I still have a ton of cleaning and paint touch up to do.

Prior to yesterday, the engine was running great, plenty of power on acceleration and was the best that it ever ran. Also, after the initial backfire and stumble, when you get to the higher RPM's the engine runs fairly strong and light black/gray (not rich thick black) smoke comes out exhaust.

I can only think of two things off the bat:

1 - check that your timing didn't slip (distributor loose?).

2 - check that a cam lobe isn't going flat.

Back firing is the result of a lean air/fuel mixture, check the accelerator pump. Maybe something is clogging the fuel line or dirty filter.

What would cause a lean mixture all of a sudden? Also, could a bad fuel pump cause the problem? How can you check the accelerator pump? One thing I noticed also is that there is minimal, if any, vacuum at the vacuum advance hose. The car has the factory TCS system on it also. Could that be causing the problem?

Weather will do it too..

I'm betting the accelerator pump diaphragm is giving up the ghost. Look down into the carb and open the throttle. You should get a pump shot instantly. If not, time for a new pump diaphragm.

Inadequate advance could certainly create the bog - not sure if that would create a backfire, though.

I'm betting the accelerator pump diaphragm is giving up the ghost. Look down into the carb and open the throttle. You should get a pump shot instantly. If not, time for a new pump diaphragm.

Would an accerlator pump just up and quit like that or would it be a slow death? The carb was rebuilt not that long ago over the winter and ran perfectly fine last week.

Could a retarded timing (not enough advance) cause the engine to backfire through the carb as well as have a bog?

I too think this a carb issue, sounds like a lean condition. Accel pump is a good place to start, as suggested. I'd also suggest checkin carb float height. Do a good visual inspection of the ignition system too, just to be sure it isn't that.

I to think it sounds like an accerlator pump. Do as suggested above, remove the air cleaner (with the car not running so you do not get a backfire in your face) quickly move the trottle from idle to full acceration. You should see two solid streams of raw gas, one in ech primary barrel of the carb. If yu have a weak or sputting stream, replace the accerlator pump.

How hard is it to replace the accelerator pump? Or should I take it back to the place that rebuilt the carb?

How hard is it to replace the accelerator pump? Or should I take it back to the place that rebuilt the carb?

Verify that it is the problem, as SS70 says. Look for two solid streams of gas as you open the throttle. They should start as soon as you start opening the blades. If it's under warranty, let them fix it. It's not hard, but you should know what you're doing. There is a good HP series book on Rochesters, would be handy if you feel like tackling it.

UPDATE: Ok, well I first tried to advance the timing more and reset the dwell to 30. Seems that the engine likes 12-14 degrees of initial timing advance. Has plenty of power now and can't wait for the engine to break in so I can really have fun with it.

Thanks again for everyones input.

See ya at the shows. If you are in the Hershey, PA area, stop by and check out or el camino which will be on display at the AACA museum.
http://www.aacamuseum.org/home.php

UPDATE.... to the Update..... Ok, after adjusting the timing to where it is really advanced, the problem seems to be a lot better but still pops out the carb when you floor it from a dead idle. I just tanked up with 93 octane ($46.00 total purchase- OUCH) and the elky is running a million times better, smoother idle, better acceleration, and best of all.... NO MORE POPPING. I am assuming since the tank was pretty much empty that it did not have a good enough octane in the gas. Looks like we are all set now for the show season. Look forward to seeing everyone at the shows soon.

I'm betting the accelerator pump diaphragm is giving up the ghost. Look down into the carb and open the throttle. You should get a pump shot instantly. If not, time for a new pump diaphragm.

It's a Q-jet. It uses a piston instead of diaphragm like a Holley. But, the arm that actuates the pump has two holes for the linkage from the throttle plates. The pump travel can be increased by moving the linkage to the hole closest to the carb float body.