My 1974 with a 350CI engine has recently started to backfire thru the carb. when the engine is cold and in gear while accelerating. It seemed to start out with one or two backfires in the carb. and finally has progressed over a 6-8 weeks to backfiring anytme I press on the accelerator while the engine is cold. Although I've always had trouble with rough idle when cold or hot, the cold idle now is terrible. When the engine is warm the backfiring stops and the engine runs OK but I get a pretty heavy exhaust smell. Have the carb idle screws set at about 3 turns, which is where they've been since I put it on and set it up. I just gave it a tuneup and still have the problem. The carb. is a rebuilt I put on about 5000 miles ago.
Can anyone please give me their opinion as to what they think might be wrong? Thanks.

 

I would start to look at 3 things
1. Vacuum leaks - get a vacuum gauge and with a basically stock engine you should be pulling 16-17" at idle. If not use the gauge to adjust out the carb for the highest vacuum reading evenly on both screws. Check timing and then chase down the vacuum leaks. Clamp off each vacuum line on the carb and manifold to see which one makes the reading go up.

2. Base timing set it to 8-10 degrees with the vacuum advance plugged. Then check the mechanical advance to be moving smoothly.

3. Run a compression check to see if you have some valves hanging up.

The heavy exhaust smell is a too rich of a mixture. If you have the idle screws out 3 turns that should not make it too rich. Too high a float level, jets, rods etc can be off. Just because it is a rebuilt - don't take that to the bank. If you have serious questions about the carb (assume it is a Q-Jet) check out the carb loaner program on the forum with Lars and see if you can get a loaner sent out and give it a try. These units are pretty good and should help eliminate one source of a problem.
Let us know what you find out.

 

Backfire up through the carb upon throttle application is indicative of a lean condition. Normally, timing will not cause a backfire upon acceleration, since the timing has to be radically off in order to cause a backfire.

Here is a common cause for this:
If you recently rebuilt the carb, a bunch of carb kits on the market have accelerator pumps in them that are not compatible with ethanol in the fuel. This causes the pump plungers to swell up, and the accel pump will seize in the bore (I know this sounds weird, but I've seen over a dozen cases of this now). Due to the spring on the pump shaft, the pump shaft will still appear to be moving up and down when viewed from outside the carb, but the plunger will actually not be moving at all. This causes a lean condition upon throttle application, and will make the carb backfire. The problem is worse when the engine is cold, since the engine needs a richer shot when not fully warmed up. To check for this condition, simply look down your carb primary side venturis with the engine off. Slightly move the throttle linkage: you should get an instant discharge of fuel out of the accel discharge orifaces upon the slightest throttle movement, and the stream should be steady and continuous as you continue to move the throttle. If not, your pump is one of the bad pumps, and you need to replace it.

Also, make sure you check your choke for proper adjustment and operation: With the choke cold and closed, your choke pulloff should crack the choke plate open exactly 1/4" upon engine startup (measured from the lower forward edge of the choke to the choke airhorn forward wall). If not, adjust or replace the pulloff.

 

Could be ignition. I had a similar problem and it turned out to be a bad ignition coil. I would flake out when it got hot and cause a backfire through the carb.