Carbureted Engine has Performance Problem!

I've got a 1977 AMC Pacer (ugly +1) which has an engine performance problem. The engine is a straight-up 6 cylinder, 258 cu. inch. The transmission is an automatic 3-speed.

The performance problem is this. While in neutral/park, the engine will start up fine and idle fine. But when attempting to rev the engine up, the carburetor backfires very badly, suggesting preignition or a valve problem. The engine will eventually get up to high speed after much effort. At high rpm's the engine purrs. But going through the mid ranges, the engine is very rough. Also, when the engine is at high RPM, and I let off the throttle pedal quickly, the carburetor will make a series of faint backfires as the engine slows down, like a *put*put*put* noise.

While on the road the engine has a serious loss of power throughout the mid range of RPMs. The loud backfiring of the carburetor is intermittent and I was not able to recreate it under a specific condition, although it happened most often when I attempted to accelerate quickly or put any type of strain on the engine.

I checked the ignition timing. It appears to be spot on. Also I personally retrofitted the vehicle with an electronic fuel pump, so no problems there.

Any suggestions as to what I should look for? I have theorized that this is being caused by a loose or misadjusted timing chain, a stuck valve, a damaged head, a bad head gasket, or possibly a malfunctioning distributor advance system. Checking all of these things though would take a lot of time.

Anyone out there have some experience that I don't, who might suggest what I should check first? All help is very much appreciated. If you have any Q's about the vehicle I will answer them as best I can.

Usually, if the carburetor is backfiring when you try to accelerate, it means it's running too lean.

sounds like your accelerator pump isn't working.

Or, there is something else wrong with the ignition.
A bad capacitor will often give the same symptoms.

IMO your cam timing gear is worn and has skipped a tooth, so all your valves are opening and closing late.

The first things I'd check are the advance. You should be able to check the simple things like vacuum and free movement of the mechanical advance at home, but you can pull the distributor and take it to a shop with a Sun ignition tester for verification of what's actually happening.

Then from there I would basically go in order of difficulty to check.

I'll toss in a few cents to maybe help narrow it down, but none of these are iron-clad. Its not always the case, but often times a loose or worn timing chain will manifest itself as poor acceleration like erratic retarded timing including backfiring, but not show any sypmtoms when in neutral revving the engine. The timing advance is less critical while not under load, so if its a timing chain, chances are it would be much worse while driving than in neutral. A similar quality happens with the accelerator pump. Fast throttle openings are when you really need the pump squirt. If the problem is less noticable when you slowly apply throttle to bring up the RPMs, but it bogs and pops when you snap it open, chances are its an accelerator pump issue.

Its also quite possible that if all of the above are consistent that its a piece of dirt lodged in a jet. That happened to me and I could get it to rev if I kept pumping squirts as I accelerated, but if I just tried normal acceleration it would pop from being very lean.

Another thing I would check is the carb body. I've owned two AMCs and both of them developed cracks in the top of the fuel bowl near the front left screw in the air horn. I coudn't see them unless I applied some torque to the screw, then I could see it start to split apart. I replaced the fuel bowl casting and never heard a peep from them since.

Don't forget, 1977 was bad for pollution crap, and the companies leaned out the mix. Maybe look into enlarging the main jet (carb) just a TINY bit. I know you're not suppossed to, but I've had good luck just drilling the jet in other engines.