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Got a 1985 with stock 305 set up for california smog. Got a new Rochester quadrajet on it. Rotor, cap, plugs, wires, ignition modual, coil, condenser. Can't find a vacuum leak anywhere. Replaced some suspect vacuum hoses. EGR working. Heat Riser Working. New PCV. Had a buddy go through carb too, just to be sure.

Problem is, it backfires, heasitates and almost dies when under heavy acceleration. Sometimes. Sometimes it does great. Once it does backfire, it pretty much keeps doing it when accelerating quickly. I can accelerate slowly and it seems to be ok. I don't know what to do. Help!

Maybe valves? Can timing chains slip back and forth? The smog stuff is all new to me so I have been studying it like crazy. Don't think it is smog anymore. Another friend told me maybe distributer pick up coil. Said it was easier to change the distributer than to change the pick up coil in the distributer. Anlyone know if that is true? Any help is greatly appreciated guys. Thanks.
 

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Backfire can be defined as a loud noise as a result of an explosion in the: i.) intake manifold; or ii.) the exhaust system. See if you can determine whether the backfire is occurring in the exhaust system or coming back through the intake/carburetor. Most backfire conditions are a result of a lean condition.

An engine can backfire for a number of reasons including (but not limited to):
1. Lack of fuel;
2. Improper timing;
3. Improper firing order;
4. Stuck or damaged valve;
5. Broken/inoperative emissions equipment

Start with the basics:
1. Check for vacuum leaks;
2. Confirm that firing order is correct;
3. Pull spark plugs and check condition (look for damaged, inoperative or unusual condition spark plugs) of plugs and spark plug wires;
4. Check baseline spark advance at idle (make sure that the weatherpack connector attached to distributor is disconnected during test) and compare to specs;
5. Confirm that the carburetor accelerator pump is operative;
6. Inspect the condition of egr valce, AIR check valve, diverter valve and catalyst(s);
7. Check distributor rotor and cap;
8. Hook up vacuum gauge so that you can monitor engine vacuum while driving (determine vacuum condition at idle, cruising, acceleration, and during backfire);
9. Do engine compression test;
10. Check for timing chain wear by determining how many degrees of crank movement you can have without any distributor movement.

Obviously as you go through this list certain issues or cylinders will pop-up and point the way to the source of the issue.
 
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