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Usually there are three reasons for backfiring thru the carb. Not enough fuel, spark timing too early, or an open intake valve. If the compression is good on all cylinders as you stated, my guess would be spark timing or mixture too lean. Could be a large vac leak somewhere too. You say you checked the ignition, but do you know for a fact the plugs are all firing at the right time ? You could pull each plug wire and check each cylinder one at a time. Did you change the rotor when you changed the cap, and is there any chance the wires are cross-firing ?
take pipe of carb turn over see if fuel comong out,if not check fuel filter ,check plugs for spark,no spark check points on distributor. fuel/spark ok check timeing.if fuel/sprk/timeing ok check carb/jets for dirt ect;
Not sure which design carb you have but here's a rule that works for all carbs. Attach the vacuum advance hose to any port coming from the carb baseplate (bottom). If you don't have a free one, you can use a "T" in another line or if there's a fitting on the manifold you could use that as well. Just make sure that the line has full vacuum at idle. (check with a vacuum gauge or just put your finger over it and feel for vacuum) Since it's pretty old, it would be good to put your finger under the advance mechanism when you attach the line (running) and make sure the small rod that goes into the distributor moves when you do that. if it does not, then the advance diaphragm is bad and you will need to replace it. Use small rubber plugs to block off any unused ports. As for the fuel line. You should have a threaded inlet on the fuel bowl. The line attaches there. If you need to make a line, you can get one from any parts house, cut it to length and use rubber fuel hose to join it together with the one already on the fuel pump. When running new line, keep it away from exhaust manifolds, belts and pulleys. Best to put an in-line filter in the hose....that will keep the carb clean. If it's a four barrel (doubtful for that engine) you would need a fuel rail that joins both carb inlets to the single line from the pump. If you have a problem understanding this, just ask and I'll clarify it for you.
NEED ADJUST SET IGNITION TIMING.MOST TIME YOU INSTALL ANOTHER NEW ENGINE.THE IGNITION TIMING AT DISTRIBUTOR OFF TIME LITTLE TOO FAR ADVANCE CAUSING WEAK SPARKS FOULING OUT SPARK PLUGS. CHECK FUEL PRESSURE SOON AS ENGINE STOP AGAIN DONT TURN ON IGNITION SWITCH CHECK FUEL PRESSURE.SHOULD BE 43 PSI. IF NOT 0 FUEL PUMP BAD OR FUEL PUMP RELAY FAULTY OR FUEL FILTER STOPPED UP.IF FUEL SYSTEM GOOD PROBLEM IN THE IGNITION TIMING.
check ur timing - MARK with a scratch tool or use a yellow majiuc marker to see where the timing is now then-
loosen the distributor bolts - if you have a distributor on that model and - turn the distributor one way - then the other and see if it improves. You can slightly tighten the bolts or carry a wrench with you and drive the car and see if a new position improves the running of the car. Also - change the plugs if you haven't done so - check them to see how they look anyway. You may have a flooding condition on your hands as well but that is a whole new thing - so stick with timing and plugs -