hello everyone, danny here, my cousin has had a 302 windsor recently rebulit running on unleaded and a 600 holley and is having problems with it backfiring. It starts fine, but has a big backfire through the carby. Firing order and timing has been checked but it still backfires and doesnt want to idle properly. The engine does not ping, BUT WE NEED HELP!!!!!
Some things to check: much too rich a mix (try leaning mix) , too lean a mix, contaminated fuel (how long has it been sitting in the tank?) leaking carbi into inlet manifold (can you see fuel coming out the carbi inside its throat when the engine isn't running?), no PCV valve or no mesh or gauze spark/fire trap in PVC system and as rings have not yet sealed blow-by fuel is getting back into carbi, too high float level, worn dizzi, inlet or exhaust valves not sealing (compression test time) , worn carbi needle valves or broken tips from screwing them in too far, failed EGR system too much nitro....
This site http://www.badasscars.com/techtips.html#Anchor34 suggests: "Back firing is usually caused by a spark plug "sparking" when either the intake valve is open or when the exhaust valve is open. The engine timing doesn't really have much to do with the whole thing. When you turn the distributor, you are affecting the timing for the whole engine, not just one or two cylinders. When a back fire occur, it is just one or two cylinders, so the overall timing usually isn't the culprit. Once the timing has been set, if you don't move the distributor or make any changes to the internals of the distributor, then the timing should never go out unless you physically move the distributor again. Backfiring through the carb (spitting) can occur in the mornings when a carb's air/fuel mixture is too lean. This goes away once the engine warms-up. It can also be caused by a bad ignition system, such as plug wires cross firing which sends a spark to a cylinder that has the intake valve open. When that plug sparks out of turn, it lights the fuel in the cylinder and the pressure has to go somewhere... if the intake valve is open, it goes right back up through the intake manifold and out the carb with a "spit" and sometimes even a flame. When the air/fuel mixture is too rich and there is unburned fuel in the exhaust system and cross fire occurs when the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes, which results in a big bang out the tail pipes. Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet. So if you have an occasional backfire, don't start messing with the overall timing, start looking for a problem somewhere else down the line."
Timing right for the cam you have? 302's and 351's Windsors Pre efi have different timimg as in fireing order.Though the efi 302 has the same as 351..
As stated above check lean conditions in particular needle seat/fuel level, accelerator pump actuation? can be adjusted by that screw and spring on the fuel bowl side..Inlet leaks,broken valve spring,bent valve.Crappy points..
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