Hey guys my 400 has had this problem for a while and it finally starting to annoy me. It will bakfire through the carb when the engine is cold if you try to give it a lot of gas like over half way to floor, but once it warms up to normal temp no problem it will take all you can give it, it practically blew apart my holley and I replaced th intake and put on a edelbrock 1406 my performance in creased greatly except the old backfire. I dont have any vacuum leaks. And I think the backfiring has caused the throttle linkage to leak fuel by now. Any help woudl be greatly appreciated.

 

Is the engine rebuilt? If so, what was done to it?

 

Think about the way a carburetor works. When you open the throttle, you give the engine more air, not more fuel. The only additional fuel it gets when the throttle opens is the immediate shot from the accelerator pump. Additional fuel doesn't start to flow until the engine speed (rpm) goes up, and then it pulls more fuel through the carburetor's main metering system.

When the engine is cold, it requires a richer mixture (more fuel, less air) to run, hence the choke. Opening the throttle too quickly when the engine is cold overwhelms the carburetor's ability to provide the fuel required, so the mixture leans out, combustion temperature rises rapidly with the lean mixture, and the overheated exhaust eventually ignites the air/fuel mixture in the manifold during the intake/exhaust valve overlap, which you experience as a backfire. The engine stumbles badly because after the backfire, most of the intake manifold has been evacuated of any air/fuel mixture, and the next two or three cylinders in the firing order get little or no air and fuel until the carburetor can catch up and refill the manifold with air and fuel mixture.

Backfiring when cold means it is leaning out when you open the throttle. It isn't getting enough fuel because the carburetor is calibrated to provide the correct mixture (and accelerator pump shot) when the engine is warm.

Try adjusting the choke some, or just go easy on it until it warms up (that's better for the engine anyway, and backfiring can damage the carburetor). Avoid opening the throttle rapidly until the engine is warmed up.

Most aftermarket carburetors have this problem to some extent. The OEM Motorcraft 2150 2V carburetor has a system called the "high-speed pullover," which supplies extra fuel when the throttle is opened and the choke is still closed. When adjusted properly, the OEM carb gives much better cold driveability than most aftermarket units.

 

Hey thanks a lot bubba f250 thats kinda what I figured on what was happening I knew it had something to do with engine temp. but anyway thanks for your time I just let it warm up more.