When its cold I have to start the engine on choke. I also have to drive on choke for a while until it warms up. If i don't use the choke, or turn the choke off before the engine is COMPLETLEY warmed up, it will fire up through the carb, kind of like missing, but not quite. It has also started doing this even after the engine has warmed up completely. Why are flames shooting from my carb and burning my air filter(lol)!!! I set the timing about two months ago bout it didnt help. Some advice? Please!?!?

 

When its cold I have to start the engine on choke. I also have to drive on choke for a while until it warms up. If i don't use the choke, or turn the choke off before the engine is COMPLETLEY warmed up, it will fire up through the carb, kind of like missing, but not quite. It has also started doing this even after the engine has warmed up completely. Why are flames shooting from my carb and burning my air filter(lol)!!! I set the timing about two months ago bout it didnt help. Some advice? Please!?!?

 

I got the same problem.

 

Generally, there is only one reason for backfiring through the carb - a stuck or burned intake valve.

 

sounds like a timing problem. has there been any work done on the chain and gears? ford used a phenolic cam gear which when worn out will allow for chain jumping

 

I agree with ranger429, time and again I've seen this with vehicles in need of a new timing set. My son's Torino did it a couple of years ago Still had the 'plastic' coated gear for the cam. Bad thing was I had to change it out in Jan. outside and it had a 3 foot snow drift around it. Wasn't going to till spring but my '66 F100 I had at the time dropped an axle and I needed something to drive.

 

yea and just think where all that "plastic" goes to when it is worn off the cam gear in little chunks

 

I would check the timing, I just replaced the timing set in my 84 460 motor, the gear had teeth acually missing. I could move the chain left and right a inch and a half when it was still on there. It had 170,000 miles.

 

YOU GUYS ROCK!! My trucks been doing this for a while now and ive adjusted the timing and everything you can adjust on the carb, and nothing helped at all! Im pretty sure that ive got the original timing set in there. I asked in the engine forum what was going on and no one told me the right thing. Thanks!!

 

The choke temporarily enriches the fuel mixture to help the engine run when it's cold. Closing the choke when the engine is cold and until it warms up is the normal procedure.

There are a couple of reasons for cold backfiring, but the problem is hot exhaust gas in the cylinder igniting the intake charge during the valve overlap (when both exhaust and intake valves are open). Engines with high performance camshafts have this problem more because of the increased lobe overlap.

Closing the choke enriches the mixture, which reduces combustion temperatures so the exhaust gas left in the cylinder during valve overlap is cooler. Closing the choke also reduces the availability of oxygen in the intake charge, making it more difficult for the exhaust gas to ignite it.

Backfiring through the carb when the engine is warm is the same phenomenon, but it is usually caused by excessively lean mixture. Lean mixtures burn hotter and somewhat slower, and the hotter exhaust gas, with more residual heat from prolonged combustion, is more likely to ignite the intake charge during valve overlap.

Bad ignition timing can also contribute to backfiring, and pumped-up hydraulic lifters that hold valves open too long can contribute to the problem.

Check your timing to make sure it's in spec. Here's a page with timing specs for 351M/400 truck engines:

http://home.earthlink.net/~bubbaf250/perf/perf10.html

 

I see the light, my 408 has a performance cam and it has to warm up for about ten minutes before it's driveable in the morning.

 

TRUCKMAN79, put a socket on the crank and watch for excessive play on the distributor rotor, crank it back and forth by hand. simple way to do a quick check on the timing chain assembly

 

It doesn't have a big vacuum leak somewhere does it? A lean condition can also cause it. I would think that it could be the timing chain though. Does it still make good power? Mine jumped 2 1/2 teeth and wouldn't go much over 30mph. Not to fast for a 460. It was actually on top of the gears since there wasn't any plastic left on the cam gear.
Jimmy

 

truckman79- I don't see where you have ever asked this question in the engine forum. So how could anyone have told you anything???

 

Check the vacuum advance.

It will do the same thing.

Just pull the hose off the carb with the dist cap off and suck real hard on the hose. If the pickup doesnt' move and you can suck air, you have a bad diaphagm. I've gone through 3 or 4 in the last 10 years.