Ok I have a 79 f150 4X4 400 and it started backfiring through the carb a few days ago. Before this it ran good, so in the past few days I have changed the Plugs, Plug wires, Rotor, Cap, Brain box,Coil, and rebuilt the carb. Still to have it backfiring through the carb, and every once and a while the exhaust. I have double check the firing order and the timing and both are dead on. Any help i could get would be great.

Thanks

 

Where is the timing that you consider it "dead on"? It sounds like a timing issue to me.

 

8 BTDC I have tried to advance it some and retard it also to the same results

 

x2 on the timing

 

Vacuum leak would be my second guess.

 

Has anything happenned to the exhaust to cause a restriction? It will backfire if it can't breathe.

I remember years ago we had a GMC that started backfiring, and we had just put a new exhaust on it. Turned out that the exhaust pipe was some kind of wierd double walled stuff, the truck got wet going thru puddles and it made the inner pipe collapse (even though the outter looked OK). The truck just couldn't exhale.

 

I cant seem to locate a vacuum leak but I will look again, as for the exhaust someone else also mentioned that to me, the truck has single walled exhaust into a cheery bomb with no catalytic converter, but I guess that is my next option.

 

Backfiring can also result from a stuck valve.

 

if a rocker arm nut has come loose it will cause the engine to back fire through the carb believe me been there done that.

 

X2 on the rocker nut............

 

Do a compression test on the cylinders.

 

I will check that, What should the compression be at?

 

Ok the results are in I have between 135 and 140 on all cylinders but one and it is at 110 is that low enough to be the problem?

 

Looks like a stuck or not properly seating valve to me. No guarantees that this will work, but we used to pour tranny fluid into the carb while the vehicle was running to try to free up stuck valves. I've seen it work, and I've seen it not work. Seafoam into the carb will also clean up the top end. If you've got a burnt valve, nothing short of a valve job will fix it.

 

wow....there for a second, i thought you were doing a test on my engine...sounds like the last time I did a comp test on mine.