Hello all, I'm new here but have been working on a '73 F250 for a long time. I've searched a lot of forums for an answer on this issue and have not found anything that quite applies. Hopefully someone can help me out. Here goes:


The '73 F250 has an FE 360. I put full length headers on it to replace the OEM exhaust manifolds. They run into glass pack mufflers. I also replaced the timing set with a set that is advanced 4 degrees. I replaced the carburetor with an OEM MC 2100 rebuilt carb. Replaced fuel pump. New plugs, new points, point gap set to specification. New distributor cap and rotor. New condenser.


When I started it back up all that I set the initial ignition timing to around 8* BTC. The truck ran pretty well at these settings, but when I let off the throttle I get a lot of backfiring out the exhaust (mostly in situations such as after a downshift). I assumed the initial timing was too retarded based on the new cam timing, so I tried to advance it slightly. This only made the backfiring worse. I also thought the idle mixture might be too rich or too lean, but I played around with the mixture and got it to run best at the OEM setting -- about 1.5 turns out from seated on the idle mixture adjustment screws. The truck runs great and has much more power than before, it idles well and runs well at higher RPMs. The only issue is the backfiring when I let off the throttle. Is this just because I have so little backpressure now, and it's something I will have to live with? I would love to hear your thoughts!

 

Welcome to FTE! Try hooking your vacuum advance up to manifold vacuum instead of ported. When you let off the gas with ported, the throttle plates close and you lose all vacuum advance. With manifold, when you let off the gas, you keep lots of advance and your backfiring may subside. Good luck!

 

Afterfire..backfire through the exhaust: Too rich a mixture/float stuck, plugs not gapped right causing unburnt hydrocarbons to fire on the exhaust stroke, a dead cylinder, timing too retarded.

It could be as simple as an exhaust leak (gasket) at the head or collector flange. Either one would suck cold air in and cause an afterfire.

Since you said your buggy runs well, the top situations might not apply...but they could be a consideration.

 

The exhaust leak is a good point -- forgot to mention that I know I have a leak at the manifold gasket on at least one side, have not had time to take it apart and fix it yet. The other side seems to seal up ok but it could be leaking slightly. I seem to get an equal amount of backfire out of either side.

 

FYI.. I ran my 360 at 10° to 12° BTDC and it popped and coughed on downshifts. 'Had to dial it back to 6° BTDC and all is good now.

I'd be leery of not setting the cam straight up on an FE.

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Update:. Replaced header gaskets on both sides and it sounds a lot better. Still have some exhaust popping on driver side only. Ran compression test and got 0 psi on cylinder 5. All the rest were 100-105. So I'm guessing I have a bent/burnt/broken exhaust valve. Couple questions:

1. Am i missing anything obvious? Does a valve seem like the likely cause?

2. I've never replaced a valve. I've done a lot of work on cars but really no internal engine work. Is this doable or should I just pay a pro? Would the valve seat need to be machined anyway?

3. If I have 1 valve replaced should I just replace all valves on that bank while head is off? Any thoughts on what a shop would charge for this job?

Thanks in advance folks!

 

Pull the #5 plug and take a peek. If it's not oil fouled chances are good the rings are good and you have a broken/cracked valve.

I would send the head to a reputable shop and let the "experts" take a look at the whole head. Depending on a broken valve, the seat might need attention. The guide might need a peek-see, too. A 3 angle valve job should be next in line - if you have the dollars.

I don't know what the shops are getting these days for a valve job...might not be too ridiculous....coupla hundred maybe?