I've got a bad miss in my 1984 F-150. It's a 302 with AOD and duraspark II ignition. It has 270,000 miles, but my son, son-in-law and I put a NAPA rebuilt engine in it about 25,000 miles ago. At the same time I replaced the radiator, water pump, fuel pump, power steering, ignition module, coil, distributor cap, rotor, wires, and spark plugs. I removed all the emission controls except the EGR valve. It ran great until this summer when I pulled a camper 350 miles from Virginia. When almost home the engine seemed to backfire through the carburetor and started to miss. I continued the 70 or so miles home and unhooked the trailer but I still had the miss. The miss was erratic, worse when climbing a small hill. When I left home it would miss bad but if I stopped for gas or coffee 10 miles from home it would smooth out. Sometimes it would be slight and sometimes so bad I thought it would buck my head off. Now it seems to miss constantly. The truck starts at the first turn of the key and seems to idle fine. I've replaced the fuel filter at the carb. The distributor cap and rotor,pcv valve, checked each spark plug and regaped (all seemed to be firing clean). I've looked at it at night and don't see any spark jumping, and I've played with the timing some. (I don't have a timing light so I go by sound.) I bought a used 1978 motorcraft used 2 bbl carb off ebay that has an automatic transmission kickdown with the thought of rebuilding it and trying that. Another thought I had is that the mufflers were rusted real bad but I was running short of money so I put them back on, could I be getting a blockage? I'm retired living on Social Security so I don't have a lot of money to put in this project. I've read in the forum about another fuel filter but can't seem to find it. By the way my mileage has dropped from 15-16 down to 11-12 mpg. What should I do next? Will removing the emission controls change what the timing should be? Should I tackle the rebuild on the carb? or should I maybe rebuild my existing carb? Help.

 

This sounds like a broken valve spring or something valve train related. I will suggest a compression test to check for the offending cylinder.
Afterwards remove the valve covers and inspect all the rockers. Crank the motor over until each valve spring is in it's highest position then check to see if you can push down on any of the valves by hand. Also try to spin the pushrods by hand, checking for looseness.
Let us know!
Steve G.

 

This is something I didn't think of. I'm going out of town for a couple of days but will try it when I get back, then let you know. If its a valve problem wouldn't the plug be fouled?
Don

 

If you take the valve covers off, check the lift on each valve. A worn lobe on the cam can cause back firing.

 

Not necessarily! (fouled plug)
Also, one thing I forgot to mention, while the covers are off, if all checks out ok.
Start the motor and observe the rockers to make sure they all have enough movement to open the valves.
Let us know!
Steve G.
(Ron clark posted while I was typing this and he is correct about the worn cam lobe)

 

I have had both problems with my Ford... timing and broken valve springs. A valve spring will be noticed at idle, under no load and under a heavy load. You will be missing on that cylinder. And yes, the plug should be really wet on that cylinder. I never had the engine back fire with a broken valve spring. It just had a loss off power and rough idle. It isn't a smooth feeling while traveling down the road. Very consistent rough running.

Now timing... your plugs can still looked fouled depending on how bad the timing is off. Ususally more than one plug will look bad. What you are describing is exactly what my truck was doing a couple of weeks ago. Backfiring, (especially under a load) loss of power, but start and idle OK. I did a lot of the same things you did and couldn't get it to run right. I finally took it to the Ford dealership and they timed it and adjusted the carb, and it runs like a top.

You should invest in a timing light or borrow one and start there. The timing by feel just doesn't always work. I would bet that is your only problem. The simptoms are very similiar, if not exact, to what I was going through. By the way, the dealership only charged me $120 and that included an oil change!

 

Sorry for not getting back sooner. Finnaly got the $$$ together to take it to the dealer, figured at least they could tell me if it was something real bad. Turns out the distributor was a notch off and had 1 bad plug wire. Total cost: $78.00. If i'd knowned that they would have had it months ago. Thanks everyone for all the help.