Hey guys, I have a 77' F150 custom Explorer with the auto 4x4 and a slightly built up 400. it's developed a rough idle and backfires through the carb under moderate acceleration and I loose power for a brief moment. I've rebuilt the carb (2bbl motorcraft), replaced the ignition module , plugs and wires, knew distributor, cap and rotor. I was battling a bad pre-ignition problem so I placed the timing at 8°. seemed to work well in the summer months but now that we are into this -30 weather is can't go anywhere without the carb poping. any ideas???

 

How old is the timing chain and sprockets? That sounds like the chain jumped and it is out of time.

If not, then you have a very lean air/fuel mix. Do you have the original air cleaner on and the heat stove hooked up? That is very important, especially is really hot or cold weather. See this: Air Cleaners - ???Gary's Garagemahal.

 

the engine was built in 93'. the guy was using it for racing when the engine was bought. I came into possession of the truck about 4 years ago and it's always had a rough low idle (about 300-400rpm) I figured it was just the performance cam that's in it. the engine only has about 60,000 miles on it. and yes all the original air cleaner components are on the truck . and I thought it was running lean but occasionally she'll puff black smoke and she smells like she's running really rich. I'm kinda at a loss.

 

Have you figured this out yet? I'm aware that your question is a week and some change old but I figured I'd throw some stuff at ya.

First, 300-400 RPM at idle is waaay to low. You want to see 650-700 RPM idle in DRIVE.

With a "performance" cam, the engine wants to have about 16* BTDC for initial timing to be happy.

Did the PO run a vacuum advance lock-out? If so, you need to re-hook it up or fix it.

Puff black smoke and smells like a lot of unburnt hydrocarbons indicates a blown power valve. Hard starting and smelling very "rich" at idle is another indication.

 

I wonder if lifter preload was ever checked ?