I have a 1970 Ford F100 with a 360 stroked to 390. I recently got it running after the past owner didn't take care of it. I had a timing problem and replaced the chain and while I was in there the water pump as well. I also got rid of the junk aftermarket electric fuel pump he put on it and put the mechanical one back in a brand new one at that. I read the correct amount of fuel pressure, I recently rebuilt the carb, and made sure there was new, good gas in it... Every time I try and run it I have the Idle set 50 high so at 600 instead of 550 cause its an automatic it used to be manual. I start the truck up and it sits there and idles from 500 to 700 just going back and forth then idles fine then does it again. It will only run smoothly for about a second or two then runs rough. When I rev the motor it just does fine, I raise the Idle to 1000 just to experiment with things a little and it wasn't as bad but still was stilling there flickering from 1000 to 1100.. I know I'm young and don't know too much about these trucks, but I'm pretty sure its still supposed to idle smoother than that. Is it?
I asked around and people were saying just up the idle.. I don't think that's necessary I rather just solve the problem rather than deal with it later down the road. Any help and ideas will help me out.

 

It sounds like a vacuum leak at first. FE's are very sensitive about the intake manifold seal because of bolt angles and the width of the manifold. Does it have the stock manifold or not, there is a pin in the china wall for alignment that needs to be removed for an aftermarket manifold? Have the heads and block been resurfaced, that will change the angles between the head and manifold? Did you use the cork gaskets, those are usually too thick and will hold the manifold off of the head? All of these will probably show up as oily plugs because the engine is sucking oily air out of the engine through gaps in between the head and intake because the gasket is not compressed enough.
Are all the unused vacuum orifices on the carb and intake manifold plugged? Are you setting the timing with the vacuum line to the dist. plugged? Here is a quick ck to do, while it is running, cup your hand over the choke, if it speeds up then you have a vacuum leak somewhere. Less air through the carb matches the air coming in somewhere else with the same amount of fuel.


Check this out, report back and we'll see wachya got

 

Another cheap but invaluable tool for you would be an automotive vacum gauge. Around 10 bucks at Harbor Freight.

 

Good stuff here... young ain't hurting you I was 16 working on my mother's '65 Fairlane wagon, and learned a lot from that old girl.

Definitely sounds like a vacuum leak. Could be anything from a carb gasket, to the PCV valve itself.

One thing I have to ask is, does it burn any oil? Like when you get on it, let off, and get on it again it blows blue smoke? That's an intake gasket leak.

 

Little update, I put my hand over the Carb Choke and it killed the engine. So I dont know what else it could be.

 

A can of spray carb cleaner is also helpful when tracing down a vacuum leak. Spritz a little at various locations such as the vac line connection to the distributor, around the carb baseplate, anywhere that little bit of extra air can be leaking in. Engine speed changes, you've narrowed it down a bit.

You also mentioned rebuilding the carb. All the gaskets installed correctly, fasteners tightened down properly? Does it have the correct gaskets between the carb (and spacer plate if it has one) and the intake? When I first picked up my 65 F100 it was doing something similar. Turned out one of the carb mounting studs didn't have the nut tightened all the way.

 

You also didn't mention if it's had a tune up. New plugs, wires, dizzy cap and rotor. Also plug off the dizzy vacum line. The dizzy diaphrams have been known to leak as are the tranny vacum diaphram if it's an auto.