This problem developed yesterday, I drove around town running errands, filled up about halfway with fuel, came home and the truck sat and cooled off. It ran fine albeit with a somewhat rough idle that I had believed to be normal. A few hours later I fired it up (by using some slight throttle, since it doesn't like running cold), but once I pulled out and stopped at the light, the engine stalled. It would then immediately fire again with the starter and run completely normal until I let it idle again, where it would then immediately stall.

Info on truck:
1979 Ford F150 4x4
T-18 4 speed
300 I6, all stock

Rebuilt carb was installed about a year ago, got from a parts store, and it does NOT have an idle speed adjustment screw, although there is a hole for one. I don't believe this is the problem since it has been that way for the last 25,000 miles.

My immediate suspicion was the fuel pump, as I have had problems with them before, but after replacement it didn't fix the problem.

The problem is prevalent regardless of engine temperature, so I don't believe it could be vapor lock.

I tested the ignition module at the local Advance Auto, and it checked out fine.

I pulled the distributor cap, and the inside was clean, with only slight wear on the terminals/rotor.

The fuel filter is the see through type, it has less than 1000 miles and is visibly clean.

Ignition wires are relatively new, and in visibly good shape.

Vacuum hoses are new and I checked for leaks, I replaced the carb base gasket and have not found any other vacuum leaks. I also sprayed brake clean around the intake ports, so I would like to rule the manifold gasket out as well. Overall I don't believe it is a vacuum leak, since there is no audible sound and the engine runs like a top when my foot is on the throttle.

My current suspicion is the ignition coil, as far as resistance goes it is 0.7 ohms over max spec on the primary (it reads 2.2 ohms and I believe max spec is 1.5) and 210 ohms low on the secondary (reads 7490, min spec is 7700 ohms). However when I tested a new replacement coil at the parts store, it was further out of spec than the current coil.

I've been diagnosing this all day today, and am getting pretty frustrated with it. I was hoping for some advice, tips to start with tomorrow.

Thanks in advance!

 

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Not really sure where to go with this?

 

It doesn't have an idle adjustment screw? First, I would think that would make for a really lean setup, since it'd always be sucking air. But, second, that's how you set your idle in the first place. Sure it's always been gone and didn't just fall out?
An idle adjustment screw isn't exactly an unnecessary component. Get that taken care of and I'm sure you'll notice some difference.

 

Means to ignore it.

I had said something but immediately changed my mind, replacing the text with 10 characters is the only easy & fast way of "deleting" it (for the time being)

 

It does have an idle mixture screw, and a fast idle speed adjustment screw, but no regular idle speed screw. There's a hole on the bracket next to the fast idle screw for the regular idle speed screw, but it doesn't go into the actual body of the carburetor.

Ah, no problem, I thought I might've missed something incredibly obvious.


Anyways, I'm now thinking it may be a float problem, any ideas?

 

It could be a float problem but that should show up more when you're driving vs. at idle, unless it's fouled and flooding out the engine at low rpm. At what rpm does it idle? How many inches of vacuum at idle?

 

Ah, I misunderstood which idle screw you meant.

When i had a fuel pump going bad, it had a hard time starting (had to pour gas into the top of the carb to get it to fire), but not idling. Doesn't mean yours may be failing differently though.

You also may have a torn accelerator pump inside the carb, which'll be giving it too much fuel and making it have a hard time idling.

 

Unfortunately I do not have a tach, and my vacuum gauge is at my parents' (I'll try to pick it up sometime this week if I haven't resolved it then). Just by my rough guess, I'd say it normally idles around 600-800 rpm, but now if it runs slightly under 1400 rpm it starts to hiccup, and any lower and it stalls.

I'm starting to believe it is definitely something to do with the carb, now that I think about it I did remember a strong smell of gasoline when it would stall on the road. I'm betting it is flooding at idle. I've got finals to study for or I'd go tear it apart right now.

 

In your 1st post, you mention that you filled up about 1/2 way with gasoline and drove home. The truck sat for a few hours and upon restarting, did not want to run right.

Could you have gotten some contaminated gasoline?