223 Start/Idle Issue

All,
New to this forum but not the Ford world. Have a '61 F-100 with a 223 that I just finished rebuilding.

Tuned the motor the other night, set dwell, timing, idle speed and idle mixture and it was purring like a kitten. Ran it for about an hour without issue.

Started it last night to back out of the garage and it started right up but after idling for approximately 30 seconds it died. Now I can't get it to start unless I have the throttle all the way to the floor. And even then it'll run at high revs but if I let off the gas it idles for 10 seconds before stumbling and dying.


Here's what I know so far:
it's getting fuel (fuel bowl is full and I get a nice strong stream of fuel in the carb)
It's got a spark (new points, condenser, cap, rotor, coil, plugs and wires). I went so far as to pull the wire from the cap and watch it spark to the block while I turned the motor over. Nice blue spark.
It's timed correctly (threw the timing light on #1 and it was right at 4 degrees while cranking).
No major vacuum leaks (Brand new vacuum advance and vacuum hose from the carb)
All my grounds are tight (battery to block, battery to body, body to block and block to frame)

The only thing I can think is that something got lodged in the idle passageway in the carb causing issues. But how would that happen (brand new fuel pump and filter) and why would it occur so suddenly?

I'm at a bit of a loss here so any help would be appreciated. Thanks!

Possible contamination from the tank that got sucked into the fuel line and stuck in the filter.

You might blow out the line back to the pump and back to the tank. Also, there is a filter at the tank pickup and it could be fouled with rust inside the tank. That is the usual cause when it runs well and then just dies while running. Then starts right up again but dies again at road speed when the fuel pickup is at max.

Try removing the idle adjuster screw (after cleaning everything around it) and blowing compressed air into the hole the screw came out of. You already diagnosed the problem in your post. If you don't like the idea of air pressure in the carb, start the engine and keeping the rpms well above idle remove the screw while the engine is running and spray Berryman chemtool or other suitable solvent in the idle screw hole. If the blockage is disolvable this should do the trick also. I have used both these methods against idle problems with a 1953 Ford 215 engine in my forklift.

Just a thought, the new gas eats up old carbs, or parts in older car rebuild kits that are still om the shelf. I had a similar problem some time back.

That too. If you have plastic/nylon internals, gas with alcohol (aka: solvent) will melt them. Also plastic/nylon parts in any element of the fuel line. Although usually not overnight.

Thanks for all of the replies.

I had thought about the ethanol in the gas doing bad things to my carb kit.

I will try your idea of pulling out the mixture screw first. If that doesn't work, $40 for another rebuild kit isn't much.

You can probably get needle valves with rubber seats and cannibalize the plastic parts from your original carb.

It sounds to me like the carb is flooding out.