...I'm about yea close to shooting this truck.

The past 3 days have been a wrenching blowout on my '78 F-100 with a 302.

We replaced the gas tank, and all associated rubber hoses under the truck.

Well, then we saw the 'booster pump' on the carb was leaking. Simple enough, get a rebuild kit.

Out of 4 auto parts stores, two dealers, 6 hours on a telephone, and 9 kits, we couldnt find one with all the proper pieces, and none had the right choke parts.

So we go to AutoZone and buy a remaufactured Motorcraft replacement, with one difference.'

It's electric.

The old one was a 'heat riser' type, with a hose from the exhaust manifold and one from the intake. The new one just has a hose to the intake, and a wire to the side of the black doohickey on the side, the name of which escapes me right now.

Got a tester, 12.4 volts at the wire with ignition on. Hook everything up, and the truck will start, run a few seconds, then die. Unless you hold the throttle open. No idle.

We're assuming there's a vacuum leak somewhere, but have no idea where. The three vacuum trees are all capped, the vacuum booster/ spacer under the carb is all capped, the vacuum ports (two of them) on the carb itself are capped, with the PCV valve hose run to the third one on the carb.

What are we overlooking? Should the electric current to the carb cut off after a certain amount of time? Should there be a vacuum line run from one (or both) of the capped places on the carb to the trees or the booster?

Thanks

 

The old "heat riser " type chokes had a small piston that was vacuum operated when the engine first started. It pulled the choke plate open just a small amount at fire up and then progessivly opened as things warmed up. There is an adjustment for this and it is sensitive. As I recall the vacuum source orginated in the carb.. It may be also that an intake vacuum source could come from the intake manifold somewhere else.
I believe the current to the choke is continuous. It warms the spring coiled inside the round housing causing it's tension to lessen with increased temp. The old housings were adjustable for spring tension on the choke. The newer housings may have a protrusion fitting in a notch that prevent this adjustment. The protrusion can be removed (look it over well and be sure about what you are doing) to allow spring tension adjustment. If you don't know carbs it is easy to get things out of whack.

 

On the old choke, you might also have a "hot-air choke" which has two vacuum lines: One comes off the air horn of the carburetor, so you plug that at the carb, and the other is an insulated thermal tube that goes back into the choke/spring unit on the carb. Now you may have a problem if the hot air choke plate is not sealed because of the vacuum from the exhaust gases going into the intake manifold. So if the electric choke on the rebuilt carb does not bolt to the old choke port on the manifold, and you have removed the hot air unit, you have a very big vacuum leak that needs to be sealed. You also have the two very long bolts that are part of holding the intake manifold which should be torqued that go through that hot air choke mounting plate. If this is your configuration, you may have to put the old choke plate back on the mainiold and plug both ends of the heating stove. I know there were a good bunch of choke configurations, so this might not be the one you have.