Gentlemen,

I have a 1979 F250 with a 351M in it - recently installed from a donor truck. These engines heated the automatic choke with a tube that went into the intake manifold, did a few coils, then one end is attached to an inlet on the choke body, the other end appears to connect to an inlet on the carburetor.

The tube on both my 351Ms is rusted out and needs to be replaced.

Two questions:

1) The inlet on the carburetor I'm talking about is on the underside of the air cleaner support, at the rear of the carb. Is that the right one? The old tube appears to be pointing at that.

2) Just how exactly does this work? It appears to be intended to pass air through the tube, heat it in the coil that passses inside the intake manifold, and heat the choke bimetal with that warmed up air. BUT - since the tube terminates at the choke, that would require something blowing air thorugh the tube, and I can't think of anywhere on a carburetor that blows in stead of pulling vacuum. How's this work?

Thanks.

PS - I've also seen carbs from a few years later that had an electrically heated choke, but that seems like a step backward to me. I mean, the electric heater is totally disconnected from the state of the engine, whereas the air through the coil gives a good indication of the state of the engine. Sit your warmed up truck outside for a while in winter, and the choke body will be stone cold long before the rest of the engine. Start it up, and you've got a choke warming up from scratch on an already half warmed up motor...no?

 

On mine there's a hookup point near the top of the carb on the underside. A rubber hose runs from there to the tube that goes into the top of the engine. From there it attaches to the bottom of the choke thermostat housing using a threaded connection. I'm not quite sure how that air travels through the tube either.

I found that a choke thermostat that's not electrically assisted didn't work very well on mine. I put my old electric one back on and it works much better. Without the electrical assist is takes longer for the choke to open in cold weather. Depending on your engine that may or may not be a good thing. On the original units, the electric choke heater goes to the stator of the alternator so it only operates when the engine is actually running. That's how they tied it to the state of the engine so to speak.

Stan

 

Yes, that confirms the connection point on the carb, thanks. That's exactly what I believed.

I'm going to at least try the original system, just to keep it original, and if it doesn't work well, I understand they have kits to convert to a fully electronic system with a temperature sensor that bolts to the block and everything for about $50CDN.

 

The carb has a controlled vacuum leak into the choke spring housing, and draws air from the air cleaner, through the heater, and into the choke housing. The part of the intake that the tube runs in is actually an exhaust crossover passage, so if the tube corrodes, it is sucking exhaust gasses.

 

Aha! A vacuum passage through the choke itself. Excellent - thanks.

 

For anyone else's future reference, today I fabricated my own new choke stove.

The '79 351M has the stove in the exhaust cross-over in the intake manifold, just under the choke body of the carburetor. The heat tuibe passes through a plate boted to the manifold and does a coil inside.

Took the plate off the maifold. The tube had broken off from rust. I drilled out the old tube, to 1/4". Used 1/4" O.D. copper tubing and wrapped it around a piece of 3/4" copper pipe. It took several tries to do the wrapping just right to avoid crimping the tube, but I found that pulling hard to force the tube to stick to the pipe during wrapping helped. A bit of heat on the tube helped to start it.

Then the two free ends through the original plate, with extra length to bend once in place. The coil fit, just, snugly, into the cavity in the intake manifold. And the 1/4" tube seals fairly well in the holes in the plate. Maybe I'll put a bead of RTV to make sure.

 

On my '76 F250 4X4 360 the choke heat comes from a tube coming out of the rear of the passenger-side exhaust manifold.