Dies when you get up to highway speeds
 

I recently bought a 76 f250 super cab with a 460. The truck ran ok in town I recently bought a 1976 F250 super cab with a 460. It ran fine in town but when I got it on the highway it flooded out and died. The air filter was clogged so I figured that was it. I replaced it and it ran fine for a few trips then it happened again today twice when I gave it more gas. Both times when I removed the top of the breather it fired right up. I was thinkin it could be bad gas but why would it start without the breather? Any ideas? I posted this yesterday on new forum but thought this was a better place.

Did you notice if the choke butterfly in the top of the carburetor throat was staying open? If that butterfly can move around and close it will choke off the engine due to lack of incoming air. Let us know what you found.

The butterfly is open. I did do some cleaning on the top of the engine. I had the top of the carb covered. And was spraying brake cleaner on a rag and wiping around the carb. I noticed this morning that the positive wire to the choke was all but broken. I'm looking for a connector to reattach. Do you know how the flap in breather works?
The one that switches from exhaust to cold air.

There is supposed to be a temperature sensor in the air cleaner housing. Some are vacuum controlled and others don't use any vacuum. When the engine is cold the incoming air to the carb is supposed to come up from the exhaust manifold through a snorkel and into the air filter. Once the sensor determines the air is warm enough it opens the snorkel to outside air. If the electric choke is not heating the element to hold the choke open the force of the incoming air will close the butterfly and starve the engine for air. I believe that is probably the only thing causing problems. That wire comes from one of the alt terminals . If you correct the electrical connection it should take care of the choke problem.

Reconnected the choke and took it out for a drive and it happened again. I get out and take the breather off and it starts back up. When I got it home I sat there and revved it up trying to get it to flood out and die but it won't do it. The flap in the breather I asked about is not connected to anything that would open it. It is just spring loaded. I did remove some duct tape that was covering holes in the bottom of the breather yesterday when I was cleaning it up. Maybe there is not enough suction to open the flap now. This only happens when the breather is on and driving at higher speeds.

As far as I know that door, as it is called, does not rely on air to make it open. If the mechanism to make it open is missing, just find something to hold it open. It is only needed when starting the engine in cold weather.

Thanks for the input. I did find the sensor in there. But I drove truck home without the breather on and it still died. I don't think it's bad gas because the truck wouldn't run fine in the driveway. Maybe a carb issue?

Are you sure it is getting overfueled? You are telling us it flooded out , that would mean being overfueled. Could it be starving for fuel ? High speed places more demand for fuel . Insufficient fuel could be caused by a partially plugged fuel filter. Did you check that out? A 460 needs a lot of fuel and maybe it isn't getting enough at higher speeds.

Please check to see if your fuel cap is vented. I think Hanky is on to something with starving for gas and a non-vented fuel tank would do that.
Eric

Thanks Eric,
Just goes to show you , you can't think of everything ! Very possibly a simple thing like the wrong fuel cap.