ok i have done a very intense cleaning and rebuild of my carb. 250 inline single barrel. truck still will not run without the choke on, even with it on cant pull a hill or get above about 45 mph. i checked vacuum in the intake and its right on 20, that is with the choke on because thats the only way it will run. if you turn choke off you can hear a sucking sound from carb and engine dies. i have checked all possible vacuum leaks found none. could this possibly be in the ignition timing??
My 1st guess if choking keeps it running is a vac leak, but since you are having problems under power as well I'd guess something else.
For sure check the timing, anywhere from 0 BTC to 16 BTC it should idle just fine without the choke. Beyond that check the whole curve, but if it is acting up, just rev it a bit with the light on it, and anything between 20 BTC total and 40 BTC total it should run and cruise and accelerate fine.
My guess is that something in the carb is messed up - either float level is WAY low, or something in the fuel circuit got dirt or a gasket or something lodged in it.
WHich carb do you have?
Rochester B or BC or BV?
Carter YF?
Or the dreaded Rochester MV?
A buddy of mine once got so mad at the Rochester MV on his truck he punched out his own windshield with his fist. The truck was not impressed and it still didn't start. But, he told me he did feel better.
its a rochester B carb. thing is you can start it, get the engine warm and push the choke off hold the accelerator to the floor and it will run very rough for a short time, then die. If drive down the road with the choke on it will run smooth but once you get around 40-45 mph and try to give it more accelerator to go faster you hear a sucking sound in carb and sounds like its bogging down till you ease back off the accelerator. i have took the carb apart cleaned it in paint thinner then with carb cleaner, blew it out with the air compressor, put in a rebuild kit, all this has not helped a bit.
It might be that a spring or ball in the power valve area might be installed incorrectly or missing a part. Go online and get an exploded view of the carb and make sure all the parts in assembled and are there.
Rap
Had an inline filter plug once an fuel flow to engine was reduced to the point that had no power and a max speed of 35 or 40 mph. Changed filter and problem was solved. Some times we over look the simple things.
x2 on the fuel filter and check fuel pump too although just about anything should be enough flow at idle.
Assembling the main jet tower on that Roch. B is tricky, the gasket has to seal and everything has to be aligned just right or the power valve needle can stick open or shut. Maybe the wrong ball is in the power valve recess(too big)? Any misalignment could force a vac leak between the tower and the carb top, and this is where all the fuel flows up from the main jet so if there is a leak there, the carb would misbehave all over, much as you describe. I'd bet on that as the leak point.
If the carb checks out, and you have gone through the fuel pump, you might also check the fuel tank pick up. Sometimes the screen tube or bag can come off and varnish builds up like a little ball of epoxy at the end of the pick up tube. allows enough fuel to get started, warm up and even drive a little but not enough fuel to step on the gas of climb a hill.
after much blood, sweat, and prayers found the problem. dont know what its called but there was a small brass plug not much bigger than a tylenol that plugs into the end of a tube on the bottom side of the air horn. it had come out, not sure what was supposed to hold it in, but wrapped it with teflon tape put it back in hole and fired right up running great far as i can tell. anyone know what that may have been. not in any of the diagrams i have.
peen it in there so it won't come back out.