Why my 1971 250 dt only idles with choke on, stalls with choke off, idles high when started?

Usually a carb cleaning will solve this. The low speed jet has a very small diameter hole and just varnish from old gas can clog it,

You don't say what make of car but it's about the same for all. Your problem is the choke adjustment. The choke raises the idle when the car is cold and after warm up drops to normal. You are doing the chokes job when you hold the pedal down, once its warmed up the choke no longer functions till cold again. So check the choke and the high idle speed adjustment. Simple fix. Hope this helps mike

I'm assuming the truck runs ok once it is warmed up. Sounds like you need to make some adjustments. The cold idle speed is set too high, and the choke is opening too fast. The cold idle should be around 1500 rpm and the choke should take at least 90 seconds to open depending on air temp.

Take the Carb off and clean the ports and make sure the float isn't sticking

REPLACE COOLANT TEMP SENSOR..

Sounds like high speed adjustment is lean. Find the high speed adjustment (probably the only one) turn it out counterclockwise one half turn should do it. Hope that helps.

ok buddy on the carb will be some sort of screw and idle screw turn this in 1/4 of a turn with the machine running until it idles at 1000rpm also the choke will stall the engine when warm the choke is only for cold starts get back to me more info or problems

clutch needs adjusting on the lever also on the carb will be a mixture screw turn this out 1/2 a turn to allow more fuel in when choke is off get back to me if no difference

You will have to check the carbs it seems the gas isn't flowing in freely into the float chamber and filling it up.

Sounds like the choke needs to be turned down slightly and the idle turned up?