When running fuel is being pushed into the carb so can't go the other way when running to even start the siphon.
Bowl being full the float needle would be closed stopping any siphon from starting.
And the big one the fuel inlet is above the fuel level with no connection below the upper fuel level in the bowl.
Dave - - - -

 

No, I mean being siphoned through the idle circuit .--not back through the fuel line. I have straight six jeep that does this because the carburetor is undersized-- in order to get a decent idle I have the throttle adjusted so far that it exposes the enrichening hole, which drains the entire bowl contents overnight. Thinking a similarly maladjusted YF could also do this?

Again, I had a 4.9 with a Carter YF and never had this issue even after sitting for a week in during a Virginia summer. I've been wrong before but I'd investigate this before resorting to diesel-gas.

EDIT: also, how is your bowl vent hooked up? A lot of these trucks around today are hacked together. If your bowl vent is left open this could definitely cause a speedy evaporation.

 

The Carb can not siphon through the idle circuit as the air bleed for the circuit it at the highest point of the siphon. the enrichment circuit is also fed from the same idle circuit The only way this could happen is if the air bleed is blocked/plugged and that would lead to a rather poor and overly rich idle. Gasoline is continually getting lighter and lighter as the refineries can get more gasoline out of a barrel of crude with synthetic refining techniques, the heavy chains are able to be modified to provide a higher volume of lighter chains most heavy crudes now have a higher market value than lighter crude a reverse from was years ago. And since all modern vehicles are using pressurised fuel systems it is not an issue having gasoline with a higher percentage of lighter hydrocarbon chains for operational considerations. So evaporation is becoming an issue in unsealed fuel systems (carburettors) the only real solution is to lower the temp of the fuel system (not always practical) or increase the vapour/evaporation point of the fuel.

 

Thanks you all, that what i would think but the line is empty so the fuel must be going somewhere. I replaced the fuel pump yet it still does it now it has me wondering if i installed fuel pump wrong. But i felt the cam lobe i put the arm on top of with flat surface

 

Maybe I missed it, but how do you know the fuel line is empty?

As Matthew has previously suggested, if the line was indeed empty (and that was the only fault), you wouldn't necessarily know. You could totally disconnect and cap the fuel line to the carb, and the fuel sitting in the float chamber should be enough for the next start, and probably good for about 45-60 seconds of run time at idle. With everything connected as normal, that time is more than adequate for the pump to refill the lines as needed and keep the float chamber filled to the correct level.

I highly suspect you have two issues at play here.

The first and most important problem is the fuel is escaping overnight from the float chamber. Lots of ideas bandied about. It could be accelerated evaporation due to heat, cheap gas, etc. The problem could also be internal leakage from the carb, such as the accel pump previously mentioned. This would leak down inside the intake manifold, so it wouldn't be obvious. Remember, there's no way for the input line to siphon fuel back out of the float chamber. Picture a shower head filling a bathtub. That's what the outlet from the float valve is like. For a siphon to work, you have to have a continuous tube immersed into the liquid. You don't have that with a float valve spraying into the float chamber from above. Even if the float valve stuck open and overflowed the chamber, as soon as the fuel level was siphoned down a little bit, the continuous path is broken, and the siphon action stops. So I wouldn't worry about any potential siphon action. Instead, figure out where that fuel is really going.

For the second problem, let's go back to the fuel line. Remember you'd never notice this if the fuel wasn't disappearing from the carb. I bet it happens on a lot of vehicles, but is never noticed. Once the pump gets going above cranking speed, it may move fuel just fine. You say you have a new fuel pump, but please don't convince yourself that it must be good because it's brand new. If the internal check valves aren't fully seating, this would allow fuel to drain back with the engine off. Please note I didn't say to rush out and buy a new fuel pump. But do please refresh our memory about how you know the line is draining back.

It would be a good idea to check some basics, too. For one, tee in a pressure test gauge at the carb inlet and see what pressure you have with the engine at idle.

 

Update problem still exist but i put marvel mystery oil in fuel plus i installed a secound valve to keep fuel from emptying or evaporating. This weekend i am going to try adjusting the float on a backup carb i have to see if maybe that can help. The idea behind that is so that it wont drop so much and maybe keep it close longer so that the gas won't evaporate or return to tank so quickly. The float gets stuck sometimes when float goes low. Then it goes back to normal.. I saw someone else posted about similar problem

 

Again fuel in the bowl can't return to the tank so adjusting the float fir that is a waste of time. If the fix the sticking then yes adjust away.

Do you have the bowl vent hose connectedto the carb as I don't remember anyone asking.
If not hooked up then hook it up, hose should go to the charcoal canisters under the battery.
Dave - - - -