Does gas syphon back out of the carb?

[jzjames]

On our trucks does the gas somehow syphon back out of the carb when shut off for a while? (overnight)? Results in long engine cranking to get the gas back up there to start.
This has been the case since I’ve owned it, I’m pretty sure it’s cranking to refill the bowl with gas before it will start.
Why would the carb be empty of gas after being shut down for a while? Is it the fuel pump?

It was true on my previous 1bbl, and now my 2100 2 bbl.

[Jbarnes79]

The only reason I can think of why this would happen is the fuel is boiling off from engine heat after shut down. That or the float valve has deteriorated.

To see if it's a heat related issue, get the engine started and let it idle just for a few minutes to give the bowl time to fill up then shut it off and wait until the next day. If it fires right up the next day then that's likely your problem.

[Fraser]

Fuel cannot syphon out of the carburetor because the fuel level is below the needle and seat, also, a cab mounted tank is rather high, not a lot of pullback. Check that all heatshields are in place, particularly the thick base gasket, if your application didn't have one, think about a custom set up. A series of aluminum plates and gaskets to create a "sandwich" used to be available, but could be fabricated. Also, the fuel line could be re-routed to a cooler location, they are sometimes rather close to heat sources, and the pump itself can benefit from added shields, it could heat soak and evaporate inside, creating a mini vapour lock that doesn't pump at first.
E15 fuel evaporates quicker than real gas.

[Fraser]

Just thought of this, if you have dual tanks, turn the valve to off when you park, turn it back in the morning before trying to start, you will have the same starting problem, but will have confirmed no fuel backflow.

Finally, a reason for that "off" position!

[jzjames]

Maybe it’s the intake manifold? The long runners on my I-6 engine.
Every time on starting (cold) I have to crank for like 10 seconds before it will fire. If engine is still warm from running it fires right up.
It’s a high mileage 300 inline six, maybe it’s got reduced compression when cold.

[cep62]

Before you try starting it look down the carb and pump the throttle . You should see a stream of gas.
If not it may have leaked out or your accelerator pump might be weak.
Also check to see if the choke closes properly when it's cold. That makes a big difference.
Do you have an electronic ignition? with a good spark ? that helps too.
A cold start requires more fuel. I've run chokless vehicles through Michigan winters ,
I would just keep pumping the gas and crank every now and then .
Eventually it would start up , then die ,repeat until it idles.
A good operating choke is a wonderful thing. good fuel injection spoils a person.
I have a 77 that I plow snow with , it has a hand choke Holley. It works good once you figure it out. but you have to
know how to drive it.

[jzjames]

Ok, thank you. It doesn’t really have a choke.
I rigged up the hand choke once and it didn’t do anything. Think I will connect it again and see if it makes a difference.
The more I think about this it might be because the compression is probably on the low side, maybe very low.
You know the sixes will run forever, right?