[Dick Boneske]

The fuel bowl does not empty through the fuel line, it empties due to evaporation through the vents on top of the carb. In warm weather, it takes a little over a week for all the fuel to evaporate. This is true of Tripowers, Quadrajets, and Holley carburators.

[AdamIsAdam]

Originally Posted by Dick Boneske

The fuel bowl does not empty through the fuel line, it empties due to evaporation through the vents on top of the carb...

That was my question and my thought. Does a check valve do anything? I guess not. Except impede fuel flow.

[Tom Vaught]

As Dick B posted, it is simply a normal physics process.
The fuel in the bowl overtime evaporates.
Then you have the idle & main jet circuit passages without fuel in them and the engine can't start without a rich air/fuel mixture on a cold start.

Many people do a pre-inspection of their vehicles (after they remove the car covers) and
check the oil, tire pressure, and fuel in the fuel tank registered on the gage.

Some have driven off with a couple of gallons in the tank and then obviously were left on the side of the road with a classic vehicle.
Rarely does the tank pick-up remove all of the fuel in the tank.

So I used to do all of those steps plus use a oil squirt can (with gas inside) and give the carb fuel bowls a GOOD shot of fuel before cranking. Seemed to work ok when you just stuck the squirt end of the oil can down into the carb vent plus a gas squirt on the blades. A EFI pump and Tanks,inc tank is a lot easier. (If you keep it full of gas), lol. I always verify oil pressure before the ignition is turned on.

Tom V.

[pfilean]

As fuel evaporates from the float bowl the float drops and allows air to enter the lines.. That will not empty the bowl but, depending on the angle the car is sitting at, may allow the fuel to drain back all the way to the tank for a long sitting car. Refilling the line will add to the cranking time to get the bowls full enough for the car to start.

A check valve should prevent this line drain back but may add some resistance to normal pull forward from the mechanical pump. But check valves are usually only added when an electric primer pump is added. They are needed in the normal line and in parallel with the electric primer pump to prevent the pump from just recirculating within itself.

I did that approach and it worked. But I thought the cross sectional area of the check valve was too small and adding resistance to the mechanical pump. So I took that out and replaced the check valve with a tank switchover valve intended for dual tank trucks. That valve had the original 3/8 fuel line cross section though the valve and also works. But I think with less resistance to the mechanical pump. Just another way to solve the problem.

[carbking]

Originally Posted by pfilean

As fuel evaporates from the float bowl the float drops and allows air to enter the lines.. That will not empty the bowl but, depending on the angle the car is sitting at, may allow the fuel to drain back all the way to the tank for a long sitting car. Refilling the line will add to the cranking time to get the bowls full enough for the car to start.

A check valve should prevent this line drain back but may add some resistance to normal pull forward from the mechanical pump. But check valves are usually only added when an electric primer pump is added. They are needed in the normal line and in parallel with the electric primer pump to prevent the pump from just recirculating within itself.

I did that approach and it worked. But I thought the cross sectional area of the check valve was too small and adding resistance to the mechanical pump. So I took that out and replaced the check valve with a tank switchover valve intended for dual tank trucks. That valve had the original 3/8 fuel line cross section though the valve and also works. But I think with less resistance to the mechanical pump. Just another way to solve the problem.

I learned a long time ago not to use the "never" word, but this should not happen with the original mechanical pump, as the check valves in the pump will prevent fuel from returning to the tank.

For starting with a squirt, I prefer to use a bottle (or oil can) with a small spout and fill the carb through the bowl vent. This allows the carb to function as the engineers designed it to function, and prevents burned eyebrows from a backfire if you use too much fuel. At my age the only hair left on my face is the eyebrows! Got to save something!

Jon.

[Dick Boneske]

Jon is 100% right on priming the carb through the vent in the airhorn instead of into the carb throat. It's safer, avoids washing down the cylinder walls with raw gas, and gives you a clean, smooth startup. I mentioned this earlier in this thread.

Attached are pictures of the "early" '57-'65 setups with the 1/4" pipe in the airhorn opening. The second picture covers all '66 and up Pontiac Tripower and 2 bbl. carbs. Both types have a hole plenty large for an oil can spout to squirt gas into. You only need to prime the center carb, the ends will fill after startup.

[pfilean]

I learned a long time ago not to use the "never" word, but this should not happen with the original mechanical pump, as the check valves in the pump will prevent fuel from returning to the tank.

I know you are right Jon. I thought that also. But I have had some that would be dry on both the inlet and outlet of the pump if you pulled the hoses. Laying under the car you would expect a face full of fuel. So I had to conclude the fuel had drained back. And I don't think there had been any alcohol in the system to screw up the check valves..

[AdamIsAdam]

In the FWIW category, I thought I'd pass along an interesting update on my car:

I finally got my new fuel system installed. It's a full 1/2" setup from pickup to pump, then 3/8" from pump to Qjet. All steel except a few inches here and there at pickup, in-line filter, and pump as usual.

Now, with the new fuel pump and no more rubber lines (I had a few feet of it before) the car FIRES RIGHT UP much faster hot and cold. And today it fired up after sitting for nearly a week. So I don't know if it's the new RobbMc pump that is just holding pressure better or a lack of evaporation/leaking from the lines/fittings or what, but it's noticeable now under every scenario.

[Tom Vaught]

Glad it is working well for you now!

Tom V.