The Stovebolt Site Forums The Shop Area Engine Shop Anyway to keep carburetor primed?

My 47 runs like a sewing machine. It sounds and performs great. All new tune up parts, NOS replacement distributor, rebuilt carburetor. Here is my problem. If it sits for more than a day the gas drains from the carburetor and fuel line. No leaks, just appears to backflow down to fuel pump, which is a new carter mechanicail pump. I have to either crank and crank and crank until the fuel fills the line again or squirt a little shot of gas into the carburetor to start it. Once it has started I can shut it down and it starts right back up. Sit for a day or more and same thing. Is there anything I can do? One other observation. I have an inline clear fuel filter. right before the carburetor. It is a new generic one that is horizontal. It is never more than half full. Engine off or on. Only about 1/2 to 1/3 of the actual filter is sitting in gas. I can't even see how it is pulling gas out to the carburetor. Maybe not an issue but it just seemed strange to me. I thought it should be full.

Felt like I was reading about my own trucks! I was dealing with this just yesterday since I don’t drive them as often in the winter.

I’ve been thinking about installing a fuel line check valve between the fuel pump and the carburetor᠁ ideally between the fuel pump and the filter as you described.

I’m sure someone will soon let us know if a check valve will do the trick or make a better suggestion.
Greg

BTW - I’ve never seen my fuel filter 100% full᠁. It always manages to have some amount of air trapped in it.

I have put an electric fuel pump in line back near the tank with a spring return switch. You hold the switch on for a few seconds & it is all primed.
It also helps if you pump the gas pedal as you crank the engine. The accel. pump pulls off or near the bottom of the float bowl. When a little fuel gets in the bowl the pump will prime the carb. Other wise you have to fill the bowl all the way for the air pull the fuel from the outlet.

George

Starting after vehicle sits for a period of time

Jon

The fuel pump itself has a check valve in it, so perhaps you need to rebuild it. The kits seem kind of pricey but they will have the new diaphragm material which is resistant to ethanol and it is a pretty easy job to do. Pay attention to which direction the check valves are installed. My trucks can sit for a least a couple of weeks before the gas in the carb bowl gets low enough that you have to crank excessively for them to start.

Regarding the "air" in the fuel filter that is normal and is not a contributing factor to your problem.

Kent

I had same issues I installed a low psi electric fuel pump at the tank hooked up to a toggle switch off the ignition switch will only power up with ignition switch on turn it on for about 10-12 seconds and truck fires right up 235 with manual choke Rochester carburetor

The small electric fuel pumps that look like a metal fuel filter with a couple of wires will flow fuel without running. A mechanical fuel pump will pull gas right through one. Use one of them with a push button switch as a boost pump to prime the fuel system on a garage queen that sits more than it gets driven. If the fuel system loses its prime in less than a week, fix or replace the mechanical fuel pump.
Jerry

I have a brand new fuel mechanical fuel pump. I loose the prime in 24 hours. I guess I will be shopping for an electric fuel pump to prime the system . The challenge will be to find a 6v one.

If you search the internet for "6 volt electric fuel pump", quite a few sources come up.

Cheers, Dean

Wasn't as hard as I thought. Amazon. 6v designed as assist or primary. Designed for unlead and alcohol based gas.

"New" doesn't necessarily mean "good". Have you tested the pump to see if the one way check valves are working?
Jerry

I have pressure tested but I am not sure how to verify the check valve.

Sorry, I don't understand the concern about a check valve.

The fuel in the carburetor CANNOT possibly backfeed to the pump (unless you guys are parking the trucks upside down - hard on the paint on the top of the cab )! The fuel inlet in the carburetor is located ABOVE the fuel level in the carburetor; and there is no direct connection for a syphoning action.

It might be possible that the fuel in the fuel line would backfeed through the pump back to the tank if the pump inlet check valve is bad; but if the fuel was not evaporating out of the carburetor, the carburetor bowl would contain sufficient fuel to start the engine, and run it until the fuel pump regained its prime.

Check out the Reid Vapor Pressure that efi will withstand (modern fuel) then check out optimal Reid Vapor Pressure for a carburetor.

Jon

There is really a very simple test to check out the above.

The next time you think that the system has lost its prime:

(1) acquire a clean squeeze bottle with a fairly small entrance/exit (mustard or ketchup squeeze body)
(2) fill the squeeze bottle with fresh fuel
(3) open hood
(4) remove air cleaner
(5) using the squeeze bottle, fill the carburetor bowl with fuel
(6) replace air cleaner
(7) close hood
(8) enter cab
(9) pump footfeed 3 times, and remove food from footfeed
(10) start truck

If it works, forget the check valve.

If is doesn't work, my apologies

Jon

That brings back memories. My mom used to use that term for the accelerator pedal when I was a kid. Of course she grew up learning to drive a Model T that only had a "handfeed". My Model A had both a footfeed and a handfeed. Mom hasn't driven for about 15 years now, so doesn't use that term much anymore. Mom is going to be 100 in a little over a month.

Kevin - thanks for sharing. And wish your Mom a really happy birthday from all of us when the event occurs.

Not quite that old, but I do remember handfeeds and footfeeds quite well. Also hand spark advance.

Jon

Learn how to use the choke properly, also. Fuel injection systems and automatic chokes have pretty much eliminated the necessity for any thinking to be done when starting a cold engine.
Jerry

I saw a very early race car the other day on tv. It had a hand pump on the side that was used to pressurize the fuel. I don’t know if it pressurized the tank but I get the impression that you to keep doing it on and off while driving!

The guy I got "my " truck from is not a car guy at all. But, in the 15 years he owned the truck, he never knew it had a manual choke. The button on the dash was replaced with a generic lawn mower looking button without a letter C on it but he was priming the carb with gas or ether for 15 years because he didn't know to pull oput the choke to start the truck.

I have posted the test procedure- - - -repeatedly- - - - -and everyone ignores it. It was even a sticky thread at the top of one of the forums for a while. Jon is right about the float bowl being unable to drain unless there are other problems with the carburetor.
Jerry

Thank you carbking. I could not understand the confusion. The carburetor fuel containment system works sorta like an old toilet fill system. The tank always contains the water until it is flushed. The fuel in the carburetor float bowl can never to back into the fuel feed line, the float pushes on the valve to prevent that. Even if the fuel level drops and the float is not pushing on that valve how do you think the fuel could then just climb up past that valve, and down that fuel line? Especially since there is a good distance that the fuel would have to jump to get there.
Now if your heating and evaporating the fuel then yeah it could disappear that way, or a hole or crack in the float bowl, pressure build up in the float bowl could force the fuel out, but those float bowl vents are not prone to allowing that.
I really don't understand what is going on and how a carburetor needs this constant fuel addition (priming) unless your in a very hot climate and the fuel is evaporating.

The Quadrajet could leak fuel from the float bowl by way of the main jet wells, rebuild kits would contain a foam gasket to help seal the welsh plugged bottom of the well. I can't remember if the 2G has the same set-up as the Q-Jet, what say you carbking?

The 2G has a couple of drive plugs, but they rarely, if ever, leak. The early Q-Jets (1965~1967) did leak at the well plugs, and the foam gasket helped the mind of the rebuilder, not the actual leak

There are now excellent "fixes" on the market or repair the 1965~1967 Q-Jet well plugs.

Rochester fixed the problem in 1968, so no need to worry about the later Q-Jets.

The fuel simply evaporates from the bowl vent.

Jon

Thank you Sir for that clarification.

Let me clarify, I didn't think gas was draining out of the bowl back into the fuel line. I should have been more clear. Once the fuel line is primed and gas is in the carburetor the truck runs fine and will start right up until the gas in the bowl evaporates. There is no indication it is leaking at the carb. I do have a small squirt bottle with fuel. I remove the air cleaner. Give the carb a squirt of gas, get in the cab, pull the choke, turn the key on, hit the starter. The truck starts right up. Sometimes it continues to run and sometimes it runs until it burns all the fuel I squirted into the carb and I have to repeat the procedure until the fuel line is filled back up with gas. Most times I don't have to repeat the procedure more than twice. Once it starts I leave the choke out, pull the throttle cable out slightly to increase the idle RPM and let it run for a few minutes. I then push the choke and throttle cable back in. the truck will sit and idol. If I turn it off, t starts right back up. Later in the day it will start right back up. 1-2 days later, I have to repeat the squirt bottle procedure. As stated before, if I just crank it long enough it does pump the gas into the line and will start but I don't like putting that kind of stress on the starter and battery. I did purchase an electric fuel plump I plan to add to eliminate the need for the squirt bottle. I will also perform the check valve test. I needed an excuse to buy a pressure/vacuum gauge anyway. Even though I have not seen any signs of a leak at the carb I am going to do a closer inspection. It is hard to believe the gas in the bowl is evaporating in 24 hours.

Yes, those brass era cars require hand pumping of the fuel pressure (unless they were gravity fed like Model T Fords). The hand pump pressurizes the fuel in the tank.

Once that's done, the fuel pump will generally be able to handle everything from that point on. Once you stop and want to restart, you have to pump up the tank again.

Dave - the length of time required for the bowl fuel to completely evaporate depends on several factors:

(1) type of carburetor
(2) type of engine ("V" or inline)
(3) to a certain extent, the ambient temperature

Aluminum bodied carburetors have greater evaporation
Carburetors with bowls directly above the throttle body have greater evaporation (a Carter AFB has much greater evaporation than a Holley 4150)
"V" engines (where the carb is directly above the "heat sink" a.k.a. engine) have greater evaporation than inline engines

Consider the Carter AFB; there are no plugs below the fuel line, and syphoning back through the fuel line is impossible.

I have two on my truck. If the engine is fully warmed up, the bowls will be empty 30 MINUES after shutdown.

After determining the issues, I modified the carbs to run with an electric pump, and I use the electric pump full time.

Jon

Thanks. I have a Carter - DTE2, straight 6. I am definitely going with the electric fuel pump. As stated earlier, the one I bought is designed as stand alone or as a mechanical pump assist. I am wiring it with a momentary push button switch.

dgrinnan, you could perform a quick test. With a cold engine, use the starter to crank and fill the float bowl, once you know the line and bowl are full stop cranking, don't start the engine. Leave the truck until the next day, remove the air cleaner, look down the throat of the carb, actuate the throttle rod and see if you have fuel by way of the accelerator pump. If the cold carb is empty in 24 hours I would suspect it is leaking. Just my 2c