Dry Carburator ????
Thread Starter
Disabled by request 8 JUN 2025
Joined: May 2019
Posts: 329
Likes: 94
From: St. Augustine, FL
Dry Carburator ????
'81 small block.......I have a quadrajet on my stock '81 with aluminum intake that was rebuilt by National Carburetor in Jacksonville,FL. When starting for the first time each day it takes a bit of cranking (choke closed) to start the car. After that it starts on the first spin of the starter. I pulled the air cleaner off today before initial start of the day with the engine stopped and looked down the carb throat while working the throttle and there was no accelerator pump working. My deduction was that the carb bowl was dry. After it started the accel pump worked fine and the car runs flawlessly. Engine temp every time was 180-200 degrees. Is it possible the bowl percolates dry after shut down?? My infra red temp gun showed carb bowl temp to be 130 degrees and thermostat housing to be 180 degrees .Heat soak?? Are idle passage plugs still a problem?? The carb gasket is the 1/4 inch open hole type provided by the carb rebuilder. Would using a 4 hole gasket help??......Lars HELP.
Race Director
Joined: May 2006
Posts: 18,734
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Check valve in fuel pump not working.
fuel filter in carburetor not equipped with check valve.
least likely, leaking plugs in carburetor body.
email lars at v8fastcars@msn.com
fuel filter in carburetor not equipped with check valve.
least likely, leaking plugs in carburetor body.
email lars at v8fastcars@msn.com
Last edited by MelWff; Nov 11, 2019 at 07:46 PM.
Drifting
Joined: Jul 2015
Posts: 1,765
Likes: 254
From: Catlett VA
I have this problem in my 68 bb convert. When the engine is hot, turned off and left to sit for over 30-45 minutes, the hot engine boils off all the gas in the fuel bowls on my QFT double pumper. (no problem if the engine is turned off while it is still relatively cold). A thick insulator gasket/spacer under the carb helped a little, but I finally changed to an electric fuel pump. Now I turn the ignition key to the "on" position, listen for the fuel pump to run for 15-30 seconds as it re-fills the fuel bowls and then turn the key to the "start" position and the car starts instantly, instead of cranking the mechanical pump for half a minute. Note: to confirm that heat sink is boiling off the gas and not some other problem, like bad needle and seat, get the engine up to operating temperature, park it in your garage or some quiet place, turn off the engine and listen for the bubbling sound of the gas percolating. I have glass sight windows on all four corners on my carb and I can see little bubbles as the gas boils.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
There are two possibilities:
1. The carb is 'dry' as you suspect:
2. The accelerator pump in the carb has a bad seal on it.
The latter item is the most likely. But here is how to test to find which one of those possibilities is really the cause. In the morning, go out to start your engine....but DO NOT depress the accelerator pedal. Just crank it so that fuel will be pumped into the carb bowl (this assumes that your carb is draining back to the fuel tank when sitting idle). Now, open the hood, remove the air cleaner, and look for getting squirts into the primaries when you pump the carb throttle.
If you get good squirts of fuel, your carb IS draining fuel back to the tank. SOLUTION: Buy a fuel filter with a back-flow prevention valve on it. A buck or two more, and that should solve your problem. P.S. Use TWO wrenches to release that fuel line nut going into the carb body!!!
If you do NOT get any squirts of fuel into the primaries, your problem is a bad seal (most likely cause, anyway) on the accelerator pump in the carb. If your seal is not of modern materials, it will swell and fall off the piston when you put ethanol-laced fuel into your tank. No seal....no squirts. You can buy just a new accelerator pump WITH an ethanol-capable seal at most GOOD auto part stores.
1. The carb is 'dry' as you suspect:
2. The accelerator pump in the carb has a bad seal on it.
The latter item is the most likely. But here is how to test to find which one of those possibilities is really the cause. In the morning, go out to start your engine....but DO NOT depress the accelerator pedal. Just crank it so that fuel will be pumped into the carb bowl (this assumes that your carb is draining back to the fuel tank when sitting idle). Now, open the hood, remove the air cleaner, and look for getting squirts into the primaries when you pump the carb throttle.
If you get good squirts of fuel, your carb IS draining fuel back to the tank. SOLUTION: Buy a fuel filter with a back-flow prevention valve on it. A buck or two more, and that should solve your problem. P.S. Use TWO wrenches to release that fuel line nut going into the carb body!!!
If you do NOT get any squirts of fuel into the primaries, your problem is a bad seal (most likely cause, anyway) on the accelerator pump in the carb. If your seal is not of modern materials, it will swell and fall off the piston when you put ethanol-laced fuel into your tank. No seal....no squirts. You can buy just a new accelerator pump WITH an ethanol-capable seal at most GOOD auto part stores.
Last edited by 7T1vette; Nov 12, 2019 at 01:02 PM.
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
Same situation on my '69. If I just start the car and quickly shut it off, it will stay full of gas for weeks. So, I've concluded that it is not leaking or draining back, but boiling or evaporating. A insulating gasket probably doesn't help much because everything in the engine compartment is really hot. Modern gas probably makes this worse.
Unrelated to this, my research back in the day, concluded that a 4 hole gasket was the best for my car for performance.
Unrelated to this, my research back in the day, concluded that a 4 hole gasket was the best for my car for performance.
Last edited by Sayfoo; Nov 13, 2019 at 02:43 PM.
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Drifting
Joined: Jul 2015
Posts: 1,765
Likes: 254
From: Catlett VA
Sayfoo- I was pulling my hair out trying to figure out what was wrong with my dry carb. On day I parked the car in the garage after driving it for about an hour and stayed in the garage working on some stuff on my work bench. I kept hearing this peculiar noise and finally tried to find what it was. I tracked it to my car and it was coming from the engine compartment. I opened the hood and this bubbling noise was coming from the carb. My QFT carb has glass sight windows on all four corners showing the fuel level in the fuel bowls. I could see little bubbles in the liquid gas and over the next hour watched the fuel level go down and down and finally there was no gas left in the carb. It still boils the gas off when the engine is hot, but my electric fuel pump re-fills the carb if I let the pump run for 30 seconds before turning the key to start.
Instructor
Joined: Mar 2011
Posts: 165
Likes: 31
From: space coast florida
"freeze" plugs
If you take the throttle plate off the carb, you will notice two freeze plugs underneath the float bowl. These frequently developed leaks, draining into the manifold. Back in the day, you used to remove those and put plugs with o'rings in there.
Sold many of those when I was selling auto parts. Probably can't find them today. Nor a real auto parts store with an auto parts salesmen.
Sold many of those when I was selling auto parts. Probably can't find them today. Nor a real auto parts store with an auto parts salesmen.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
it is not floating up out of the bowl and disappearing past the needle and seat to drain back into the tank. the mech fuel pump has 2 one-way valves to stop fuel in the line from draining back. it is either evaporating or the leaking plugs let it drain into the intake.
Last edited by derekderek; Nov 15, 2019 at 05:46 PM.
Thread Starter
Disabled by request 8 JUN 2025
Joined: May 2019
Posts: 329
Likes: 94
From: St. Augustine, FL
"The most common cause of Q-Jet start problems, if it occurs after the car has been sitting overnight or longer, is that the fuel standing in the fuel line between the pump and the carb is draining back through the fuel pump checkvalve due to a slight leak in the pump's internal checkvalve. Since the Q-Jet's needle and seat is in the bottom of the float bowl, the fuel draining back through the fuel pump will actually siphon fuel out of the carb's float bowl.".....Quote from one of Lars papers.
Race Director
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