Gas syphon in back out/ fuel pump?
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More Turbo
Joined: Feb 2001
Posts: 526
Likes: 80
Gas syphon in back out/ fuel pump?
‘syphoning’ !
On my 4.9 Inline Six I have a Motorcraft 2bbl carb.
Every time I have to start the truck for the first time of the day, it cranks for a long time. 15 - 20 seconds, before the carb gets gas and starts. Anybody else experienced this?
It’s done this for the 25 years I’ve owned this truck. It seems like the gas is siphoning back out of the carb on shutdown. And the fuel pump needs to pump until it fills the bowl back up.
Would like to solve this problem, I don’t think it’s an ignition or engine problem.
Would low compression cause this? It is a very high mileage engine and probably has considerable blow by.
On my 4.9 Inline Six I have a Motorcraft 2bbl carb.
Every time I have to start the truck for the first time of the day, it cranks for a long time. 15 - 20 seconds, before the carb gets gas and starts. Anybody else experienced this?
It’s done this for the 25 years I’ve owned this truck. It seems like the gas is siphoning back out of the carb on shutdown. And the fuel pump needs to pump until it fills the bowl back up.
Would like to solve this problem, I don’t think it’s an ignition or engine problem.
Would low compression cause this? It is a very high mileage engine and probably has considerable blow by.
Last edited by jzjames; Apr 20, 2026 at 04:41 PM.
Hotshot
Joined: Jul 2020
Posts: 10,519
Likes: 2,654
From: Near Salt Lake City
More likely it is "boiling" or "percolating" out
Not a whole lot you can do about that
It might have a leak in the power piston allowing all the fuel to drain into the intake where it also evaporates out
If you have not overhauled that 1bbl carter Motorcraft carb, I'd start right there
The main power piston in those carbs controls the metering rod traveling into the jet, and also is the accelerator pump
You'd be surprised how well they run with a new diaphragm in them
Not a whole lot you can do about that
It might have a leak in the power piston allowing all the fuel to drain into the intake where it also evaporates out
If you have not overhauled that 1bbl carter Motorcraft carb, I'd start right there
The main power piston in those carbs controls the metering rod traveling into the jet, and also is the accelerator pump
You'd be surprised how well they run with a new diaphragm in them
Thread Starter
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More Turbo
Joined: Feb 2001
Posts: 526
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It’s a 300 I6. With the Offenhauser intake and a Ford 2bbl carb.
‘Runs great but won’t start until the fuel is pumped back up into the carb - every time when cold.
Now I’m wondering if the low compression engine is causing this because when engine is cold the compression would be the lowest, I.e. longer cranking to get it to fire.
‘Starts on first click when hot. IDK.
‘Runs great but won’t start until the fuel is pumped back up into the carb - every time when cold.
Now I’m wondering if the low compression engine is causing this because when engine is cold the compression would be the lowest, I.e. longer cranking to get it to fire.
‘Starts on first click when hot. IDK.
Fleet Mechanic
Joined: Aug 2007
Posts: 1,834
Likes: 260
From: Piney Woods of East Texas
Manual choke or electric? Low compression will not effect how long it takes to get fuel. That is based on the output of the fuel pump. May want to try a thermal insulator under the carb to minimize the boiling of the fuel. You could add an electric fuel pump, then it would only be a few seconds before the bowl is full.
Hotshot
Joined: Jul 2020
Posts: 10,519
Likes: 2,654
From: Near Salt Lake City
The 2bbl has a leaking power valve ?
I believe that can drain a 2bbl Motorcraft
They also might have well plugs that can leak
It's been a while since I've looked at one
It still could be percolating or evaporating out
I believe that can drain a 2bbl Motorcraft
They also might have well plugs that can leak
It's been a while since I've looked at one
It still could be percolating or evaporating out
Hotshot
Joined: Jul 2020
Posts: 10,519
Likes: 2,654
From: Near Salt Lake City
It’s a 300 I6. With the Offenhauser intake and a Ford 2bbl carb.
‘Runs great but won’t start until the fuel is pumped back up into the carb - every time when cold.
Now I’m wondering if the low compression engine is causing this because when engine is cold the compression would be the lowest, I.e. longer cranking to get it to fire.
‘Starts on first click when hot. IDK.
‘Runs great but won’t start until the fuel is pumped back up into the carb - every time when cold.
Now I’m wondering if the low compression engine is causing this because when engine is cold the compression would be the lowest, I.e. longer cranking to get it to fire.
‘Starts on first click when hot. IDK.
See if it fires right up on ether, cold
Mountain Pass
Joined: Dec 2020
Posts: 203
Likes: 24
Try a phenolic spacer it stopped my fuel percolation mine was enough to do what your explaining.
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More Turbo
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’See if it fires right up on ether, cold’
I am going to try this. I dont think I have a boiling fuel situation with this carb, But I could be wrong.
All this long cranking every day has me worried what it’s doing to my ring gear! I keep trying to figure this out.
Wouldnt a low-compression condition take more cranking when cold ? Fires right up on restart.
(No choke on this carb! I put a manual choke on it at one point and it made no difference. Pump pump pump. Crank crank crank.)
I am going to try this. I dont think I have a boiling fuel situation with this carb, But I could be wrong.
All this long cranking every day has me worried what it’s doing to my ring gear! I keep trying to figure this out.
Wouldnt a low-compression condition take more cranking when cold ? Fires right up on restart.
(No choke on this carb! I put a manual choke on it at one point and it made no difference. Pump pump pump. Crank crank crank.)
Last edited by jzjames; Apr 22, 2026 at 01:41 PM.
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Hotshot
Joined: Jul 2020
Posts: 10,519
Likes: 2,654
From: Near Salt Lake City
Yes
What is the compression reading?
Hot restart or cold restart, the compression readings should be the same
How long have you had it running with the Offy intake and 2BBL on it?
Long enough to get the rings broken in, right?
I would assume the engine is fine until you check the compression if it starts and runs
you could check the "wet" compression
We used to fire engines up, that have sat for a long time, on trans fluid and ether or gasoline down the horn
Those had low compression until the rings got seated
What is the compression reading?
Hot restart or cold restart, the compression readings should be the same
How long have you had it running with the Offy intake and 2BBL on it?
Long enough to get the rings broken in, right?
I would assume the engine is fine until you check the compression if it starts and runs
you could check the "wet" compression
We used to fire engines up, that have sat for a long time, on trans fluid and ether or gasoline down the horn
Those had low compression until the rings got seated
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