Backfiring through carb
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Freshman User
Joined: Apr 2010
Posts: 28
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From: Alabama
Backfiring through carb
ive been racking my brain over this for a couple weeks now while i searched through dozens of old posts.
Backfiring through the carb is a sign of being to lean correct?
Well ive Turned the set screws almost completely out and no change!!!
I can rev it nice n high with no problems. Acc pump has been replaced and i can see fuel when i look into the carb and give it gas.
New Fuel filter on carb along with a inline fuel filter.(one of the clear ones).Cap,Rotor,Plugs n wires.Nothing seems to help.timing is dead on at 10deg.
Ive checked for vaccum leaks and have plugged all the ones i find.
Any suggestions would really help! Thanks guys
Oh and its a 77f100, 360 auto. with a 2150
Backfiring through the carb is a sign of being to lean correct?
Well ive Turned the set screws almost completely out and no change!!!
I can rev it nice n high with no problems. Acc pump has been replaced and i can see fuel when i look into the carb and give it gas.
New Fuel filter on carb along with a inline fuel filter.(one of the clear ones).Cap,Rotor,Plugs n wires.Nothing seems to help.timing is dead on at 10deg.
Ive checked for vaccum leaks and have plugged all the ones i find.
Any suggestions would really help! Thanks guys
Oh and its a 77f100, 360 auto. with a 2150
Fleet Mechanic
Joined: Feb 2007
Posts: 1,326
Likes: 48
From: Middle TN
A lean carb when suddenly floored will fall flat (like you cut the key off for a second) then it will backfire through the carb and then many times it will come on strong for a few seconds before leveling off.
If this is how its acting it's probably starving for fuel.
One thing I would check is to make sure you are getting flow from the tank. The ethanol sometimes attacks old rubber fuel lines, make sure they are all flowing.
Next I would pull the carb and rebuild to find a blocked passage. Maybe a blocked needle valve or main jet.
If this is how its acting it's probably starving for fuel.
One thing I would check is to make sure you are getting flow from the tank. The ethanol sometimes attacks old rubber fuel lines, make sure they are all flowing.
Next I would pull the carb and rebuild to find a blocked passage. Maybe a blocked needle valve or main jet.
Thread Starter
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Freshman User
Joined: Apr 2010
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From: Alabama
Bart thats exactly what is happening. Could the float do this to?
Possibly the arm to the acc pump? what hole should it be in on the throttle linkage?
Ive followed my fuel lines and havent found any kinks or pinholes.
Ive pulled the carb already and cleaned all the passages
Possibly the arm to the acc pump? what hole should it be in on the throttle linkage?
Ive followed my fuel lines and havent found any kinks or pinholes.
Ive pulled the carb already and cleaned all the passages
Fleet Mechanic
Joined: Feb 2007
Posts: 1,326
Likes: 48
From: Middle TN
The needle valve for the float could be sticking in the hole and not releasing. This is common and would cause problems. I would change the needle valve. As the float drops does it force the needle pull down off it's seat?
The float could be bent and rubbing the side wall. Not common but I have seen the composite floats swell and start to rub, of course brass ones won't do this.
Generally if the acc pump is not working it will just not accelerate like it should, I wouldn't think it would pop back on you, but every carb is different.
On the rubber fuel lines the rubber deteriorates on the inside, flakes off and causes a tight place or blockage. They look fine on the outside. i have seen a couple go bad, the rubber lines coming out of the fuel tank.
The float could be bent and rubbing the side wall. Not common but I have seen the composite floats swell and start to rub, of course brass ones won't do this.
Generally if the acc pump is not working it will just not accelerate like it should, I wouldn't think it would pop back on you, but every carb is different.
On the rubber fuel lines the rubber deteriorates on the inside, flakes off and causes a tight place or blockage. They look fine on the outside. i have seen a couple go bad, the rubber lines coming out of the fuel tank.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
When does it happen? Anytime you're accelerating, or only when you start the truck? Does it depend on whether or not the engine is warmed up?
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Freshman User
Joined: Apr 2010
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From: Alabama
fmc- its only when i accelerate. and it does it hot or cold
Bart-Can you buy the needle valve seperate outside of the rebuild kit???
It will accel if you ease into the throttle. but if you give it 1/4 or more of the pedal itll backfire,pop and try n die unless you get off it.
Oh and it has a brass float
Ill have to check into all of your suggestions after the weekend. ill be in Atlanta till tomorrow night
Bart-Can you buy the needle valve seperate outside of the rebuild kit???
It will accel if you ease into the throttle. but if you give it 1/4 or more of the pedal itll backfire,pop and try n die unless you get off it.
Oh and it has a brass float
Ill have to check into all of your suggestions after the weekend. ill be in Atlanta till tomorrow night
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Senior User
Joined: May 2002
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From: Gatlinburg
Hopefully you do just have a carb issue. Make sure your coil pickup & coil are good.
Not to try to be the barrer of bad news but if nothing else seems to help. Pull your valve covers off one at a time, and put a long piece of cardboard at the bottom below the valve springs to keep oil from dripping out.
Run the engine at idle and see if you see any rocker arms that are barely moving up and down. If so, you could have a flat lobe on your cam.
A flat "worn" lobe will defintely cause a spat back up through the carb.
Not to try to be the barrer of bad news but if nothing else seems to help. Pull your valve covers off one at a time, and put a long piece of cardboard at the bottom below the valve springs to keep oil from dripping out.
Run the engine at idle and see if you see any rocker arms that are barely moving up and down. If so, you could have a flat lobe on your cam.
A flat "worn" lobe will defintely cause a spat back up through the carb.
Thread Starter
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Freshman User
Joined: Apr 2010
Posts: 28
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From: Alabama
30over i pray to god your not right about a flat lobe.
I took the carb off and bought a bucket of carb cleaner so its soaking right now. we'll find out tomorrow if it helps at all. and i pray to god it does!!!!
Can anybody take a pick of the wiring and plug for the pickup coil. P/O broke off the plastic for the plug and jus has the wires pushed into the male plug.Id like to make sure its correctly pinned
I took the carb off and bought a bucket of carb cleaner so its soaking right now. we'll find out tomorrow if it helps at all. and i pray to god it does!!!!
Can anybody take a pick of the wiring and plug for the pickup coil. P/O broke off the plastic for the plug and jus has the wires pushed into the male plug.Id like to make sure its correctly pinned
Thread Starter
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Freshman User
Joined: Apr 2010
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From: Alabama
Well Master Mechanic you were right about the box bein bad. took to oriellys and it failed miserably. Cant pick up another one right now. so itll have to wait.
Gonna finish cleanin the carb today and put it back together.
Gonna finish cleanin the carb today and put it back together.
Freshman User
Joined: May 2010
Posts: 36
Likes: 0
From: Jacksonville
even though the box can be bad you're probly still gonna have the same problems, my '74 f100 is doing exactly the same thing, i replaced the box, the coil, the spark plugs and plug wires,i even went from a Motorcraft 2 barrel to a Holley 4 barrel and there was no change performance wise, i myself am looking into converting the distributor into standard points so i can have constant power to the plugs.