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Backfiring through carb

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Old 05-21-2010, 01:28 PM
76BigBlock
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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
 
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Old 05-21-2010, 01:38 PM
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Check your cam timing (if you replaced the timing chain) and also mechanical advance vs vacuum advance.
 
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Old 05-21-2010, 01:39 PM
76BigBlock
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havent touched the timing chain or cams
 
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Old 05-21-2010, 02:15 PM
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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.
 
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Old 05-21-2010, 02:37 PM
76BigBlock
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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
 
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Old 05-21-2010, 09:11 PM
4x4 Bart
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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.
 
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Old 05-22-2010, 03:24 AM
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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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Old 05-22-2010, 08:02 AM
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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
 
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Old 05-22-2010, 08:48 AM
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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.
 
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Old 05-22-2010, 05:51 PM
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im having exactly the same problem with my '74 f100. but i dont remember which screws are which as far as the fuel/air screws are on my Holley 600 on my 390
 
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Old 05-23-2010, 07:28 PM
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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
 
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Old 05-23-2010, 10:47 PM
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Wait if i had a flat lobe wouldnt it also affect it when i rev the engine in park????
 
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Old 05-24-2010, 04:54 AM
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your problems could also be in your ignition system. i had the same symptoms your describing and it turned out to be the duraspark box was no good.
 
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Old 05-24-2010, 11:23 AM
76BigBlock
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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.
 
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Old 05-24-2010, 08:14 PM
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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.
 
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