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F250 w/400 backfire

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Old 09-22-2018, 11:53 AM
Fordmaicoman
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F250 w/400 backfire

Good morning. I'm sure this has been asked before, but my 77 F250 w/400 I bought recently backfires/sputters when accelerating. It idles and accelerates great off the start, but I have to baby the gas to accelerate or it will backfire and sputter. It has an aftermarket newer edlebrock carb. Is this a timing issue, or carb issue? Also, when driving down the freeway when I step on the gas, it will accelerate and make a sucking/vacuum sound when I let off the gas. Does this go hand in hand with the backfire issue? I haven't tried any fixes yet. I wanted to talk to the pros first! Thanks all!
 
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Old 09-22-2018, 12:13 PM
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Yes carb & timing issue could be the cause.

Timing is easy put a timing light to it.
​​​​​​Make sure base, mechanical & vacuum advance timing are all good.

Om the carb, motor off, pop off the filter assy. And using a light look down inside the carb when moving the throttle by hand.
You should see gas squirting if not you will need to look into the carb accelerator pump and why it dies not work.

After checking that out let us know what you find and we can help from there.
Dave - - - -
 
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Old 09-22-2018, 12:19 PM
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Hmm. So it has an aftermarket edelbrock carb and you say when you give it gas, it sputters? You should check the accelerator pump setting on the carb. They have 3 different holes on the pump lever, that you can change the position of the throttle linkage to pump more or less gas when accelerating. Look up Edelbrock accelerator pump adjustments.

It be possible that it's also running too lean during acceleration, or too rich, you may want to try both directions

For example, my truck has an Edelebrock 1405. It was set to the "default" hole position - when I would try to accelerate hard, the truck would hesitate and bog down. I determined that is was probably running too lean during acceleration. I moved the accelerator pump to force a larger squirt of fuel during acceleration and now it works properly.

Where does the backfire occur, exhaust or carb? I think exhaust means it is running too rich, whereeas carb backfire means too lean. If I remember correctly.
 
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Old 09-22-2018, 12:21 PM
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Could be either/both, vacuum leak, etc. Backfire through the exhaust tailpipe correct?

You'll just have to run through the basics, there's no silver bullet. It could be something as simple as the spark plug wires installed in the wrong firing order. Be sure to check for this. Pretty common mistake.

Vacuum leaks are probably the big bugaboo with good engine tuning. Incorrect ignition timing. Misadjusted carburetor. Not necessarily in that order. Try disconnecting and plugging the vacuum advance canister port on the carb. Then drive it and see if it runs better.

A lot of times what you'll find is a combination of tuning errors and defects in every system, and they stack up, and then the "golden screwdriver" got pulled out to try and tune around it. Start at the beginning and use a process of elimination. Find out what the problem isn't, because generally each adjustment is done in a certain order because it affects the other adjustments down the line.. If the ignition timing is way off, the carburetor won't adjust correctly. If the fuel pump is defective the float height won't adjust correctly. If the distributor is installed wrong it won't time correctly. It's amazing that some engines will even run, with everything completely buggered. "Oh, it just needs a carb rebuild."
 
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Old 09-22-2018, 12:24 PM
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To little of a squirt will make it do this I have never seen too much given on carbs unless someone "fixed it" that way.
I go with too little.

I should add if too much gas is given you would see black smoke out the tail pipes do you see that on take off?
Dave - - - -
 
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Old 09-22-2018, 01:50 PM
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Originally Posted by niko20 View Post
Hmm. So it has an aftermarket edelbrock carb and you say when you give it gas, it sputters? You should check the accelerator pump setting on the carb. They have 3 different holes on the pump lever, that you can change the position of the throttle linkage to pump more or less gas when accelerating. Look up Edelbrock accelerator pump adjustments.

It be possible that it's also running too lean during acceleration, or too rich, you may want to try both directions

For example, my truck has an Edelebrock 1405. It was set to the "default" hole position - when I would try to accelerate hard, the truck would hesitate and bog down. I determined that is was probably running too lean during acceleration. I moved the accelerator pump to force a larger squirt of fuel during acceleration and now it works properly.

Where does the backfire occur, exhaust or carb? I think exhaust means it is running too rich, whereeas carb backfire means too lean. If I remember correctly.
It seems to backfire from the carb. If I baby the throttle it will catch itself up and accelerate ok, but id you give too much gas too quick, she falls on her face.
 
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Old 09-22-2018, 03:14 PM
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If the timing is correct, the first thing I would check is the accelerator pump.
 


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