351m constantly backfiring
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351m constantly backfiring
im getting sick and tired of this. my 77 f250 highboy is my daily driver, and i love it. but when the motor is cold i can feel it bogging down and if i dont quickly floor it, it backfires out the carb. i cant figure out whats causing it, because it still will do it when the motor is hot. allthough not as much. and it doesnt bog down, it just backfires, even at highway speeds. my entire engine compartment is covered in black soot. i think it may be my carb, it is an edelbrock 1406 600cfm 4bbl. i hate it!! i have4 run many holleyes and never had this problem, thats why im stumped!! anyone have this problem? ohh i just thought of one thing, i covered a few vaccume leakes with the correct plugs, and about a month later all the plugs i put on are gone. any help please!
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Wouldn't hurt to check your float height and make sure its not staying open. You might be running too much fuel to the carburetor--that could explain the soot and the bogging down.
I had a similar problem--mine was much worse and my carb constantly gushed fuel out the fuel bowl vents. I had to rebuild the carb because the needle valve was gummed up. When I got the carb back on it shot a loud backfire out when it started up, I'm guessing that had to do with the engine being flooded so bad beforehand.
I had a similar problem--mine was much worse and my carb constantly gushed fuel out the fuel bowl vents. I had to rebuild the carb because the needle valve was gummed up. When I got the carb back on it shot a loud backfire out when it started up, I'm guessing that had to do with the engine being flooded so bad beforehand.
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i have found that the soot, is definatly coming from the backfireing, because on the air cleaner there was a very thick layer of black "goo" that i cleaned off. it was like when you hold a lighter at just the right distance from something. i was told by my local speed shop that edelbrock carbs have problems with dirt, and they said put 2 fuel filters on, but i allready have that. how do i check the float hight? i've never worked on a carb before. i think it is getting to much fuel, because the exsahst has a very heavy gasoline smell. but why is it that if i feel it bogging down and i floor it it goes away? wouldnt that just be adding more fuel?
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
That heavy gasoline smell in the exhaust is another sign you're runinng very rich. When I had my problem, I could fill a parking lot with clouds of raw fuel.
I haven't worked on an Edelbrock carb so I don't know the exact procedure to set the float height. You have to take the airhorn off, and it seems like the float is on the underside. Someone else will have to help you there.
Your needle valve could be gummed up too. If that is the case, the needle that keeps the fuel bowl from overfilling won't be able to seat, and it will be stuck open.
As for flooring the pedal to keep it from bogging down, I'm going to guess this works because your throttle plates are wide open and since there's so much airflow, the excess gasoline gets burned more easily.
Might be time for a carb rebuild...just my 2 cents
I haven't worked on an Edelbrock carb so I don't know the exact procedure to set the float height. You have to take the airhorn off, and it seems like the float is on the underside. Someone else will have to help you there.
Your needle valve could be gummed up too. If that is the case, the needle that keeps the fuel bowl from overfilling won't be able to seat, and it will be stuck open.
As for flooring the pedal to keep it from bogging down, I'm going to guess this works because your throttle plates are wide open and since there's so much airflow, the excess gasoline gets burned more easily.
Might be time for a carb rebuild...just my 2 cents
Last edited by fmc400; Apr 23, 2004 at 03:43 PM.
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
Bogging usually indicates too lean. If it does it cold and not hot it is definitly too lean. Backfiring out the carb can be lean or rich.
First look for vacuum leaks, spray carb cleaner around the base of the carb, and the manifold. Look at all vacuum lines and fittings for leaks.
If there are no leaks, go one step rich in cruise. Change the metering rods to #1456 (0.073 x 0.037).
First look for vacuum leaks, spray carb cleaner around the base of the carb, and the manifold. Look at all vacuum lines and fittings for leaks.
If there are no leaks, go one step rich in cruise. Change the metering rods to #1456 (0.073 x 0.037).
Last edited by danlee; Apr 23, 2004 at 06:59 PM.
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it backfires mostly when its cold, as soon as the thermostat opens, it doesnt do it much, but it will do it when its hot sort of randomly. as for the bogging, its definatly only when the motor is cold. the thing that has me stumped, is that its exhibiting simptoms of running lean and rich. how the **** is that possible? LOL. when i gave it a tune-up, my father said the plugs are normal, but they were kind of white, does that mean anything? and if its running lean, why the rich smelling fumes? i dont think it needs a carb rebuild, it was new 2 years ago. previous owner did a frame off resto.but i'm beginning to think that it just got dirty and the float is not ajusted properly. i can always wait untill i finish building up my 408. 
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
Edelbrock says bogging is due to:
Air Leaks
Too Lean
Too Rich
Backfiring is due to:
Air Leaks
Ignition/Timing
Choke Adjustment
I think that backfiring can be also due to Rich or Lean Mixture.
Does it backfire at light cruise or under power?
Also check your oil for level and smell. If you are flooding you will have gas in your oil.
Air Leaks
Too Lean
Too Rich
Backfiring is due to:
Air Leaks
Ignition/Timing
Choke Adjustment
I think that backfiring can be also due to Rich or Lean Mixture.
Does it backfire at light cruise or under power?
Also check your oil for level and smell. If you are flooding you will have gas in your oil.
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Check your vacuum advance on your distributor. Sounds like the diaphram may have a hole in it. Take the carb end of the vacuum hose that goes to vacuum advance loose. Take the distributor cap off. Now suck on the vacuum line and see if the advance moves. If it doesn't move and you keep pulling air then your diaphram is leaking and not advancing your timing. Also, the advance could be stuck. If it is, penetrating oil should free it up.
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Joined: Jan 2003
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From: Addison,Texas
If the plugs are white, then they're running too hot. Really hot plugs could inadvertantly ignite the fuel while the intake valves are open. (And the backfiring through the intake manifold would certainly tend to blow the vacuum caps off.) I'm guessing it only backfires through the manifold, from what you're saying. In which instance you're not running too rich, so much, as far far out of time. I suspect the bogging comes from the early (very early) ignition.
Had you changed anything before this started? Have you looked at your timing?
ash
['Hrmm.']
Had you changed anything before this started? Have you looked at your timing?
ash
['Hrmm.']
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the only thing that changed was the outside climate. i had been driving the truck as a daily driver for about a month maybee 2. then it started getting cold in the morning and it would backfire. i am going to check the timing soon. one other thing that i forgot to metion, maybee its just coincidence, but i blew out a donut gasket at the end of the header collector. the reason i think its coincidence is that it never backfires through the tailpipe. only through the carb. the inside of my carb was covered in a thick layer of black soot. i used carb cleaner to get rid of it. but i know its from detonations in the carb/intake area.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
If the only thing that has changed is the climate, then the choke is definitely something to look at. On mine when it starts getting warmer outside I have to lean it out a bit.
Timing is definitely something to look at. It is another explanation for the bogging down and the backfiring, and the white plugs make it even more suspicious.
Timing is definitely something to look at. It is another explanation for the bogging down and the backfiring, and the white plugs make it even more suspicious.
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