Decel backfire
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Joined: Apr 2006
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From: Kansas City, KS
Decel backfire
1986 F150 300 I6, one barrel carb.
Starts and runs just fine...when I take my foot OFF of the gas pedal to decelerate, it backfires...
I used to know what that means...this CRS is difficult to deal with...
Any ideas?
Starts and runs just fine...when I take my foot OFF of the gas pedal to decelerate, it backfires...
I used to know what that means...this CRS is difficult to deal with...
Any ideas?
Fleet Mechanic
Joined: Jul 2011
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He's right... timing is always the biggest thing that causes a backfire.
Then, check your fuel mixture. Did you set it with a vacuum gauge? I had a lean pop/backfire at idle that would go away when I enriched it.
I'll assume you've been driving it a while and it's not a burnt up exhaust valve.
Also, what kind of muffler do you got? Not trying to imply that you're not intelligent or such, but glasspacks are known for popping, and I've heard even Flowmaster 40's do the same thing on decel.
Also, I'd pull codes just to be sure.
Then, check your fuel mixture. Did you set it with a vacuum gauge? I had a lean pop/backfire at idle that would go away when I enriched it.
I'll assume you've been driving it a while and it's not a burnt up exhaust valve.
Also, what kind of muffler do you got? Not trying to imply that you're not intelligent or such, but glasspacks are known for popping, and I've heard even Flowmaster 40's do the same thing on decel.
Also, I'd pull codes just to be sure.
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From: Kansas City, KS
Thanks all.
I don't know what type of muffler it is, but it IS a muffler, not a glass pack... and I'll get to playing with the timing this weekend....
For all I drive the thing, it's not a huge deal...just annoying...
I don't know what type of muffler it is, but it IS a muffler, not a glass pack... and I'll get to playing with the timing this weekend....
For all I drive the thing, it's not a huge deal...just annoying...
Fleet Mechanic
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Lead Driver
Joined: Jul 2009
Posts: 6,477
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From: Sheffield, AL
tcbofade,
The 1986 4.9L originally was computer controlled with a feedback carburetor. Is yours still so equipped?
This particular engine is very sensitive to vacuum leaks and came with small plastic vacuum lines which over time get brittle and crack from heat and vibration. Also, the carburetor is held down by just two bolts, which tend to vibrate loose. Grab your air cleaner with your hands and see if you can rock it back and forth. If so, the nuts have loosened.
Get back with us and we will try to help.
The 1986 4.9L originally was computer controlled with a feedback carburetor. Is yours still so equipped?
This particular engine is very sensitive to vacuum leaks and came with small plastic vacuum lines which over time get brittle and crack from heat and vibration. Also, the carburetor is held down by just two bolts, which tend to vibrate loose. Grab your air cleaner with your hands and see if you can rock it back and forth. If so, the nuts have loosened.
Get back with us and we will try to help.
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Post Fiend
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From: Lost
Exhaust leaks near the engine are notorious for causing this to occur. Lifting off the throttle causes a sudden decrease in flow thru the pipe, and any spots that are not well sealed can allow fresh air to be pulled in by the higher velocity of the exhaust flow suddenly dropping. This fresh air can allow unburned fuel to ignite inside the pipe, and results in a "pop" to be heard from the end of the pipe.
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From: Kansas City, KS
Yes, the feedback carburetor is still on it...no, none of the feedback equipment is...PO removed most of the EGR and emission systems... I removed the rest and cleaned it up a bit.
I have replaced every vacuum line and resealed the valve cover and the carburetor a little more than a year ago.
I STILL have vacuum leak, so I think that I'll have to pull the head off to reseal that...and I'm just not sure that I care enough...
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From: Kansas City, KS
(safely...)
If that IS the case, I'm not hurting anything to drive it as is, correct?
Fleet Mechanic
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EDIT:
You say you removed the rest and cleaned it up a bit, did you go ahead and do the DSII swap already?
/EDIT.
Do a Duraspark II swap. It's plug and play. You'll need:
A non-computerized Carter YFA carburetor
The wiring harness (found at junkyards for cheap) that has the ignition module, coil, distributor, oil pressure, and water temperature plugs on it
The correct coil from a DSII 300
A Duraspark distributor with vacuum advance (if the EGR is eliminated, you could also change the springs and weights on the inside of the distributor, and change the vacuum advance canister to give you a better timing curve for more power/mpg's and eliminate any possible ignition knocking that could be occurring due to lack of EGR function)
And a DSII ignition module
The upside to this conversion is that your motor won't be running rich anymore (which it's doing right now, I guarantee...) and you can upgrade/add on some performance goodies down the road if you want (headers, efi manifolds, 4bbl, etc.).
Even if you don't do any upgrades, swapping to the DSII (entirely plug and play btw, no wiring necessary) will let you get your motor running right like it should be.
You say you removed the rest and cleaned it up a bit, did you go ahead and do the DSII swap already?
/EDIT.
Do a Duraspark II swap. It's plug and play. You'll need:
A non-computerized Carter YFA carburetor
The wiring harness (found at junkyards for cheap) that has the ignition module, coil, distributor, oil pressure, and water temperature plugs on it
The correct coil from a DSII 300
A Duraspark distributor with vacuum advance (if the EGR is eliminated, you could also change the springs and weights on the inside of the distributor, and change the vacuum advance canister to give you a better timing curve for more power/mpg's and eliminate any possible ignition knocking that could be occurring due to lack of EGR function)
And a DSII ignition module
The upside to this conversion is that your motor won't be running rich anymore (which it's doing right now, I guarantee...) and you can upgrade/add on some performance goodies down the road if you want (headers, efi manifolds, 4bbl, etc.).
Even if you don't do any upgrades, swapping to the DSII (entirely plug and play btw, no wiring necessary) will let you get your motor running right like it should be.
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