Carb backfire - 460 Ford Forum
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post #1 of 30 (permalink) Old 03-24-2019, 08:00 PM Thread Starter
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Carb backfire

Canít figure this out. 1985 460 motor. Duraspark system. All stock except edelbrock 1406 carb. It backfires though carb in park when I apply throttle kinda fast. I have removed valve covers and nothing seems broken or loose. Cleaned carb. New gaskets. Mixture screws at 2 turns out. I donít see a big change in rpm when they are all the way seated in or turned out. Moved plunger to top hole. New cap n rotor, cables, coil, plugs (wich are now black and dry). Timing at 8 btdc as stated on valve cover. Was mentioned to
Me about timing curve but I donít know much about that. The idle is kinda ruff. Thanks for any help
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1978 ford bronco with stock 1986 460 motor edelbrock 1406 carb
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post #2 of 30 (permalink) Old 03-24-2019, 08:50 PM
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Originally Posted by Sudsmcduff View Post
Canít figure this out. 1985 460 motor. Duraspark system. All stock except edelbrock 1406 carb. It backfires though carb in park when I apply throttle kinda fast. I have removed valve covers and nothing seems broken or loose. Cleaned carb. New gaskets. Mixture screws at 2 turns out. I donít see a big change in rpm when they are all the way seated in or turned out. Moved plunger to top hole. New cap n rotor, cables, coil, plugs (wich are now black and dry). Timing at 8 btdc as stated on valve cover. Was mentioned to
Me about timing curve but I donít know much about that. The idle is kinda ruff. Thanks for any help


Look for vacuum leaks.

Timing curve is initial plus mechanical = total and in at what RPM

Vacuum advance is at part throttle cruise and very light acceleration.


From my site.


Duraspark_distributor_recurve_instructions_index



SJ
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post #3 of 30 (permalink) Old 03-24-2019, 09:48 PM Thread Starter
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Quote:
Originally Posted by The Mad Porter View Post
Look for vacuum leaks.

Timing curve is initial plus mechanical = total and in at what RPM

Vacuum advance is at part throttle cruise and very light acceleration.


From my site.


Duraspark_distributor_recurve_instructions_index



SJ
used 2b RHP


Thanks for the reply. Any advice on wich recurve kit and where to buy?

1978 ford bronco with stock 1986 460 motor edelbrock 1406 carb
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post #4 of 30 (permalink) Old 03-25-2019, 08:41 AM
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HI, STOP.. get the kit if you want but your problem is a vacuum leak.. turning the mixture screw in should lower the idle if not kill the motor. if it does NOT then there IS a vacuum leak. ether carb gasket or a vacuum line somewhere.. use carb cleaner and spray around under the hood at carb and vacuum lines and if the rpms pick up then you found the leak,..
Check the big aluminum spacer leaks there and is hard to seal.. if you have the wrong gaskets..
good luck
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post #5 of 30 (permalink) Old 03-25-2019, 04:57 PM Thread Starter
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Originally Posted by pedal2themetal45 View Post
HI, STOP.. get the kit if you want but your problem is a vacuum leak.. turning the mixture screw in should lower the idle if not kill the motor. if it does NOT then there IS a vacuum leak. ether carb gasket or a vacuum line somewhere.. use carb cleaner and spray around under the hood at carb and vacuum lines and if the rpms pick up then you found the leak,..
Check the big aluminum spacer leaks there and is hard to seal.. if you have the wrong gaskets..
good luck
tim
Thanks i will check around for leaks today

1978 ford bronco with stock 1986 460 motor edelbrock 1406 carb
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post #6 of 30 (permalink) Old 03-26-2019, 04:15 PM Thread Starter
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Thanks i will check around for leaks today
My fuel pressure is at 6 psi. Didnít find any vacuum leaks. After turn motor off and give it full throttle, while looking down into
Carb canít see any fuel squirting into carb.

1978 ford bronco with stock 1986 460 motor edelbrock 1406 carb
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post #7 of 30 (permalink) Old 03-26-2019, 06:25 PM
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HI yea no squirt tells me accelator pump (inside carb) not working and that can cause the back fire as well.. rebuild time.
If it is working you should see a squirt of fuel just as you move the linkage. I bet its hard to start cold as well. It needs that squirt of fuel to help in starting especially cold.
good luck
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post #8 of 30 (permalink) Old 03-26-2019, 06:31 PM Thread Starter
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Originally Posted by pedal2themetal45 View Post
HI yea no squirt tells me accelator pump (inside carb) not working and that can cause the back fire as well.. rebuild time.
If it is working you should see a squirt of fuel just as you move the linkage. I bet its hard to start cold as well. It needs that squirt of fuel to help in starting especially cold.
good luck
tim
I just tried another carb. It backfired again. I’m guessing it’s bad intake valve now. I don’t know how to check for that or possibly the duraspark black box is going bad?

1978 ford bronco with stock 1986 460 motor edelbrock 1406 carb

Last edited by Sudsmcduff; 03-26-2019 at 07:55 PM.
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post #9 of 30 (permalink) Old 03-27-2019, 06:41 AM
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Hi, to check for a bad valve you use a vacuum gauge.. I dought its a valve, did you check to see that the other carb's accelator pump working?
Here's a link to how to read a vacuum gauge
Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

good luck
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post #10 of 30 (permalink) Old 03-28-2019, 07:28 PM
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I know that you mentioned you set the timing. But I have seen this happen when timing is retarded. Just for the sake of experiment try advancing the timing to 10 degrees BTC and see what is does. I run the 460 in my ranchero at 12 degrees BTC and it runs great. (mine is stock too)
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post #11 of 30 (permalink) Old 03-29-2019, 10:36 AM Thread Starter
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I just realized that it was a egr motor. Previous owner removed all vacuum and egr stuff. I set timing to 8 btdc cuz the valve cover said so. With it all removed I bet itís not supposed to be at 8 anymore. Thanks. I will try it at 10-12
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1978 ford bronco with stock 1986 460 motor edelbrock 1406 carb
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post #12 of 30 (permalink) Old 03-29-2019, 12:22 PM
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Originally Posted by Sudsmcduff View Post
I just realized that it was a egr motor. Previous owner removed all vacuum and egr stuff. I set timing to 8 btdc cuz the valve cover said so. With it all removed I bet itís not supposed to be at 8 anymore. Thanks. I will try it at 10-12

EGR engines only require different ignition timing at part throttle cruise. Less vacuum advance.


Even a completely OEM 460 benefits significantly from a properly curved distributor.

12 to 14 initial and 34 to 36 total depending on location and altitude.

Vacuum advance needs to be dialed back to about 12 to 14 degrees additional at part throttle.


The OEM ignition curve was emissions focused. Aftermarket reman distributors are all over the place.



SJ
used 2b RHP



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post #13 of 30 (permalink) Old 03-30-2019, 04:37 AM
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In reading your initial post you mention new cap, rotor, and having had the valve covers off. Did you check to be sure you have the plug wires in the correct firing order? You didn't say whether or not this started after the tune up or it was doing it before....it's a common mistake I've done it to myself more than once over the years....

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post #14 of 30 (permalink) Old 03-30-2019, 11:16 AM Thread Starter
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In reading your initial post you mention new cap, rotor, and having had the valve covers off. Did you check to be sure you have the plug wires in the correct firing order? You didn't say whether or not this started after the tune up or it was doing it before....it's a common mistake I've done it to myself more than once over the years....

Dan
Yea I triple checked it. Iíve made that mistake many times as well

1978 ford bronco with stock 1986 460 motor edelbrock 1406 carb
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post #15 of 30 (permalink) Old 03-30-2019, 01:55 PM
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I was thinking of this post as I drove the '75 Country Squire earlier. It used to behave exactly the same way. It has a 460 with the Edelbrock carb and Duraspark II ignition. I did end up installing a pre-1972 timing set in it to zero the cam, but after that, I could not seem to get rid of the bog/backfire that you describe. What I found was that setting the ignition timing to factory settings was not working.

I finally connected a gauge to manifold vacuum and advanced the timing until the gauge hit its maximum reading (20-21"). At this point, the engine was starting to miss, so i backed off the timing until she smoothed out and left it there. It was idling faster, so I turned the idle screw back down and adjusted the mixture for smoothest idle. This was all done with the engine at operating temperature and the advance hose connected to the gauge rather than the distributor. I also re-purposed one of the vacuum valves on the water outlet prevent vacuum advance at cold start-up.

This works well and the engine is a powerhouse. It has smooth, even acceleration and no longer hesitates/backfires. If you time it this way, you should be able to drive it warm without any detonation. If it does ping, retard the timing in small increments and test drive until the pinging is gone. My engine doesn't ping at all, but some slight pinging under heavy load is acceptable.

If it starts/cranks hard when hot, turn the ignition on and wait a few seconds, then crank it. This seems to allow the module to charge and retard initial timing as it was designed to do during starting. When I follow this procedure, the engine fires right up. Hope this helps...

1978 Lincoln Continental Sedan 460 V8

Last edited by landyawt78; 03-30-2019 at 01:59 PM.
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