Backfiring when letting off the gas

[mopar12372]

timming would do this ONLY IF YOU REMOVED THE DISTRIBUTER or touched it ! what is the timming set at ?
and yes the exhaust will grumble or have a pop with a exhaust that is flowing good (this is not a bad thing dont worry ) . look for a exhaust leak around the heads , at the conection on the bottom of the exhaust manifold or collector gasket , sucking air from conections. the flow masters are known for this too . lets start with the easy things then move on to the more complicated stuff .
timming usually backfires through the carb and popping back fireing is usually carb issue . i step at a time change one thing at a time .

[ghjedi]

I dont want to muddy the waters but are you sure that your are going in the right "direction" when you are looking at the current firing order? Dont big block disturbutors rotate in the opposite direction (CCW) compared to a small block (clockwise)?  Maybe you are just looking at it "backwards" ?

I think you are right-- the manual indicates that the firing order goes counter clockwise for 440 engines and that's what I went with. 

So here is my firing order that was working reasonably well (except for the one-time exhaust bang when transitioning from 1st to 2nd gear).  1-8-4-3-7-6-5-2.

Now that I look at it closer and compare it to the manual's setting (1-8-4-3-6-5-7-2), I see that I'm off on the 6, 5, and 7 cylinders.  I did not play with that sequence, so I'll try that combo later.

When looking at it the wires again last night, I noticed that one of them was loose and resting on the header.  It now has a pretty good burn mark through the insulator jacket.  I'm thinking that this could be the source of my issues, so I'll need to take care of this one first before messing too much further with the firing order.  I saw on another post that such a condition could cause the spark plug wire to arc to the header and thus cause a backfire.  Either way, I know a burn mark through to the wire isn't good and needs to get fixed first.

[JayBee]

Just thought I'd throw this picture into your thread, it's from my '70 shop manual. Kinda curious what the final solution will be too.

[PlumCrazyRTSE]

Wow, it appears that you may have just about the same problem I had.  When you fix the burnt wire and change 5,6 and 7 around I'll bet your backfire issue goes away.

[ghjedi]

Thanks JayBee- that picture matches what I have in my 72 manual, so I appreciate the confirmation.

PlumCrazyRTSE, I sure hope you're right.  The really odd thing is that the car runs great and idles great, except for this backfire.  I can reproduce it quite easily under a very specific scenario (in 1st gear accelerate quickly to 3500 rpms, let off gas and put in the clutch to transition into 2nd gear-->pow! shotgun blast!

I saw that one wire sitting up against the header and I then remembered (your?) post about a burnt wire causing a backfire.  On closer inspection, it sure enough turned out to be a serious burn through the outer jacket.  I had just replaced the wires a few months back and I thought I had been careful to tuck the wires away from heat sources. Well, there goes a few more $ down the drain.  I got a new set on the way.

I'll provide an update in a few days, once I get the new wires and get a chance to play with it again.

[PlumCrazyRTSE]

The really odd thing is that the car runs great and idles great, except for this backfire. 

Yeah, mine seemed like it was idling all right as well and it seemed like it had ample power too but I could sure tell the difference after it was corrected.

[ghjedi]

Update: I installed the new spark plug wire set with the original firing order that I had before (i.e. not per the manual).  It fired right up, but there definitely were several pinging backfire sounds in my exhaust, so I promptly shut it down.

I then reset the 3 spark plug wires that were out of sequence per the manual and fired it up.  Bingo- it sounded great and no backfiring!  I will take it on the street in a few hours and give it a good run to verify that my original problem (occasional backfiring when shifting gears) is resolved.

I'm attaching a photo of my burned spark plug wire for reference.  I'm starting to think I had 2 issues going on here-- the burned wire and the wrong spark plug wire sequence.  A good run later today will tell me one way or the other.

[ghjedi]

Final Update: I took the car for a good spin and guess what?  No backfiring at all!     

The more I think about it, the more I believe I had 2 issues going on here.  The first one being the burned wire and the second one being the incorrect firing order.

I don't understand why the car ran pretty well with the wrong firing order (except for the one-time backfire when shifting under heavy acceleration), but I'm not going to worry too much about that now.  The car is running great without any issues with the correct firing order per the manual and that is something I can understand.

[JayBee]

  You might even experience better gas mileage and horsepower now too.

[ghjedi]

You might even experience better gas mileage and horsepower now too.

I think you're right.  I need to test it some more, but it did feel a bit more powerful.

[PlumCrazyRTSE]

I don't understand why the car ran pretty well with the wrong firing order (except for the one-time backfire when shifting under heavy acceleration).

440's produce a lot of power, even when they're not hitting on all cylinders.

Glad to hear that you've got it all sorted out.