Odd backfire

[squid]

So I am experiencing an odd backfire.  I bought my 440 cuda last year, and I'm a little rusty on diagnosing anything (I have had late model cars for the last 20 years).  It seems I forget more than I learn these days.  Anyway my car has a few odd symptoms (which I will get to), but the backfire is most annoying.

It started backfiring last week.  However, it only backfires on a warm to hot day, and after the engine warms up fully.  I will be driving along, the engine will hesitate (almost like it has no ignition) for 2-3 seconds, then bang the quintesential gun shot is heard round the world.  Also, when this occurs there will usually be a large puff of black smoke.

After it backfires once, I rarely does it do it again.  However, it may do it if I drove it longer, but it always seems to happen a few miles from home.

I figured I would list my few other odd symptoms in the chance they are related.

First let me say that the 440 is an original and stock HP 440 up to the heads.  It has had a valve job, cam, timing chaing, intake manifol, and carb replaced about five years ago.  The cam is a mild performance cam from Mopar Performance, and the carb is a Holley 750 with a manual choke. Also, I have an electronic ignition from Mopar Performance.

Okay, the other symptoms really apply to the exhaust.  When the car is cold and idles, I get a black soot in the exhaust it appears to be carbon like.  Once the car is warm I don't see any smoke from the exhaust.

If I get on the throttle I will leave everyone in the dust and the smoke.  Litteraly this car will move fast, but there will be enough black smoke when the secondaries open to kill all the mosquitos that live here in Memphis.

I also thought I would mention the recent changes, and things I have noticed about the car.  I changed the cap, rotor, plugs and wires a couple months back.  I did notice that the Charcoal Canister doesn't look like it's plumbed right, but it seemed to be working, so I didn't change anything (if someone has a pic of how this should be plumbed, I would appreciate it).

So that's the story...  Any assistance that is given will be greatly appreciated.

-squid

[MEK-Dangerfield]

Just something to check here... can you pull a spark plug or two and see what they look like? I don't know if it has anything to do with your backfiring, but it sounds like you are running super rich. Possibly sticking choke? 

  Mike

[71383bee]

  definately rich.  Does it burn your eyes as it idles or drives?

Sounds like it is dumping tons of raw fuel into those cylinders which has probably washed them clean.  You really need to do a compression and/or leakdown test to find out if your rings are shot and check the damage. 

The backfire is probably caused by raw fuel loading up in your exhaust system and then igniting when a hot spot is reached.

Get the timing adjusted to around 32-38 total and check where initial is.  If it's around 12-18 then leave it and start working on the carb.  Begin by idling it down as much as possible and reset the mixture.  Start at 1.5 turns out from full seat.  adjust for best manifold vacum signal in gear at idle. 

If this improves things then shut her down and change the oil and filter.  If it's running that rich then you got gas in the cranckcase and Gas and oil are a bad mix that will drastically decrease the lifespan of your bearings. 
 

[Grancoupe]

Definitely running rich. Sounds like your running too rich. Check your oil too. Smell it for fuel. If its contaminated you want to change it and lean out the motor. You can damage your bearings if your oil gets too thin.The backfire is probably due to loading up fuel in the exhaust and detinating it.

[JCWCuda]

If it's running that rich i would check to see that Choke is working properly and check your floats in the carb aren't  sticking

[Bullitt-]

re: charcoal canister, you never stated year. I have the '73 manual , the diagram tells little but it looks simple. Middle line to tank, LH (drivers side) line to carb. called canister purge line (shown on front of carb.) RH line to carb. called carb. fuel bowl vent line (shown on the RH top of carb.).
If these are blocked off & you still have a sealed gas tank cap, vapor lock can be created which will starve fuel to the pump.
Also there is a fiber glass filter in the bottom of the canister that requires replacement.

[Mt.St.BigBlock]

  I will be driving along, the engine will hesitate (almost like it has no ignition) for 2-3 seconds, then bang the quintesential gun shot is heard round the world. 

IMO, there are two(or more) problems here. First off, I agree the carb is set too rich. I don't believe that alone is causing
a backfire. I've been under the impression that a lean condition will cause a backfire before a rich one. The main clue is
in the line quoted above. Either the ignition coil or the pick up coil in the distributor is breaking down due to heat and
killing the spark for a instant or two and when the fire comes back..."CHIC-BOOM" (my dad used to do that on purpose
with his old truck coasting down hill in gear with the ignition off and then turn it back on...CHIC BOOM my brother and I
called it!!) If you still have the old coil, I think you said you swapped it, put it back in and see if new one was bad. Could
also be something else killing the ignition but the coil might be a easy starting point. I had the distributor pickup coil
go bad in my Mirada and it started out with the intermittent crap.Goodluck and let us know what you find.  Jeff

[Bullitt-]

might trow in a new ballast resistor for good measure just $5. at Advance Auto

[ShelbyDogg]

I would watch the tach to see if it drops down to zero, when the problem happens. If you don't have a tach, I would run a little tiny light up to the windshield, under the hood. one end to ground the other end to the + side of the coil. When you lose ignition, look at the light.  It could be the firewall connector that runs wires to the Ballast res, alternator wires, etc. Make sure it's plugged in tightly.
About the running rich, Check that your power valve isn't blown and that it maches the vacuum of your cam.
Rob

[71383bee]

 

What happened?  Did the problem fix itself....I hate it when people do this.

None of the symtons seem to be describing a lean backfire because it is coming from the exhaust.  Not to mention the other symptons described as sooty tailpipes, black smoke on acceleration, etc.  I doubt if the pickup is dying.  The plugs could be loading up though.  They are probably fouled too and should be replaced.   

The choke is a manual so if it's not open all the way when hot it could be a problem.   Something to check.

I still say adjust timing then re set the carb and go from there...if we ever get an answer 

[MEK-Dangerfield]

What happened?  Did the problem fix itself....I hate it when people do this.

None of the symtons seem to be describing a lean backfire because it is coming from the exhaust.  Not to mention the other symptons described as sooty tailpipes, black smoke on acceleration, etc.  I doubt if the pickup is dying.  The plugs could be loading up though.  They are probably fouled too and should be replaced.   

The choke is a manual so if it's not open all the way when hot it could be a problem.   Something to check.

I still say adjust timing then re set the carb and go from there...if we ever get an answer 

It looks like the two of us are in agreement here about the problem. Let's give the guy the benefit of the doubt for a few days. Maybe he went away for Easter or something. I can assure you, THAT problem didn't fix itself. 

  Mike

[pink panther]

What happened?  Did the problem fix itself....I hate it when people do this.

None of the symtons seem to be describing a lean backfire because it is coming from the exhaust.  Not to mention the other symptons described as sooty tailpipes, black smoke on acceleration, etc.  I doubt if the pickup is dying.  The plugs could be loading up though.  They are probably fouled too and should be replaced.  

The choke is a manual so if it's not open all the way when hot it could be a problem.   Something to check.

I still say adjust timing then re set the carb and go from there...if we ever get an answer 

love to see the results, might happen to all of us one of these days, with this thread it would definatley help.

[asm74]

I just had this EXACT problem with my 67 camaro, and it turned out to be 1 of two, possibly 3 things:  1) dizzy cap and rotor were in bad need of replacement, carbon build up on terminals, etc. 2) My coil's ground wire was shorting when the car heated up.  Its a coil-in cap style hei, and apparently its pretty easy to install it so the ground wire can get clamped(or even cut) without knowing it.  3) the third issue was wire guage and resistance.  In making sure all the connections on the wiring harness were up to the job, i decided to replace the harness entirely with a new one, with better guage wires all around and better heat insulation.  This was all happening 2 weeks ago, and since then there has not been a single issue, and the cars been running strong while I build up my Challenger.  Hopefully you'll forgive the GM context, and it could offer some help.


Looking forward to finding out what the solution is!

[squid]

Okay,

Yes, Easter did interupt my work!

I spent some time with the carb, and this is what I found.  I didn't have a vacuum gage handy so I resorted to the age old "engine speed".  Well... No matter where I set it, the engine ran the same.  I did note that the entire inside of the carb was wet with fuel.

I called the local Holley shop and spoke with them.  They suggested a complete rebuild, or preferably a different set-up.  I asked about the set-up, and they recommended I switch from the 750 double-pumper to a 670 street avenger.  Their reason is that I mainly use this car for cruizing, and the occasional burnout when neccessary 

Anyway, I opted for the new carb.  This sure took care of the smoke problem.  The idle is clean, and so is the exhaust when the secondaries open up.

Now to the back-fire.  The car still has the same symptom.  It cuts out for a coupld seconds, and then back fires.  I haven't got to look at the tach, but I will attempt to do so today.  However, with the new cab, I have been able to detect another symptom which may have been masked by the rich condition.  Now there seems to be a barely detectable hesitation or missing when driving a constant speed.  I also have a flat spot when coming of an idle when going between gears or from a start (I have a 4-speed). The missing or hesitation is almost like a carbon trace, but I replaced the wires, plug, cap and rotor about three months ago.

I haven't got a chance to check the timing.  I have six kids between 5 and 12.  They can sure tie up an easter weekend, but I love 'em for it.

Oh, last thing..  The year is '72.

Sorry about the length, but there has been so much help, I thought I better give a pretty good explanation of what has happened.

[Bullitt-]

vacum advance?