[Adrian Worman]

Hi there,
I am a new member,registering in last few days,I have a '72 hardtop with 4400, 4 speed, and rally dash......and have a major problem which I can't seem to resolve.
The problems are these:-
The car will always fire up and run perfectly,even after a month of no use; however,after car has warmed up (about 10-15 mins) it starts to backfire, sometimes so badly, that even at tickover it sounds like gunshot from tailpipes; the aircleaner gauze gets distorted and eventually tickover drops to below 600 and is so weak that slight throttle application causes engine to die. After a few minutes the car will always restart and appear to tickover normally. It will take full throttle, seem responsive but will still have slight backfire.
The car will run and will drive on light throttle but on too hard acceleration it will backfire constantly until throttle is backed off and within 3-4 mins will lose power,grinding to a halt and will have very weak tickover on re starting and will not recover; this inevitably results in a tow/push back to house-the novelty of which wears thin!!

The engine is a '72 440HP block, 906 heads, '68 factory intake, 750 Edelbrock performer-brand new, factory mechanical fuel pump, half in fuel line back to tank sender unit. The exhaust is 2.5 inch pipes back to axle, 2inch over axle,2.5inch tail pipes with resonators and Hooker headers. The ignition is Chrysler electronic ignition with chrome box.

I have tried timing the engine at different setings, ranging from 0-12 degrees of advance;this has made no difference. I assumed there was a problem with the chrome box or maybe the coil and pick up in the distributor so I used one of the big Chiltern repair manuals to system test the ignition circuit and according to test results via multi-meter everything checked OK. However, when engine goes into weak tick-over and have tried testing circuit the voltage readings are much lower ,so, I installed a new electronic voltage regulator, started the car from cold, only to get same series of problems again.
In desperation, I bought a Mallory Dual Point from ebay,modified the wiring to suit,and car started and ran,appeared to be lot better ticking over for 10-15mins-even appeared to rev well and be responsive,but upon leaving the drive and onto main road, the backfiring started again under any more than partial throttle; but this time managing to limp home.
I have installed a new dual ballast resistor and made sure I have decent ground on both voltage regulator and chrome ignition box.
I have had problems with wiring loom because the car didn't originally come with rally dash and that was already installed when car came to me.
The installation of the loom was very poor and I've had loom out several times, chassis and engine, in order to get things to work properly.
One thing I have noticed, when the electronic ignition is installed, if the key is on 'run' position there is very audible buzz from distributor.

Sorry to rattle on sooooo much; I'm desperate to enjoy my car after taking 15 years to rebuild and risking divorce several times!!!

Please can someone help me????
Adrian.

[dave-r]

There are so many things that could be wrong I don't know where to start.

You are saying the air cleaner distorts? Why? Is it getting mega hot or is it blocked somehow so the engine cannot breath? How does the engine sound without the air filter?

Is the choke coming off OK? what is your mixture like? Too rich and it will bang and fart all over the place. Any carb leaks into the manifold? Are the springs on your carb metering rods matched for the manifold vacuum the engine is making? Too little vacuum for the springs and the carb will be flooding the engine.

If the distributor is buzzing that sometimes happens when you have a weak coil to dizzy lead. Dual point distributor? Are you kidding me? Dump that sucker and put the real distrubutor back in.

Get the timing up between 12 to 14 degrees advanced while you sort out the problem. Block of the vacuum advance for now too. Just to start ruling things out to keep it simple.

You absolutely sure everything is wired up OK? You sound as if you know what you are doing with sparks though.

That is all I could think of in two minutes. I wish you UK Challenger owners did not all live so far away from me.

[ianandjess]

is the back fireing through the carby or the exaust if its through the carby it could be intake valves being to tight or a valley cover leek i have had both these problems in the past
cheers ian

[Eddie]

Bet it's either the carburetor or a vacuum leak. What's a "tickover"? Sounds like the woman that lives next door to me!

[Adrian Worman]

Thanks to everyone for their ideas after my posting yesterday.
In answer to the points you raised
The air cleaner gauze distorted after a couple of backfires; I've tried running without the cleaner on but this made no difference-in fact I only screw cleaner back on when taking car out on road to avoid debris getting into carb.

The 750 Edelbrock is manual choke and I've checked linkage allows full operation on the secondaries. The idle mixture I set by the Edelbrock installation sheet, which I know is fairly basic, but when car is first started and warms up slightly it idles very smoothly at 800-900rpm.
As far as I can find there are no leaks into the mainfold. I've tried spraying carb cleaner onto the intake manifold where it joins the head, to check and see if there is any difference at idle. If any leak I would have expected engine speed to rise for few seconds?
I experimented drawing vacuum from things like brake booster; this makes strong vacuum at idle.
If I seal top of carb while engine running, after few seconds I can stall it.
The carb is installed straight from box-no adjustment made to metering rods, accelerator pump etc.
The buzzing carried on even when I installed new lead and good known coil.
I haven't gone nuts...I only installed the dual point as it was £25 cheapie in order to rule out electronic ignition probs.

Dave....I am sure of nothing with the wiring!! Electricals are not my strong point but am flattered I sound like I know what I'm doin!

The backfiring is very rarely through carb, always via tailpipes. When this gets really bad it sounds like bucket load of bangers going off.

What really confuses me is why the idle speed suddenly drops so low and eventually cuts motor when for previous 10-15 mins it idles and revs well on throttle,except for under load.

Unfortunately, I have the flu and have just shut my finger in car door so won't be touching my beloved Challenger until the weekend.
Any more feedback really welcomed....Thanks for all advise so far...you're are great bunch.

AirfuelEddie:- Tickover = idle...strangely like the woman next door to me too!
Adrian

[Eddie]

Adrian, Have you thought about valve springs? Pop a cover off and carefully look for a broken valve spring on the exhaust valve, if that checks out okay you might want to consider a compression test with a quality leakdown tester. A burnt or burned valve or valve seat could cause this also. As the engine warms metal contracts and any sealing irregularity in the cylinder will be get worse as it heats up.

[dave-r]

If it backfires through the carb the timing must be out somewhere? It can only do that if the intake valve is not closed I would think? So maybe a bent valve that is not sealing??

This is a tough one. But that's the way these things usually go.

[71_DEPUTY]

my 2 cents. sounds like a voltage drop somewhere

when it does this- check with a volt meter at the dark blue wire at the ballast resistor for around 12 volts or so- depending on battery voltage at idle- coil will be around 8-10 volts. check the battery voltage too- should be fairly close to the dark blue wire voltage. also when this happens- check the connectors that feed the steering column- heavy wires- ing. switch, as there are smaller ones for the turn signals. they should be room temp- if not you are lossing power via heat- bad connection. at same time check the bulkhead connectors for heat/melted plastic.

check engine grounds- from battery to engine, battery to rad support- nice and clean and tight with no paint behind the bolts. also I always add a braided strap from the rear of the block to the same bolt as the electronic ign. module to help in keeping the path short for the ing. and voltage modules.

John Mac

[Adrian Worman]

John
Thanks for the tip on the wiring from the ign.switch.Found a very hot bullet connector in the big white conn.block from the column, repaired connection and it has made some difference. At least that's one down,I'm almost happy. :)
Pulled the pllug's and they are all covered in sooty carbon eexcept no.7 which was sopping wet.If I pull no.7 lead off the dit makes no difference and aquick comp.test on that cylinder still hot would barely make 95psi.
Thnks to Dave and Eddie that one.
Cheers , Adrian