Mopar Muscle Association UK

Now a backfire has appeared. I have altered nothing since last week (no backfire!) except main jets from 68-66. Not exactly major tuning. Backfire appears under load at one third to half throttle. Getting very fed up with this car. All help appreciated as ever and sorry about grumbling.

Keep the faith man, that's one of the best engines in the world!
First thing I would check (and don't take offence!) check that you don't have a couple of plug wires crossed over.

If it's not that obvious, I would have to say that your timing is retarded and because it's backfiring under load at about a third throttle, I wouldn't mind betting that the vacuum advance is sticky or leaking...maybe as simple as the hose has dropped off!

Ivor, no offence taken! plugs are correctly wired. I'll check the vacuum springs although the hose is good. could it be wrongly adjusted valve lash? The new cam is 800 mile old now. I confess to lack of knowledge here as I thought hydraulic tappets were self- adjusting.

I think if there was a cam/hydraulic lifter/valve lash problem, you would have been hearing it clacking away like a good 'un.

The Hydraulic lifters are self-adjusting, assuming your push rods are the correct length!

Ivor,
Not valve train as you say. I clicked the dizzzy weights and springs, checked all 'leccy connections and richened mixture a touch. 30 mile round trip- no backfire! Thanks, one day i'll understand!

Result!

Hi Miles mate,

Again no offence, and it might sound obvious, but this worked for me on a Nova and someone we both know on a Trans Am.

Wait for it to go as dark as possible and if you can away from street lights, start the car and look under the bonnet very carefully and you might see electrical tracing between the leads or something else on the engine. Take time doing it because sometimes its faint. I had two leads cross firing (they werent touching), that I could only see in damp dark conditions. The Trans Am was tracing from a lead to the air cleaner body!!
Try and pull leads away from other items with H T lead grips (Ive got some if you want to borrow em) or a ruddy good pair of rubber handle pliers while you doing the above. Beware severe electric shock though!!
Also check you that this isnt the same problem you mentioned in another post regarding leads and plugs.

If you get a shock from your HT leads just by touching them then you need to get some new and better quality leads.

ANY leakage is wasted power and economy.

Happily, no shocks off the leads. As for testing them in the dark, its a good idea that i've used in the past. Only problem is, it dont get dark 'till late this far south!
ps James, what will follow your Challenger if you sell it?