intermittent carb backfire?

Adam · Post by **Adam** » Mon May 03, 04 9:42 pm

Friend just bought a 72 Charger SE with the 400 (Alex will know the car). He's tickled to bits with it, but about every 3rd time he stands on it, it falls flat on its face and backfires through the carb. It has a Holley 600 vac sec, Edel dualplane, headers and MP electronic ignition with a blaster type coil. Sounds like it has a mild cam in it too.

I checked all the usual stuff, and found the initial timing to be a bit high (18 deg), but retarding it to 10 deg didn't seem to help any. No visible dribbles from the carb, but I did wonder about a ruptured or weak power valve. I'll swap out the Holley for my 750dp to rule out the carb next time.

In the meantime, has anyone got any bright ideas?

Holly · Post by **Holly** » Mon May 03, 04 9:49 pm

Is that the gold one? There was a red one somewhere too ...

Alex · Post by **Alex** » Mon May 03, 04 9:53 pm

Is this a yellow car? check float level and the shooters are working right,if it is the car I think it is,it had 12 deg initial timing before so something must have slipped,also check the reluctor gap in the dizzy at 90 deg sections and also check for shaft float in the dizzy..

TrevD · Post by **TrevD** » Tue May 04, 04 8:32 am

I had the same problem a few years ago on a 360 after months of trying to sort the carb ,i found out it was the distributor , it had too much play in the shaft i put a new bush in the dizzy , set the air gap to 0.008" and it ran 100times better and never gave any more trouble

Adam · Post by **Adam** » Tue May 04, 04 7:02 pm

Yes, Alex it's the yellow one. Float levels and shooters are fine. Timing mark was wobbling a little: 10-12 at idle after I retarded it, but no worse than most. I'll check the reluctor air gap before I swap the carbs though. It's the intermittency

of the problem that made me think it might be the carb. Anyway, thanks for your advice guys.

Alex · Post by **Alex** » Wed May 05, 04 7:49 pm

I can't remember if there is a fuel filter on that car, if there is then change it out, no idea what is in the tank.....

Another thing I have noticed from modern imported cars is the strainers in the tank are pretty locked on a lot ofthem, food for thought

Adam · Post by **Adam** » Thu May 06, 04 9:44 pm

Yes Alex, there's a little Halfords plastic special perched above the intake for maximum vapour lock potential. Looks clean enough though, and the fuel bowls are full up to the float levels, so I think it's getting enough fuel. Thanks for thinking about it some more.

Alex · Post by **Alex** » Thu May 06, 04 9:51 pm

Ahh, float bowls are full at idle, but what is happening under load ?

Funny is this as I drove that car and it went pretty well, that's why I am wondering if it picked some dirt up on the way down south?

Holly · Post by **Holly** » Thu May 06, 04 9:58 pm

Very nice! Thanks Adam

Kev · Post by **Kev** » Fri May 07, 04 2:02 am

Hey, Alex! Perhaps it's running on better air, darn Sarf!

Adam · Post by **Adam** » Fri May 07, 04 7:02 pm

Good point Alex, but it will backfire if you snap the throttle open from idle at a standstill in neutral. In fact it's more prone to doing it when NOT under load. It certainly shoots an impressive flame up the carb throat without the air filter on - it singed my eyebrows!

You may be onto something though, like maybe dirt in the pump squirters or main jets. Andy said it was fine on the drive down and for the first day or two. It has got progressively worse since, but it may just be that he was being gentle with it initially. I did wonder about the blaster coil malfunctioning - not sure whether it's one of those MSD type ones that fires lots of sparks for each ignition. Anyway I'll swap the carb tomorrow to rule that out.

Another pic for James, with the cool Rallye bulge hood...

Anonymous · Post by **Anonymous** » Sat May 08, 04 6:26 pm

That does look nice, I love that colour.

Adam · Post by **Adam** » Sun May 09, 04 9:53 am

Yes James, it's the correct colour too - FY1 Top Banana. A genuine, High Impact, big block B body...
Didn't look quite as sharp when it first arrived though, eh Alex?

Holly · Post by **Holly** » Sun May 09, 04 2:28 pm

My car was that colour back when Bill Mason had it! (two paintjobs ago) Nice to see I'm not the only one with a cowl induction effect to clear the wipers

Adam · Post by **Adam** » Fri Jun 18, 04 4:07 pm

Thought I'd give a quick update on my 3rd attempt to tame Andy's Charger's backfire. Found lots of things it isn't, but still can't diagnose it

Firstly, I took the dizzy cap off to check the reluctor gap, and found... a set of points! Yep, the electronic ignition module on the firewall fooled me into thinking that it had an electronic dizzy, but someone has swapped a points dizzy in. The module's wires are taped into the wiring loom, but don't actually go anywhere. Anyway, the points looked new, but the gap was about 10 thou, so I re-set the dwell (and timing, of course). It ran better generally, but still spat back through the carb once it warmed up (is that a clue?).

We then swapped the 600 carb for my known Holley 750, which made no difference at all, so the carb is ruled out. Fuel pressure is fine too.

Vacuum test showed a steady 17" at idle, and behaved as it should when the throttle was opened and shut quickly. So, I'm pretty sure there aren't any vacuum leaks, but we test blocked all hoses anyway, including the brake servo.

I'm starting to think it has to be electrical. It has high quality new plug wires on it, but we made sure they are separated properly. Andy will try a new condensor, but after that he will take it to a place that has diagnostic equipment.