440 backfiring during highway driving only

I have a 68 Satellite wagon with a stock 440 in it with the exception of Edelbrock carb. Motor was fresh before I bought the car. I had only driven it around the neighborhood and on local roads until today and it ran fine with no issues. Today i took it out on the highway and within about a mile of doing 65-70 it bagan to backfire loudly through the exhaust with black puffs of smoke. I lost power and had to pull off the road. Car did not stall however and after i sat for a couple minutes everything seemed fine so I got back on the highway. Within 1/2 mile the same issue. This happened over and over until I got off the next exit and headed home on back roads doing 35-45 and there were no issues. The car does not backfire or sputter under load from stoplight to stoplight even under heavy acceleration. Just on the highway. I pulled a few plugs and they are nice and tan so before I start using my limited skills to diagnose I was wondering what Moparts members thought. Timing issue? Fuel? Vacuum? Car idles fine. Just want a good Starting point before i begin my easter egg hunt.

Check your timing and see if it wanders. Also, sometimes a new engine will have a timing chain issue due to improper installation or a sloppy chain due to machine worth such as align boring etc.

Starving for fuel @ highway speed ?,wrong fuel cap,fuel pump drive rod worn out,possibly something along those lines ck.fuel PSI.& volume.

I am going to check the fuel pressure. Going to make sure something silly like fuel filter isn't bunged up some too.

I agree, maybe weak or bad fuel pump or worn pump push rod. Sounds like it is runny out of fuel.

coil/condenser (or ECU). EDIT I am the odd man here, if fuel I'd say partly plugged tank sock/fuel filter further upstream/carb mini inlet screen filter

I have a good spare fuel pump out in the garage somewhere. I will throw that on tomorrow and see what happens. Hoping that's all it is because dialing in a timing issue is not one of my strong suits.

It does sound like a fuel problem. Change out the filter first. Check the fuel pressure, you should be able to get a lender gauge at a parts store like Auto Zone if you don't have one.

Fuel filter

Something very easy to check; disconnect and plug the vacuum advance to rule out too much timing under cruise conditions.

Changed fuel filter. Plugged vacuum advance. Same issue. I also had it happen while going about 45-50 today. The guy in front of me was slowing down so I had to let off the gas and then depress the pedal after slowing. Well, he was driving like an idiot so I had to do this several times and the car began to hesitate and I lost power. Car stays running but won't accelerate. Pulled off for 30secs or so and back on the road with no issues at 40 mph the rest of the way home. I will say that the engine also wants to fall down for a second when you start out in first gear. I really have to goose the gas and clutch to get a smooth start. Taking it to the shop tomorrow to try to get to the bottom of this.

What ign do you have? I feel lucky!

MSD 6AL

Hi,
Had similar issue.
Can it be sticky carb float?. With today fuel(Éthanol)it doesn't lube the Inside carb anymore.
Once in a while, I drop auto-trans oïl in fuel tank before fueling. Old-timer(uncle) suggest this to me and it work.

Just me. Reggie

Have you been inside the carb? It could be a piece of junk in a main jet. The engine would run better when you are into the secondaries and the power circuit. It happened to me, although it was a Holley carb.

R.

After much messing around over the past few weeks I have finally got the car running better but only with the vacuum advance plugged. Tried another distributor but problem was the same. Checked timing and found it was at 38. Backed it off to 36, plugged vacuum advance, adjusted accelerator pump and now no issue but I still feel like I should be able run the vacuum advance with this 6AL box. No more backfiring but is all this telling me my MSD is putting out erratic signals at cruising Rpm's?

Yes but you will have to limit the amount of vacuum advance it's giving you. you cant run 56 total like the old days at light throttle maybe only 40 then it's why bother. 3/32 allen key in through dizzy vacuum port turn CC till it runs best with no pinging. I'd take 4 turns out to start. maybe go back to 38 if it don't ping with vacuum line unhooked and work from there. get it right and you will get 1/2 mile to the gallom better on highway maybe a bit more.

rotor phasing (one of my fav's in additn to quench) might be borderline & causing the problem & ONLY vac adv changes RP. You might find the steady state RPM that it backfired with, with the can hooked up and note the timing at that RPM then unplug the can & bump up the initial to where the timing is now back to where it originally was at the same RPM in your driveway & take it out for a cruise & see if the problem went away which if so would confirm RP as the culprit. Take off the cap & see what cap terminal the rotor is closest to & make a mark on the dist top rim just below the CL of the dist cap outside bulge then take the cap back off & hand turn eng slightlytill the reluctor tooth is dead even with the magnet & the rotor is close to or at that nearby cap bulge location & the springs will be retracted (should be) & see how close the rotor tip is from dead on pointing toward the mark on the dist rim. be carefull as slop in the intergear slot/dist tang interface can skew this with too much play. the rotor is wide and the cap terminal is wide so so you have alot of circuferential circular distance movement to still be OK till the distance becomes excessive & it starts to miss & rotor radial tip distance affects this also (Echlin MO3000 rotor (NAPA) has a .060" longer tip & helps with this. note that RP shifts the rotor tip CW from its at rest position on a BB (opposite on a SB). when the combined (radial/circuferential) tip to cap terminal distance becomes greater so the required voltage is greater than what the available voltage can handle then it misfires. As said it might be just too much timing, dig in & find out what's going on. EDIT cap/rotor/plugs/plug wires OK? MORE EDIT since RP shifts the tip CW on a BB, unplugging the can can make misfiring better or worse depending where the tip is when you check it with the cap off & even better is to drill a 1/2" hole in the dist cap top flat 2/3 of the way between the center tower and the #1 wire cap terminal then shining your light down on the rotor while running & you can see the rotor in action & see it retract CW (BB) when you nail it & see the circumferential distance change between full vac (off idle if ported) & no vac (ported or can unplugged/capped). drill your cap (good for letting ionized molecules out) or a junk cap if aesthetics is important in that area. you'll see that only the can changes phasing

My new to me 67 Coronet with a LC 77 440 had a similar issue, it had an intermittent miss at idle and a miss/stutter when cruising. Turns out the previous owner had the vacum advance hooked to manifold vacum (pulling full vacum advance at idle), and had initial set at 36 degrees, so I figured I had close to 60 degrees at idle, and probably approaching 90 degrees cruising! Poor distributor couldn't figure out which plug it wanted to fire. Put inititial at 12 (38 total) and put the vacum advance back on ported and it's better. The 1411 Edelbrock still has a lean bog at tip in but that sounds par for the course

It was pinging at 38 with vacuum advance disconnected. Thanks for the great insight, guys. Looks like I know what I will be doing this weekend.

be sure & post what it ends up being