1971 Road Runner Running Issue

Hey guys, so I'm about ripping the last of my hair out trying to figure out what exactly is going on. Here's the scoop:

I took my car out and drove it on the highway for about an hour and had a total loss of power. Pulled over, started up and twenty minutes later, same thing, loss of power. While this is happening, mind you, I'm getting severe backfires. I get home and troubleshoot. Swap out the ignition module and install one of those orange ones. I'm wire for 5 prongs, but have used 4 prong ignition modules in the past. I also have a 4 prong ballast resistor. I took it out the other day and it ran fine. Today, I take the car out and it ran well until I decided to accelerate. I got exhaust pop on deceleration and the engine seemed to run rough (shaking a bit) when accelerating. I swapped out another ignition module, ballast resistor and pickup coil to see if it would remedy the situation. Nothing. Any thoughts?

Sounds like a weak fuel pump to me. Had this same thing happen on my carbed jeep. I had a vapor return style fuel filter on there and I figured out it must have been returning more than my weak fuel pump could push when it got hot. I clamped off the return line completely and that was a temporary fix until I replaced the fuel pump with an electric one back at the tank.

First do a resistance test on your ignition coil to see if the resistances on primary and secondary side of the coil are within tolerance. Check the coil service manual for ohm resistances to check. If it is a factory coil, check the FSM for ohm resistances to check. If they are not, junk the coil. If they are, continue with a dynamic test and wear rubber gloves and eye protection. With your car running, pull the high tension lead from your coil at the coil very slowly. The spark should jump a gap of an inch or so visibly and audibly with no trouble. If it is not, if it is sputtering and quiet your coil is fried possibly because of your 4 prong ballast which does not match your existing coil.

Had the same thing happen to my 1971 Roadrunner with the exact same symptoms. It may be worth a try.

"Loss of power". Are you saying the car would die and not even crank for a little bit?

I just started it up again. Starts up fine, idles fine. When I accelerate, it seems like the engine can't get past a certain rpm. It's happening just under 30mph. It's the most bizarre thing.

What coil would match a 4 prong ballast resistor? Right now I have a 12v red MSD coil.

8203 msd coil comes with the resistor that they want you to use with it.

I just looked at the fuel filter too. Barely has anything in it.

Not one of the cheap clear filters with chrome ends on it I hope. I have seen them look like nothing in them and not let hardly any fuel through.

no, it's a napa gold filter that I've always used.

do you have another ign coil that you can borrow/sub in for a quick test.

yes,i swapped out the msd with a yellow accel super coil. car doesn't seem to be acting now.

All good or still acting up? EDIT Accell wants a .7 ohm ballast with that big yellow beast & your 4 terminal "dual" ballast will for sure have more than that. You might wire it in parallel (just add a blue loop on the other end also) to get the most out of this combo. That will work fine with your 4 pin ecu.

Whatever coil can work when matched to the resistance of your 4 prong ballast. If you really want to keep the 4 prong, test it to make sure it really does have a resistance of??? i dunno...let's say it's supposed to be 1 ohm. If you read 1 ohm on your multi-meter, shop around for a coil that will work with a resistance of 1 ohm. It should be marked on the underside of the ballast but if not there do a Google search to see what it's resistance should be. But first make sure your coil is bad before you start doing all that.

Sounds good. can you get the spec for the yellow Accel if you are going to leave it in the car? It would be nice to know what ballast resistor is needed with that coil.

Thanks for all the info gents. I will get the specs for the yellow Accel. I think I'm leaving it. The car is now accelerating without issues. Some exhaust popping, but that's on the deceleration. I can tune the carb up some. I will keep you guys updated.

Did you say what kind of distributor you had?

It's a mopar performance distributor (brown cap). It came with the orange box and 2 prong ballast resistor

the last 4 terminal "dual" ballast I parallel wired gave me 1.1 ohms which will work fine & is above the .7 ohms Accell wants for a minimum with their supercoil & that amt (1.1) is lower than what your 2 terminal "single" ballast will have. EDIT Assuming you don't have the 641 MP "race" 2 terminal "single" ballast (has .25 nominal ohms).

Your problem is ignition not fuel. Did you change out the orange box or visa versa? My bets are on the box. Do you have any other spare boxes? Also, it may be the box is not grounding. I had a similar issue with an orange box. Is the box bolted to firewall? You may want to run a separate ground wire from the box to a common ground. I did that and it was problem solved.

I swapped out the orange box. installed a five prong black box. it is bolted to the firewall. I'll try the ground wire idea out. Thanks!

An OE 5 pin box (has a functioning 5th pin) requires the 4 terminal "dual" ballast & its attending wiring. Aftermarket ones with a dummy 5th pin (has no continuity to anything) works with a "single" or "dual" ballast/wiring. The 5th pin is the terminal that the red/green wire is on in the pentastar connector.

My dad had a car that did things like that, it turned out to be the winding in the distributor coil had an open and when it got hot it quit.

Very helpful info! Thanks.

Check for exhaust leaks.

Hey guys, car starts right up. Idles strong and drives normally until I get up to around 35-40mph. The engine sounds like its starving for fuel. I checked my fuel filter and there was hardly anything in it.

get a gas can & a length of common 5/16 neoprene fuel hose & plumb the gas to the pump inlet with the can in the eng compartment or inside on pass side & retry it. some carbs have a mini coarse filter behind the inlet nut. next is what Dayclona said, try another pump. that would cover about everything fuelwise that I could think of. didn't read back thru all posts but float level is good, I dont think that could be it (unless a WOT issue) but K.I.S.S. is always a good plan