Is it probably the ECU?

['74 Sport]

The Dart's 360 with Holley double pumper was broken in and tuned using an MSD box on the dyno. When the engine was installed in the car with a Mopar orange box, it took us a little while to realize we needed to reset the timing because of the difference in the boxes. Got the engine running great with good throttle response. Took the car to the transmission shop on a trailer and it suddenly decided to begin having intermittent starting problems. For a quick fix, I tried a box from Autozone and it fired right up and has continued to every time since. However, now it has an initial stumble and sometimes pops through the carburetor on hard acceleration from standstill. Is it a timing issue, once again because of the different ECU?

 

[Abodybomber]

Start with an charging systems check . Battery in the trunk?/aftermarket cooling fans? This info, is needed, how did you wire it? This info, helps to start....

 

['74 Sport]

Nope, nothing fancy. Everything is basically stock...wiring harness, grounds, distributor, coil, mech fuel pump. Only change was the ECU to get engine starting again. My question really boils down to whether or not differences in the ECUs would necessitate a change in ignition timing. I'll probably know the answer when a new replacement orange box goes back in. Was just curious to know in the meantime.

 

[dodge71demon]

Just a thought maybe the MSD had a hotter spark possibly effecting the timing and carb adjustment --- check both but I would start at the carb - check the plugs for fouling more than likely it's something simple

 

[tekslk]

Orange boxes are junk, Chrome a little better. Best thing is look up trailbeast he has a hei conversion that eliminates those boxes ballast etc and has a nice hot spark. I think his kit is $150and real easy to install, he has a video and will give you support for it.

 

[nm9stheham]

Based on the symptoms it could be either carb or ignition. But I would certainly look at ignition since that is the last thing you changed....

If ignition related: It is perhaps the timing; that is an easy enough thing to check, right? Is this happening at all RPM's or just low or high RPM's? Otherwise, it sounds as if the spark may be getting weak per the symptoms. Could be the box, the coil, the coil wire, spark plugs or plug wires, rotor, distributor cap.

What coil are you using?

BTW, when you originally changed timing for the orange box, how much did you have to change it from the MSD setting? It would be no surprise at all for there to be timing differences, based on circuitry differences. They could easily have different internal delays or even trigger off opposite edges of the pickup pulse.

Any way that you could have flipped the polarity of the pickup connector on the distributor when you installed the orange box or the new box?

 

[67Dart273]

Any way that you could have flipped the polarity of the pickup connector on the distributor when you installed the orange box or the new box?

This was my first thought. Google "rotor phasing"

 

['74 Sport]

What coil are you using?

BTW, when you originally changed timing for the orange box, how much did you have to change it from the MSD setting? It would be no surprise at all for there to be timing differences, based on circuitry differences. They could easily have different internal delays or even trigger off opposite edges of the pickup pulse.

Any way that you could have flipped the polarity of the pickup connector on the distributor when you installed the orange box or the new box?

1. Off the shelf from parts store. Not sure of the brand.

2. Timing was several degrees out. Would barely start and ran like crap. Fellow with dyno was the one who realized just what you said... MSD was triggering on opposite edge of pickup.

3. Don't know how to flip polarity. Just simply unplugged and unbolted orange box from inner fender, then plugged in and bolted down replacement box.

The stumble is at low rpm from standstill. At 20 mph, kickdown does its job and it goes.

 

[bOb shingler]

first thing, toss the new box back at the guy in the store. second, get a new chrome one like some one suggested. three, clean the grounding surfaces and the edges of the box.

 

[AJ/FormS]

Where are your timings at. A lazy idle timing will do all that.
Reverse polarity is easy to spot with a timing lite; the rate of advance goes bonkers instead of its usual steady rate of increase. To reverse the polarity, the connection between the dizzy and the ECU has to be flipped, either with an oem jumper(I have one) or an alternate mag pick-up(different colored leads) or by cut and splice.

 

[nm9stheham]

^^ Yep, flipping the polarity is easy; just reverse the 2 wires at any connection between the distributor and the ECU. If any of your connections distributor-to-ECU connections can be flipped either way, then it would be easy to do by mistake.

Could be weaker spark or messed up timing for the low RPM problem you describe; but could be other things too. It is just the change of boxes that you report as coincidental with this problem that points to the ignition.

Sounds like you would be helped to invest in a timing light and use it....

 

[Bad Sport]

However, now it has an initial stumble and sometimes pops through the carburetor on hard acceleration from standstill.

The stumble is at low rpm from standstill. At 20 mph, kickdown does its job and it goes.

Have you checked pump shot? With the engine not running, crack the throttle quickly as if you're mashing it from a standstill while looking down the carb throat.

Do you see an INSTANT shot of gas, or is there a slight delay?