Could a weak battery or one that is going south be the cause of a rough idle or miss at a hot idle? Everything is smooth with the idle, revs nicely and idles great up until the temp reaches 180 deg and then it wants to idle down really rough to the point of a miss and surge situation.
Engine is a 451, solid cam 254/.520, MP electronic ign, MP distributor, chrome box, 800 Eddy carb, 20 deg initial timing. It will idle at 1000/1100 rpm when cold but as the temp comes up it will idle down to 600 or so. It runs really good except at idle. I'll have the battery tested here in the next day or so but just looking to see if this may be cause?

Possible? Yes. But the simple determinant is to measure the volts at the batt. Got 13.5 +?
While engine running.
If the batt is old and likely on its way out....replace it.

It could if it was drawing your system voltage down, like PacNorth says, what's the voltage?

I cant find my meter, so I'll take it out and have it tested.

That's not going to tell you the whole picture. If you get 12.5 give or take on the batt, great.

But the real story is the volts, while the engine is running rough.

OK, I see. If I cant put my hands on the one I have I'll get another one and test it running.

I'm thinking elsewhere. I would start by removing the idle mixture screws & shooting a 2 second blast of brake kleen in the (2) ports with the thin red straw then reinstall the screws. turn them in to lightly seated before removal & count the turns to easily return them to their original positions when done & they might need richening. carb might need cleaning. Keep us posted.

I went through the carb and rebuilt everything, it was spotless inside when I opened it. The screws are 1.5 turns out and have always stayed right I have them set.

this just happened correct? Something changed (dirt or vac leak I'm guessing). I would still blast out the idle mixture ports (easy to do & not invasive)

Its been in this situation for a while now but I noticed it after it had sat for an extended time. The car can sit for months before I drive it. I can try a blast of cleaner in the screw holes but it was doing the same thing before I rebuilt the carb.

Bad gas? The new stuff goes bad a lot quicker.

The gas here is pretty bad but I do put a few gallons of fresh gas in it when I get it out. I have ran it enough to use any old stuff that was in there in there and replace it with fresh gas and the problem is still there. The part I don't understand is why the rough idle at operating temp.

Low voltage to a Orange box will eventually kill it. It's struggling to draw amperage from a weak system. As it starts to go south the motor may start backfiring. Put a good battery in the car and change out the box.

Think it runs bad, try running an MSD with a bad battery.

I hope it hasn't started to wipe out the box. I do have a spare orange box that I ran for years with no issues. I just found the sticker on the battery and its much older than I thought, April 2010! I really don't know how it has lasted this long. Batteries here are good for about 2 years tops. Needless to say I will be getting a new battery.

Do you run a choke? Maybe it masks a vac leak. Check ground for your ECU, but likely that would just kill it and not run rough. While at it check to see if VR is getting a good ground too.

I would say that your box is toast. Doesn't take much for them to go. I've seen them loose their ground and pop. If you need a spare I have several laying around.

My experience has been that the Mopar electronic will just quit, not cause stumbling and crap at op temps. I think that this has to be fuel related.

Check fuel pump pressure.

Doubt vapor lock, but if fuel line is in contact with any hot engine components, all bets are off.

Fuel filter ok?

Is engine running on fast idle cam when first start because of a closed choke, choke opens engine off fast idle cam on regular idle screw setting which may be to slow?

Bad coil failing as it warms up?

Cracked or otherwise damaged distributor cap or rotor.

What does a vacuum gauge read at idle.

Do you have another carb to throw on it for testing?


Engines normally will run perfectly even without a battery if charging system is in order, doubt if it is failing battery.

Engine ground sound?

Run a jumper wire from battery positive to coil positive to rule out any ignition wiring issues.

Does engine have a ballast resistor in good condition.

Coil wire to dizzy sound?

Coil wires nice and snug?

After I replace the battery Ill swap in the old orange box and go for a drive.

I have a bracket I bought from Hergfest that bolts to the firewall. I have the ECU bolted to the bracket. I also run a separate ground from the box. The bracket also allows for an airgap between the the box and the firewall.