I am trying to track down a misfire/strange idle on my 1971 Charger/440.

It is idling along a 900 rpm in Park, then it will idle up to 1,200 for a few seconds, then idle back down, and then repeat. It's annoying as anything. It starts to do it pretty much as soon as I start it, it doesn't only happen warm or cold.

One thing I noticed while adjusting the floats is that when I loosen the adjustment screw on the primary side (engine off) to raise or lower the float level, gas leaks out, but it does not do this on the secondary side. I thought maybe the needle was sticking or something, so I put a brand new needle/seat assembly in, it made no difference to the idle.

Is it possible the primary side is getting overpressurized or something and dumping fuel until the pressure drops? It's a Carter mech fuel pump with a QFT carb. The float levels appear to be spot on at idle

Any other ideas what would cause that type of idle issue? I am pretty sure there are no vacuum leaks, but maybe I am missing something along those lines. I also made sure all my plug wires are isolated and I am not (I hope) getting a crossfiring thing happening.

All help is appreciated.

Greg

Mostly stock engine? Big cam? Can only guess without more info.
Misfire and changing idle speed are different things.
900 RPM idle is pretty high for anything near stock when warm.
Could be advance weights, advance curve, vacuum leak, throttle blade adjustment...
A little leakage at the float adjustment can happen when loosened; note it has a gasket.
Sounds like you have a Holley-type carb: Carter mech pump shouldn't bother it.
Holleys will take more fuel pressure than most mech pumps deliver.

I know, there's a million things...

It's a pretty basic 9:1, iron headed 440 with a mild cam. The distributor is a brand new Summit one. I replaced it because I thought the timing was jumping around at idle, but it was just my dial-back timing light making it look that way. It made no difference to the idle

I could try idling it down a little and see if that helps, but it seems like the type of problem that shouldn't happen whether its idling at 900 or 700 (but who knows). Maybe too much transfer slot exposed?

I guess just adjusting the idle down a little is easy enough to try.

Thanks for the help.

Greg

What fuel pump and what pressure is it running at?

Its a couple year old Carter mechanical pump, I think its rated at 8 psi, but I don't have a gauge, I didn't think I would need one with a mechanical pump. The car has run fine in the past.

An erratic idle could caused by erratic fuel delivery, right? Maybe the pump is failing somehow.

How steady should the fuel level be in the sight glass when it's idling? It looks pretty steady to me, considering the car is vibrating as it idles.

sounds like a lean idle to me. back the mixture screws out some and slow the idle down a little. With that said you should get fuel leakage at the secondary lock screw when loosened. when they are lean they surge like that. What happens when you put it in gear. I bet it wants to stall.

I will give that a try, I have spent some time tuning the carb, but maybe I need to go back to square one. It actually does not want to stall in gear though, it's totally driveable.

Tuning a carb idle mixture is a bit of an art form. When I got my 51 Plymouth it was idling at 1200 rpm, spec is 450-500. Dwell was way off, points ignition, timing was way off as well.

I ended up just starting from square one rebuilt the carb as it was leaking fuel all over the place. New plugs, wires, cap, rotor, points, condenser, etc.

Then I started tuning it. A brief write up on how I did it.

http://www.yourolddad.com/tune-up

Remember, idle mixture, idle speed and timing are all interrelated and you need to check/adjust all three until you find a happy median.

I was just thinking 900 is pretty fast for a mild combo. Alot of people will just crank up the idle to keep it from stalling.

Well guess what? Instead of eating lunch, I went out an worked on it, basically resetting and readjusting the idle speed and idle mix screws (for the 10th time it seems).

It appears, fingers crossed, to be happily idling along at a steady 750 rpm. I am guessing it's not perfectly tuned, but I am not going to mess with it until I either drive it, or it starts acting up again.

Thanks for all the help as I hack my way through owning my car!

A happy mopar is worth a little starvation.