After 13+ yrs restoring my challenger I ve been out driving it over the past two weekends! Feels great and I have fewer bugs to work out than I expected!

One problem I am hoping to get some help with is tuning my Holley 5150 600 dp.

My experience fiddling with carbs is limited, however this is something I would like to become better at. So here's the issue...

As I shift into gear (first->second or second->third) and heavy on the throttle, the car pulls fine but as I let off the pedal and coast, car stumbles and sometimes it dies sometimes i can save it by feathering the gas. If car does die, it starts up again with out much issue, then i can continue driving ok, but i can reproduce the problem almost everytime.

Some more details....
Exhaust smell burns the nose, lean/rich?
I took a plug off after idling in the garage and it looked fine to me, maybe a bit lean. Ive checked the float levels and they were ok, ive hooked up a vacuum gauge and adjust the 2 idle mixture screws for MAX vacuum ( 14-15 is as high as i got), adj the idle screw so it idles ok about 800-900rpm. Car runs pretty cool (180-190) although its been hot here recently.


The car:
70 Challenger, 4speed OD, 3.73 rear, elect ignition.

The Engine:
69 340, bored .030
MoPar M1 dual plane intake
holley 600cfm
2.02 valves
CAM advertised dur 268/276 lift .429/.444

Ive found some help on the Holley website, but was hoping to get some more direction from the experienced folks on moparts!!
Any ideas where to go next

You might take out the idle mix screws & shoot a 3 second blast of starting fluid into each port but it does idle OK at rest. The unburned fuel can be too rich or too lean (lean misfire & you smell the fuel that is there at the tailpipe which ain't being burned). What's your initial timing. Reluctor gap OK. rotor phasing OK. fuel level at bottom of sight holes. floats not hanging up or waterlogged. Kind of WAG here but wanted to toss out some thoughts. It acts up coming back to idle when you let off the gas but it idles OK at rest which does point a bit toward the carb. holler with any news

Get a fuel pressure gauge and keep it at no more then 6-7 psi also lower floats to point where gas does no come out of sight hole without rocking raise idle to 1000.

If by the way the Holley you have may have plastic floats {not the brown ones} Make sure the limiter spring is on the arm other wise the will cause a issue

Back in the old days, there used to be a device that set between the throttle and the throttle stop (I believe it was called a dash pod? been a long time), That device simply cushioned the throttle from slamming close when you left off the gas pedal. This was used because when we went from wide open, to instantly letting off, the throttle plates would effectively close off all the incoming air into the motor creating a momentary fuel over load from the fuel that was flowing into the intake and through the carb. Since the fuel weighs more then the air, it takes longer to get it moving, but it also takes a little longer for it to quit moving. The dash pod would hold the throttle blades open just long enough to allow the fuel to stop moving before the air was completely stopped.

If the problem your experiencing is the motor dieing when you abruptly let of the gas after running wide open, this could solve your issue.

As far as the burning of the eyes from gas fumes at an idle, its likely a cause of the cam. High lift, long duration with a big valve open overlap are not real efficient at idle. Its the nature of the beast. Gene

I had the same issue with a 413, I pulled the holley, put on an edelbrock 600 and the problem went away. Also fixed the very rich exhaust smell. I even tried 2 other holleys before making the switch... the engine just liked the edelbrock I guess.

I ditched the Holley 600 that was on my 340 Cuda the first year I owned it (20 years ago this summer). I put on a big bore Thermoquad, best engine improvement I have made on it. Love the sound when those huge secondaries open.

I forgot to mention a Cagle Fuel pressure regulator They reduce fuel pressure auto with vacuum

*dashpot

Mine will stall if I get off the gas too fast while up in the revs. I think it's just the nature of the beast. A dashpot would help but even better is just don't close the throttle so fast, or like you said give it a squirt so it doesnt stall. Sometimes I'll let the clutch back out to keep the motor spinning through a potential stall but I'm thinking you run an auto?

Good luck

My 6 pack was doing the same ...kind of, it would idle low when coming to stops and almost want to die. It was a combo of lean and idle to low make sure you have at least 15 to 20'initial, holleys never seem to idle great low i would try for a idle speed of about 950 to 1,000 at idle neutral and even though you have your idle screws set turn them out a 1/2 turn this got my 6 pack to act correctly at stops and off throttle.

Ya this is a 4spd car.
I will start with the simplest and easiest solution first....bumping up the curb idle to 1000rpm and see if I can duplicate the problem...
I am swamped busy for a couple days here, but I promise to report back.

Thanks for the ideas this far everyone!

Ok ive finally gotten the car out for another drive. I adjusted the idle up to 1000rpm. seems better but I feel like it has to do with operating under a load, Its definetive when i honk on it up a hill, say upwards near 3000-4000 rpm, and when i let off the gas, still on the hill, i will stall. but if I keep on the gas right thru to the top of the hill its seems ok......any thoughts?

Float level....................

yep float level. too high. easy to adjust on a dp.

update: went for a quick rip, definetly seems to have helped by lowering the float level! THanks everyone for helping out!!