Car back fires

Carb could be needing a rebuild-firing order okay and plugs/wires Check the ignition timing if okay-try adjusting the A/F mixture and curb idle speed-

Misfiring? Surging?

Are your points perfect?

The Medic

Are all of you vacuum lines connected?

"Empty" Carb Ports plugged off?

What is your vacuum advance hooked to. Manifold Vacuum or to Ported Vacuum?

Can you tell if mechanical advance weights are still springed in? Not missing their springs.

You should be able to idle around 600-700 RPM

Sounds like at 1200, you are on the mains, not in the idle circuit at all!

Is your PCV operational? It too, provides combustion air at idle, if it's plugged off. You are starving idle of air!

Gotta ask this one. Is there some sort of mechanical device, preventing the throttle plates from closing more?

Is it possible the incredible Hulk has screwed your mixture screws in so much/ past the "Stop", that now your orifices are wallered out?

Your Turn

The Medic

Check the choke opening and pull off adjustment-seems to me its raising the rpm due to the choke plate is not fully opening/vertically-if its not it will pull more fuel from the fuel nozzle.

Check the conditon of it and make sure its gapped accordingly to specs

I asked about misfires/ skipping/ and stumbling (really just the 1st!)

On my '46 Willys Jeep (my avatar)

I bought and installed new points, I was told they were the "Cat's Meow", by the parts fellers!

They were "Vented Points", meaning they had a hole thru both "anvils". Two little metal doughnuts, smacking each other!

They ran OK for about 500 miles, then I developed a constant stumble, felt both at idle and at speed

Filing/ readjusting/ even trying to bend the anvils to mate more evenly did not help.

I'm gullible! I replaced with the same thing. Same problem

Never will use 'em again!

Your problem really sounds like idle mixture or combustion air. Too much or too little---Not Stumbling----The reason you cannot get your speed down, is it's running on the Mains (adequate fuel air mixture there)

When you close the throttle plates to lower your speed. Soon as you get into the idle circuit. Correct fuel air mix is un-acheievable!

Was there Never a PCV on that thing originally? Or you just didn't stick one on?

If the carb is ported for that amount of air, that the PCV would provide, you need to install one. If it never had one, and your carb is an exact replacement, Not a more modern carb. Your combustion air at idle, w/o PCV should be calibrated to the "Old Carb"

Confuse you with all that?

Adjusting your mixture screws at 1200 RPM, is probably doing very little to get the idle mix right, probably not even close. You gotta get the speed down, off of your mains and onto your idle circuit.

The Medic

Just for giggles

Create a small or growing vacuum leak,

Maybe uncap a port, add a piece of hose (maybe controllable, using pliers to pinch it).A simulated PCV

See if you can idle it down some, and adjust your mix screws!

The Medic

Remove the mixture screws and inspect its tip, also make sure the throttle plate is fully closed not being open by sticking linkage/cable and no vacuum leak. Back off on the idle speed speed screw and see if it'll drop-also could be the dashpot out of adustment that's if it has one-

Recheck the Carb and also if the base gasket is correctly installed and not leaking -You have 3-types of vacuum-that is variable venturi, ported and manifold-how many vac hoses do you have on the Carb attached to and where is it coming from? Pic's will be perfect

Your PCV system is the downdraft type-that don't have a PCV valve

new carb was cleaned and rebuilt. Might not be rebuilt properly?