440 idle goes up and down, and backfires during idle

[jenkins71]

Hi Guys. I have a 71 Charger with a 440. It has an MSD 6A box and a Mopar distributor.
I just replaced the spark plugs and plug wires, and put in a new carb. I used Autolite 85's and Summit brand 8mm wires. The new carb is a 750 Holley with vacuum secondaries.

I recently purchased this car. It has been sitting for a while. All I've done so far are the plugs, wires, and carb, and before these new parts and after these new parts I have had the same problems with idle.

The idle seems to go up and down in RPM's, and also there is a pop-pop sound every once in a while, through the exhaust. It can be heard mainly around the mufflers.

Any ideas what it is? Ignition stuff? Or idle mix setting? I think my idle mix is set up about as good as I can get it though.

THANKS!
Sam

 

[Dragon Slayer]

I just had this issue with a 383 RR that sat. I had to replace many of the vacuum hoses, and finally replacing the PCV valve grommet in the valve cover solved many of my issues. I also had carb idle mixture out of whack, but you have a new carb. Look for vacuum leaks. G

 

[WP29440SE]

could be a stretched timing chain

I would definitely try a different , known, good running carb first.

 

[bigmanjbmopar]

Have you checked the firing order? sounds like timing is off check it also you may have a blown power valve in that carb.

 

[coloradodave]

Since it is a new purchase, the first thing I would do is to take a compression test to check the overall condition of the motor. Next, hook up a vacuum gauge and check the reading. It also will help you set the timing and idle mixture correctly. The point gap or reluctor gap should also be checked. A little diagnostic work can go a long ways toward finding a solution without throwing parts at it and hoping...also, depending on how long it has been sitting, the gas may be stale which can contribute to the problem. Good luck.

 

[jenkins71]

I just had this issue with a 383 RR that sat. I had to replace many of the vacuum hoses, and finally replacing the PCV valve grommet in the valve cover solved many of my issues. I also had carb idle mixture out of whack, but you have a new carb. Look for vacuum leaks. G

I'll check more for vac leaks, but this car doesn't have much of the vacuum stuff remaining as it was set up by the PO to be a street/strip car

 

[jenkins71]

got my A/F gauge hooked up. At idle, it runs around 14.7, then gets leans here and there sometimes going up to 18. More to come...

 

[67 coronet]

good way to check timing chain is with a timing lite.start shooting,rev it up to 3k or so and watch as it comes back to idle.mark should be fairly stable as it comes into place.if it jumps up and down,then the chain has slack and should be replaced.
also,try slowly covering the carb(air cleaner off)with your hand.if the idle speed goes up at all,then there is a vac leak.
just a couple simple things to try.