Just completely rebuilt my 383 for my 71 Roadrunner. Engine is running really rough. went to check timing and found that he timing is varying by about 30-40 degrees? Vacuum advance disconnected or connected no difference still a lot of variation. Checked for vacuum leaks and went through distributor. Ideas?

Thanks
paul

I'm assuming the engine sound stays the same (timing ain't changing) but it is an electrical/timing light issue? is this an MSD/dialback light? otherwise you might flip the clamp that goes around the plug wire & try clamping it to #6 plug wire & on both ways make sure it is away from nearby plug wires that can crossfire

If it is not the timing light acting up. I would think the springs for the mechanical advance are broken or too week?

Thanks guys.... however, Engine running horrible not a timing light problem. took distributor apart couldn't 'see' anything wrong, Springs not broke but could be weak? no shaft side play but does have about .030" of end play?

the end play is a non issue. what is your initial? on ported or manifold? rebuilt with same cam or hotter cam? did it run OK before but was rebuilt cuz it was tired? same dist? more info please #1 WHAT items/parts were changed in the rebuild other than normal stuff? EDIT & settings

car went through a complete resto. Engine was apart when I bought it. Block was hot tanked and rebuilt to factory specs except for mild head work, 0.030" over, Cam is a Mopar Purple shaft 284/284, 0.484/0.484". Stock Holley carb (600cfm?)and stock intake. Distributor is factory that came with the car I disassembled/cleaned it up installed new points and condenser, cap and rotor. Coil is a OEM factory unit. Using new reproduction wires......

Does the idle speed also vary?
I've owned a 383 that sounds like your combo except an AVS off a 440, and while the idle was choppy, I wouldn't say it ran roughly. In fact it pulled really hard above approx 3,000 RPM. But because it had a fairly tight converter, low-RPM response was rather soggy.
"Rough" - depending on how rough, how consistent, and when - might even be in the valvetrain, but 1st I'd want to try a different timing light and double check the harmonic balancer to see if it's failing. Are you sure - if by rough you mean the thing's shaking - there isn't an external balancer or flex-plate/flywheel on it?

Maybe a vacuum leak at the intake?

sounds like the only parts change that could be affecting this is a wilder cam & possibly how the untweeked carb is reacting to the lower vac/less stable idle. as said a vac leak is possible. can you set the initial at 17.5 (halfway between the 15 & 20 on the tab) & see if you can get a reasonably stable idle (as slow as you can get) & what idle speed & what the timing marks are doing. what springs are in the dist (OE light/heavy with elongated loop on one end?)

Original factory distributor with 1971 electric retard timing box along side vacuum advance diaphragm ???

If this is the case

Cam is way too big, I had the same one in a mostly stock 440 and took it out.
Car ran like crap. Mine would not idle below 1300-1500 rpm consistently.

anyway,
if the idle RPM is jumping around so will the timing. Have someone hold the throttle to about 3000 RPM and set the total timing around 37 deg. then make minor adjustments to see if the idle improves.

Triple check you have the firing order correct. Substitute another carb to eliminate that possibility, and of course look for the obvious vacuum leaks... One time I pinched a wire under the intake...

I run the original 284/484 purple cam in my numbers matching 383 magnum

Idles perfectly at 700 rpm s all day long , starts and runs cold at idle even without a choke plate in my factory carter avs carb

16 degrees initial at idle

20 degrees mechanical advance all in by 2400 rpms

I replaced the slotted t bar within the distributor , taken out of a slant six to shorten the slots for mechanical advance

36 degrees total - No vacuum advance hooked up even though it looks hooked up at carb

And if he is running the factory original 1971 383 magnum distributor with factory electric retard timing box on side of distributor like I mentioned earlier , pretty good chance from all the ones I pulled and ended up replacing , that's your problem

Convert it to Mopar electronic ignition

Or find a points distributor without the 1971 electric retard timing

your balancer could be seperated.

I'd check the balancer as Mr. Yuck suggests just to be sure. I had a jeep that separated the balancer and could never time it correctly except by ear. If I used the numbers then it ran like a turd and if I went back and checked it 2 hours later it would be slightly different. Then one day I shut it and my ears hear an old school telephone ring coming from under the hood???? The balancer had come completely loose and the outer rim of the balancer would continue to turn on the inner ring after the motor was shut off (making the ringing noise). Finally all those symptoms made sense.

Appreciate all of the feedback!!! never have experienced a balancer issue but will definitely check that tonight. don't recall seeing a 'electric retard timing box'. Finally have some time tonight to go over everything again, will look at all of these suggestions. thanks again!!!

cant recall what mine looks like but it would have a wire to it & is differently shaped than a regular vac can is