I just put a ZZ502 in my 68. The engine seems to run rough at idle. I have the idle set at 950 and it moves up and down around a 100 RPM at idle. I have had several people look at it and most of them think that it is just the size of the cam that is doing it. It is the stock cam that came with the motor. So I guess my first question is how does your motor idle? What do you have your base timing set at? Are you using ported or full vacum for the vacum advance. And lastly how much vacum do you have. I am holding a steady 15 inches of vacum. Any and all feedback would be very much appreciated. Thanks!!

I don't have a 502 but what ignition are you running..... It could be a bad connection to the distributor or box if you have one, i just found this problem yesterday on my BBC 498. My MSD connector the pins weren't in all the way.

With 15 inches of vaccum it should idle nice. A little fluctuation is normal as conditions change. You should be able to get a steady idle rpm however. Conditions that would change it would be air temp and engine temp. You need to dial in your total timing without the vaccum advance hooked up. Pretty sure it's 36 as recommended by GM. I would guess full vaccum but double check that. I'd also dial in your 4 corner idle mixture screws.

I am running a complete msd System. 6Al box, blaster 2 coil, and a ready to run 8361 Dizzy. I thought the same thing with the ignition so I totally elimanated the msd system and dropped in a GM dizzy with the coil in the cap and the idle problem stayed the same. The car has a Retrotek EFI sytem on it. I have worked with them to dial it in and according to the computer eveything is set fine. The GM instructions say to use ported vacum but it runs much better and cooler with full vacum. I set the total timing at 36 at 3000rpm with out the vacum hooked up. I can tell you that as soon as you crack it off idle it runs great. I have gone over all my vacum lines and cant find a leak. I have spread the intake and throttle body with starting fluid and found no leaks. PLEASE HELP.

The crate zz502 I deal with has a carb and the supplied hei dizzy.There is no idle swing at 850 rpm with the weeny marine hydraulic roller shaft.Manifold vac is high and steady @ 15hg in gear with a 3k converter.
I can not speak for your efi system..but I can say the zz502 is not overcammed by any means underneath a carb.
Perhaps the map sensor is skewed at idle vac or do you depend on speed density sans map?

I installed a new ZZ502 in my 67. I've not had the car on the road yet
but I can tell you what I've encountered in the yard. I had a few loud lifters and decided to do a complete valve adjustment on the engine. The conclusion was that the engine now runs very quiet and very smooth. I was amazed at how far off the valves were. Until warm the engine is a bit rough but within minutes it is running smooth at about 750-800 RPM. When warm you can punch it and it comes to life just as you'd ask for - as smooth as glass. Hope this helps.
Ken