I've got a 400 bored .040 over with T-Meyer pistons. I've got stock 2V heads that the thermactor humps were ground out and bigger valves with some porting. I'm running a Holley 600 on an edelbrock intake. The only vacuum I've got hooked up is the HEI distributor that has been curved for this build, hooked up to ported vacuum in the side of the carb, the PCV to the base of the carb, and the vacuum for the temperature controls. I have hydroboost for brakes so no vacuum there. I have the stock air cleaner on, so I had to put a 4 hole 1" phenolic spacer to clear the distributor. I want to hook up the vacuum for the temp switch for the air cleaner, but not sure what to hook that to. Guessing manifold vacuum. My trans is an E4OD so I have an electronic throttle position sensor from Holley on the Choke side and no vacuum modulator.

I'm not fully tuned at the moment and the engine only has 600 miles on it. I'm having trouble with the automatic choke since I put the throttle position sensor on. I had to take the spring off the fast idle screw to even hit the choke cam and I don't think it's dialed in. I had it turned in a bit too much and it idled at over 1800 but impacted idle even when you kicked it off fast idle. I have the electric choke set at 2 notches, but I saw a sticker that said 3 notches should be the setting. But it seems ok if I manually raise the idle by pressing the gas. When warmed up it idles 580-640. I've also found that I get a little bit of valve rattle when the engine is really warmed up at highway speeds only if I put my foot in it but not under normal or even moderate conditions. I tried getting the carb tuned like the videos say to tune the Holley, and It just isn't working the same. I put the vacuum gauge on, and at idle it is saying late valve timing. I can get it right in the sweet spot for vacuum by advancing the timing, but then its a ping monster. I've got it set right at 10 and that seems to be the most I can get I will say when I had it advanced it's got gobs of power until it starts to rattle and ping. I'm guessing the custom cam maybe is causing that vacuum issue. I don't know. Needle doesn't bounce. Adjusting air/fuel doesn't move it much, but I did adjust that like the Holley instructions say. I can tell something is off because I did have it to where it would start with just a bump of the ignition, now it takes a little cranking. On occasion, the truck will diesel when I turn it off. Seems harder to start when truck is warmed up. Plugs are Motorcraft I think. I cant recall off the top of my head. I gapped them to .053.

Any suggestions?

 

A "custom cam" will lower idle manifold vacuum quite a lot. So it's tricky to try and judge idle manifold vacuum compared with a stock engine. A healthy, stock engine in good tune - with a stock cam - should pull 19" to 20" steady needle at factory ignition timing spec. At sea level. A performance tuned engine won't pull numbers like that but incorrect camshaft phasing or a stretched or worn timing chain will reduce manifold vacuum below spec. But it wouldn't ping or knock on acceleration, that's more a likely problem (leaving aside an ignition timing problem for the moment) with an excessively advanced camshaft. As camshaft is advanced the cranking compression will tend to increase.

Disconnect and plug vacuum advance. Check the total mechanical or centrifugal timing advance. Should see 34 to 36 deg. something like that. Spool it up and make sure it's done advancing all the way. May need a timing tape on the balancer, or a dial back light. Try a test drive with vacuum advance disconnected and plugged and see if the problem remains. Distributor might not be curved as well as you thought, or, there is excessive vacuum advance. The vacuum advance canisters are adjustable. It would not be unusual to see close to 50 deg. with vacuum advance hooked up. Excessive Vacuum advance is more of a part throttle rattle, not under heavy load or initial acceleration. It can come in quick though. Disconnect and plug to find out.

If all that checks out then maybe it's time to take a look at camshaft phasing. Was it degreed in, or just installed dot to dot? Can fairly easily check for at least the rough camshaft phasing by setting up the #1 piston valves at overlap using a straight edge and observe the TDC mark on balancer and pointer for either generally advanced or retarded phasing.