Did a little tuning on the truck today since it has been gradually running worse and worse over the last few months. It had gotten to the point where it wouldn't even bark the tires from a dead stop...just make a sick sounding "BLAGHHH" out the exhaust. The last time I tuned it was during this last winter because the winter blend gas they sell here just made the truck run terrible. During the last tune I had to really crank up the idle speed screw to get it to idle decently in gear. I knew I had way too much idle transfer slot open on the carb. I got the idle transfer slots set where they needed to be (about .040 showing), but to get it to idle at 650rpm in gear I had to use 16* initial timing, and used ported vac for the vac advance. I had never set the initial timing this high before, and was totally shocked at how this thing runs with 16*. Throttle response went from "ok" to lightning quick, it will smoke the tires at 1/2 throttle from a dead stop, and from a 10 mph roll it will turn sideways at 3/4 throttle. But, with 16* initial I get a pretty heavy ping when driving at light throttle...especially when it shifts gears. Also, it struggles to rev over 5200 rpms with that much timing, but gets to 5K quick in all gears. And, with 16*, it sometimes crank slowly when trying to start it. And the idle sounds off too...like it is missing (not lope). I set the timing back to 13* with manifold vac for the advance, tweaking the idle mix screws, and it still runs much better than before but isn't quite as crisp as 16* and ported vac advance. What is this telling me, and what should I do to get that wonderful throttle response back without the pinging?

Carb rebuild? Does it have a catalitic converter - could be plugged. Timing chain streached out?

-dnult

No cats, carb has been disassembled and cleaned, timing chain better not be stretched as it is a double roller comp cams unit with less than 20K miles on it.

Perform a cylinder leakage test, s/b better than 6% leakage, check timing chain slack, as dnult suggested, and try full vacuum on vacuum adjust and adjust base timing accordingly with total timing in mind.
Run a can of BG 44K Fuel System cleaner, maybe a can of GM's EOS in crankcase, doesn't hurt.

Sounds like you need to limit the travel of your mechanical advance mechanism. Whatever you add to the initial timing also gets added to the total. Have you measured the total timing?

The card that came with the distributor said it had 11* at 2200 rpms...I can only assume that means distributor degrees, which would be 22*@4400 rpms? That was with the heaviest springs (pre-installed in the distributor). I am using the medium springs now as even one light spring causes the idle to go all over the place. Today I was in the process of installing the vortec performer/650 holley spreadbore and went ahead and put my MSD pro-billet HEI in there too, even though the distributor is for the nova. This distributor has mega adjustability...this way I can find the best curve and get the old HEI curved to those specs. The curve I am starting with is 20* total in at 3200 rpms, and will try 15 initial to see how that works. Unfortunately, before I could get everything hooked back up, it started raining cats and dogs. Looks like the rain may be here a while too.

When you say 20* total do you mean 20* mechanical? That's probably a good place to start, the engine should have liked 16* initial + 20* mechanical = 36* total.

What does your old HEI have for mechanical advance? Some of the later smogger motors have a lot of mechanical advance built in I think --- don't remember exactly how much, but they set the base timing close to TDC and total around 28 or 30. Just wondering if your engine doesn't like less mechanical advance and more initial.

To limit the mechanical advance in your old HEI you can weld the slot in the stop plate to reduce the amount of mechanical advance. Its trial and error though unless you send it in to someone with a distributor machine.

ps. you might check for vac leaks too.

Well, it quit raining finally, so I got the performer/650 spread bore installed, along with the MSD HEI. 14 initial, 20 mechanical@3200 rpms, and manifold vac for the advance. This allowed me to back the primarys off enough for the idle mix screw to function, and I gained .5" idle vac. The difference is flat unbelievable...throttle response is so crisp it just leaps when you blip the throttle. It breaks the tires loose without even trying, and screams to 6000 rpms like its a$$ is on fire. And it was still revving at 6K but I backed off...cast pistons don't like that very much. I had planned on swapping in a bit smaller cam to gain some torque and off idle response but I see no need to now. This is so much stronger than the performer rpm and 600 or 750 edelbrock that I kick myself for ever getting the rpm. I guess edelbrock was serious when they said they don't recommend the rpm for heavy vehicles. I know that holley q-jet replacement helped too. It is a late LT-1/L-82 replacement so I figure it should be pretty close to perfect, since my engine specs are quite similar to an L-82. Man I am a happy camper right now :D

Oh, and the gas mileage even ought to improve quite a bit. I'll try and make a mileage run this weekend to see if I can beat my old best of 15.5mpg@65 mph.