Steve81

I read Lars new tech article and decided to fool around with my vac adv again (it's been disconnected since mid-summer because of spark knock on very hot days). i decided to reconnected it back to the ported source on the Demon 650 (it had been on the mainifold source) and noticed the rpms went up. Checked the timing and it was pulling 20 degrees at idle (same as mainifold). My understanding was with ported vac adv should pull timing as the throttle opens and have zero at idle. If that is correct, What gives?

Also, Any of you ZZ4 guys know what the advance curve is on the distibutor, what RPM the advance starts and is full in? (800-3200?)

Vetterodder

[QUOTE]My understanding was with ported vac adv should pull timing as the throttle opens and have zero at idle. If that is correct, What gives?
QUOTE]

Your understanding is correct. Sounds like you may have the throttles opened far enough at idle to expose the transfer slots, which would allow a vacuum signal to the ported outlet. What is your idle rpm? As far as the pinging issue with the vacuum advance, there shouldn't be any difference between manifold or ported as they both function the same once the throttles are opened. In some cases it's necessary to limit the total amout of vacuum advance with a different canister or an adjustable unit that allows yo to limit the amount.

Smokehouse69

I'm only running about 12* at idle with my ZZ4, initially I had a problem getting enough advance into the distributor at the right time. I tried a different set of springs and weights and I still didn't get what I wanted. In fact the advance would only go up to about 28-30 degrees at 3500-4000. I was stumped, I talked to Lars and he suggested trying at little more advance at idle, that didn't do anything but increase my total to 30-32 degrees. He also mentioned that some GM HEIs were very limiting on the amount of advance allowed by the slot in the plate. While looking at the advance slot to see if that was the problem I noticed the when the mechanical advance was as far as it would go, there was still room for more in the slot.
Then I started looking at the weights and the slot. The weights had some rough spots on them and the slot was slightly narrower toward the top of the curve.
I smoothed out the weights with a wire wheel and filed the slot a bit with a small file. (I had the distributor out of the car and disassembled.) After I got it all back together, the advance operated way better, with 12* initial advance I had no problem getting to 34-36 degrees at 3000 rpm. In fact it is all in by 2600-2800. This summer as it heated up I had to drop my intial back a couple of degrees to 10* to prevent pinging under light load. I am using ported vacuum on my vacuum advance.

Steve81

Your right. I forgot i had to increase the idle without the mainfold adv. Duh :rolleyes:

Was this with the stock weights and springs or the ones you swapped? I want to have all in by 25-2600 per Lars paper and was wondering if i needed lighter springs. I usually just crank it up to 4K to time it and was not sure when the timing was all in on the stock dist.
I'm running 16 int +20 mech +20 vac = 56 total, that's probably why it was pinging on hot days. Think i'll order the adj. vac can so i don't have to keep adj. the timing.

Thanks - Steve