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This is the situation... I have a Holley 4 bbl, vac secondary, 600 c.f.m. on my freshly rebuilt 400 small block. I'm running a basically stock H.E.I. distributor that i found for 60 bucks. I thought i was getting a sweet deal because it had a MSD coil in the cap. Without hesitation, i replaced the cap and rotor before i installed the distributor. The motor fired up just fine and after initial engine tuning, i took it for a drive. The motor pings under a load. I assumed that i got the timing off so i looked to reset everything. It all landed on the money. Idle @700 r.p.m. Timing @ 14 degrees (per Edelbrock cam specs). I questioned the idea of cam timing but 2 people assured me that it was right because they were looking over my shoulder when i installed the timing chain and gears. After verifying that there were no air leaks, i turned my attention to the distributor itself. I was thinking that maybe the mechanical advance was causing the problem. I inspected the assy and found that the bushings were wasted. So i replaced the weights, springs and bushings figuring that this would fix the problem. I elected to go with the heavy springs to slow the initial advance. This helped to the point that it would only ping if i really tried to get on it. So i checked the initial settings again thinking that the reworked mech advance affected it. While i was dealing with the settings, i noticed that now when i reconnect the vac advance, the idle increases. I was kicking myself thinking that i was tied in to a manifold vac port all of this time. It turns out that i was actually correct with a ported vacuum. This tells me that I'm getting too much of a vac signal at idle. With an idle of 700 r.p.m., i shouldn't get a signal at all to cause any advance. To help alleviate the problem i started to play with the settings. Right now, the timing is somewhat retarded to help minimize the pinging and the idle is adjusted to try and compensate for the weird timing. It doesn't idle as smooth as it should at idle and it's tough to start when hot.
My line of thinking now is that there's too much air flow though the primaries at idle. Opening the secondaries should reduce the air flow thru the primaries and adjusting the mixture screws will give me a smooth idle. But I'm wondering if i open the secondaries, will it draw a fuel mixture to keep from causing a lead condition at idle to the rear cylinders?
The 650 c.f.m. carb should handle the work considering that the motor doesn't exceed 4500 r.p.m. (the truck is fitted with 2.75 to 1 gearing)

Id like to hear any thoughts on this. I plan to add a MSD ignition system soon but Id like to resolve this before i start upgrading stuff. I may not be able to troubleshoot the problem to an existing carb setup or dialing in a new ignition. Thanks...

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Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

The truck is crap. Give it to me and I'll dispose of it for you...

Are there not any vacuum advance adjustments? Usually there are shims you can put in it.

A quick google search on the problem sent me to a mustang site.
http://www.mustangmonthly.com/howto/26823/

I dont think that the vac advance is the problem. I looked at the Mustang article and they made no reference to any vacuum draw at idle but getting the mech and vac advance to work smoothly to each other. Vac advance operates but it shouldn't move at idle. A stock block, mild cam, intake and headers shouldn't cause such an effect to the ignition system so i can only think that the problem is the vac draw at idle.
With the situation i have right now, if i set the timing @ XXbtdc without the vac advance connected, it will instantly go to XX+4 btdc once the vac line is connected. Trying to adjust around this is rather useless. Either tune the motor to idle smooth and have it ping hard or set the timing to get rid of the pinging with a very, very rough idle.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

Well, I'm no expert, but I personally would change a few things. But to be sure we need to know a little more about the motor. Compression, open chamber heads or closed chamber, cam specs, manifold, just to get an idea of what kind of timing curve it would need.

OK... this is what it has:

smallblock 400, hypereutectic pistons/.030 oversize with stock heads -76cc chambers. 9.62 to 1 comp ratio.
Stock ratio, roller tip rockers and stock push rods.
Stock crank and rods turned. Hi vol oil pump.
Edelbrock 7101 Performer RPM intake (1500-6500 r.p.m.)
Edelbrock 7800 timing chain set with a 2103 "performer plus" cam kit.
288 deg int, 288 deg exh, duration / .295 int, .295 exh lift. Hyd lifters. It'a a mild cam.
Holley 600 c.f.m. 4 bbl carb, vac secondary.
Hooker/LUV headers with 2" dia. exhausts
Basically stock H.E.I. distributor.
Stock torque converter in a rebuilt TH 350 trans, 2.75 to 1 gearing.

And yes i know that the intake isn't quite suited to the build. But with the minimal performance upgrades over stock, it really isn't an issue, especially with a 600 c.f.m. carb.
The exhaust is somewhat restrictive but thats what the Buick V-6 ran and it's on the "things to do" list. Neither of these points should have an effect on basic tuning but it does restrict the motors potential.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

I bounced around the Internet again looking for some answers figuring i would find contradicting info and i would step away from the problem as i have every other time. But this time i found something new to me. I found a .pdf file that has all P/N's and specific rates of vacuum advance units for GM H.E.I. distributors. From what i've read, the 400 motor pulls only 5 degrees max vac advance. Since i have a used H.E.I. unit, it's most likely a non 400 setup with possibly 10 or more degrees of advance.
I had not considered the concept of different vac advance rates for different displacements and configurations. So now i have a direction that can fix the problem. Hopefully the vac advance i have now is way wrong for the motor.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

I would do away with the vac advance all together.. just un hook it .. rev the engine up to approx 3000 rpms and set your total timing to 32* or 36*
you may have to play with it a bit but it will be real close with this setting.

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if you aint skeered just a little bit THEN YOU AINT GOING AS FAST AS YOU SHOULD BE!!!!

GOTTA GET A NEW LUV!!!!!!!!!

i dont know if this would work, but you can buy an adjustable vacuum advance unit on ebay for like $20 or less, i think. its just the vacumm can that goes on the distributor that you can turn and i think it gives you 10* of adjustment both +/-.

heres an auction for one on ebay.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33689&item=7976533711&rd=1

hope it helps


neil

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"but its that gut feeling of you're drawn to what feels right for yourself and what you're passionate about..."

If it turns out that i've got a vac advance unit that's not to factory spec, i'll most likely go with an adjustable aftermarket unit. That way i can eventually dial in a setting that suites the engine. If i tried to replace it with other GM units, i could go thru 4 or 5 before i find the right one for my application. Plus i dont want to dump too much money on a ignition system that will eventually get replaced. Im not sure about eliminating the vac advance all together. The truck is for street use and any advantage to gain m.p.g. is needed nowadays. But mostly i'd like to find out why it's not right. Once it's fixed, i've learned something new and it would benefit me in the future wether i help someone with timing problems or knowing ignitions in more detail when i upgrade to the MSD system.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

As far as the mechanical goes, like Spanky said, I set mine "backwards" by revving the motor to over 3000 RPM and setting the total at what I want. In my case it's about 36. Then when it idles down it drops to 18, but the timing at idle is not what I'm interested in. I'm setting the total at what makes the most power.

You'll probably want more advance because of your big combustion chamber, but you'll have take some off for your small cam and high compression. You may end up about 36 or so. Maybe more, 38 or even 40. You'll have to flog it and see what it likes.

From what I've seen in testing, most motors don't need more advance after 3000 RPM. But most people building motors for performance don't use the big chamber heads, so I'm not sure about your case. I've seen some stock "smog" distributors that keep on advancing all the way past 5000 RPM! From what read, that's not what you want. In fact, MSD makes a timing computer that is locked at 20 degress of advance which happens from 1000 to 3000 RPM because that is such a common curve.

Anyway, the adjustable vacuum advance sounds like the way to go. And I think you have more info than I about that.

I've personally never seen any benefits from a vacuum advance, maybe just because I drive my vehicles with a lead foot?

Well... This is how things are right now. I'm back home and the first thing was to order a new vac advance unit. It's an AR 12. This thing needs twice the vacuum draw as the old one to function. I installed the unit and i also installed timing tape. I tried to set the timing to 36 degrees maxed out mechanicaly but it would only go up to 24 degrees. The reason for this is the fact that i installed the heavy spring set when i worked on the mech advance bushings and weights. I elected to keep this as it is. I'd rather have soft performance right now than deal with the pinging problem. Baseline timing @ idle without the vac advance connected was 12 degrees. I then connected the vac line to the ported fitting at the carb and there was no change in idle. I thought this was right on track. I shut down the motor to button things up for a test drive. The motor wouldn't crank. The battery was weak. I thru a charger on it and i figured within an hour or so it would be back up. It was still throwing 4.5 amps to the battery. I took the battery to the parts store to get it tested. They said it was just low on charge. It didn't fail under a load test. So i bought some new terminals and lugs to rework the cables. The battery stayed on the charger overnight. This morning the battery was down to taking 1 amp. With reworked cables it all went back together. It lit right up. I took a drive and it still pings under a hard load. I went to check the timing again and found that the $8.00 timing tape installed was long gone. I couldn't help but laugh. I should have known it wasn't going to work. Later i'll plan to scribe a mark 2.51 inches from TDC to have my 36 degree mark. The next step was to try manifold vacuum. It caused the truck to maintain speed with a very light gas pedal but the pinging was still there under a load.When i got back to the house, the idle was rather high. I tried to adjust it down but i ran out of adjustment at the screw. I realized that i had the secondaries opened a little too much. I closed the secondariy side of the carb and the idle dropped to the point that i had to adjust the primary side up to get a basic idle. I decided to look at the timing again even though i couldn't reference it without the timing tape anymore. At idle, with the vac connected to manifold, it read off of the timing tab. I decided to reduce the timing to 14 degrees at idle with the vac still connected. I adjusted the mixture screws to see if i could get things smoother. I'll try again tomorrow when i go get a state inspection...

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

The truck had some low beam problems so i had to fix that before i'd try to get it inspected. A new relay and it was good to go. I drove across town to the shop that i always go for an inspection. On the way i flogged it and it didn't ping. I wasn't ready to get my hopes up with just one romp. The truck sqweaked thru inspection with questionable tires. When i left the shop, the mechanics were ready to see what the truck could do. I jumped on it and the motor roared to life! The truck got skittish as hell and smoke poured from the drive tire. It was spooky fast for about 100 yards when i let off. No pinging at all. I think it's going to work like it is now. At least now i have a stable platform to work with.
So if anyone has a smallblock chevy motor with H.E.I. ignition, make sure that the weights, bushings and springs are fresh and you're using an AR 12 vacuum canister with full manifold vacuum. These items have given me hell for the longest time and no amount of adjustment can correct these.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.