Unless I missed something, I would think an important question here is is the OP running ported or manifold vac.?

Manifold vac. can boost timing at idle, helping idle quality and letting you lower idle rpm's. Ported vac. was factory in the day, but that was emissions stuff, and also even hot factory cams had a LSA of around 115* to help idle, most after market hot cams, similar lift specs to a hot factory cam, but have a LSA of around 110*, which makes smooth idle difficult to attain at times.

 

Hi, Manifold vac for acc, no power brakes or vac advance, cam is 108*

 

Post 19.

 

11:1 CR BBC, 15 degree initial timing with 36 degrees all in at 2800rpm, 250 at 0.050 SR, vacuum advance on manifold vacuum, idle 750-900rpm. No run on with 93 octane.

Vacuum advance makes the engine run cooler at idle. No vacuum advance is for drag/race cars that are at WOT most of the time.

 

It took me a while but, I finally trained my wife on how to kill the motor with the clutch if it runs on after shutting off (950RPM idle speed.) Our 70 LT-1 with a pretty big cam and 10.5:1 compression is right on the borderline of run-on detonation, on my 72 LT-1 there is a factory idle stop soleniod to prevent this, I think the 71 also had one. You can source a soleniod kit that should bolt right on to your Holley by looking up the parts for a LT-1 Vette, even the 69 L-88 with an automatic had idle soleniod on it.

 

The roller cam really helps. With it you can run similar duration @ .050 with less advertised duration and better vacumn. This is a tough fix and a common issue with a flat tappet cam. A b28 vacumn can would work. It will allow less initial but kick the advance up at idle helping idle quality and reduce run on. I was Elle88's builder and we did what we could with the timing without the availability of a B28 can. He found one and I believe he is going to install it. I think we were at 18 or 19 initial and 38 all in by 2500. This gives him a little mechanical at idle allowing better idle but run on is an issue then quite possibly from higher temperatures. Babbah had a similar setup. Here is the thread.
http://forums.corvetteforum.com/c1-a...m-advance.html

 

the car needs a higher stall converter so you can get it down to a reasonable idle...solid lifter engine 850 is a desirable idle....it should be working on blowing the U-joints the way its currently set up from shifting 'park to any gear...should just turn it off and the engine quietly stops

 

Kevin
got the B28 vac can but not yet installed because little time and because i have a track day on sat 31st. I prefer to take the car there with engine without vac can to avoid possible issues ( i will make in car video too, stay tuned)
but i believe the vac can will help in general use of the car and possibly will reduce the tun on too.
i have just a doubt about the vac advance. what happens when you let off the gas pedal with car going fast ( the name is "coasting" probably) ? vacuum is high, advance is high , engine still running fair hi rpm but with little air and fuel ... a non ideal condition?

 

I would have though that as the engine is not 'under load' this wouldn't be an issue. When you add-up initial+mechanical+vacumn the total can look quite 'intimidating' - like in the region of 50 degrees!

 

I'm at 8.54 DCR, right on the ragged edge, but with aluminum heads. Running B20 can.

That thread you reference is perfect illustration of my experince with vacuum advance. Engine runs cooler at idle and cruise. Higher initial timing will aid idle temp as you mention, but won't affect cruise since total remains the same. I don't know if this is a proven fact, but it seems logical that cleaner running engine develops less carbon build up at idle and carbon crud is a contibutor to run-on as a hot-spot ignition source.

L71 and L89 make 4 hour run to Carlisle together, both essentially stock with same rear gearing. Iron-headed L71 should run hotter, but has vacuum advance. L89 has no vacuum advance and must run slower with periodic stops to cool down engine.

Less primary throttle opening keeps you from uncovering too much of the idle transfer slot and makes for a more stable idle. On Holleys, use the secondary throttle stop adjustment to open the secondary a little and then you can reduce your primary throttle opening. Try to equalize the throttle opening from one side to the other. I noticed better throttle response when I did this.

My engine will idle OK at 750, but better at 900-950 and with better oil splash on the cam.

 

With your cam and compression your DCR is in the safe level. That coupled with aluminum heads and good A/F ratio I don't forsee any problems. You will need to make adjustments. Dial initial back a few degrees, add a couple degrees of mechanical to maintain yout total you have now, install a little stronger spring to eliminate idle mechanical advance, all in by 3000. Check total with vacuum to make sure your not over 53. It will idle better, run cooler and get better mileage with less carbon buildup and no run on. Tried to get this before we shipped. Been waiting for the in car video!

 

A b28 vacuum can would work.

 

Mighty demon carb fitted and tuned, run on nearly solved