C-2 Vacuum Break

[Larry S.]

1)I have a '66 BB 427-450 with a Holley 4150 (original carb and just rebuilt) that I have installed, and the engine starts and runs okay. I have adjusted the idle screws using my Vacuum guage and am showing about 13-14 in. vac. at 800-850 rpm on full (not ported) vacuum. The timing is at about 12 deg advance.
My question is I dont understand the purpose of the choke vacuum break and, also, when should it draw or pull, and not pull?
Any info would be appreciated and perhaps where I could find more info in the archives.
2) I also intend to replace the main shaft in the distributor, as the 2 pins on the top of the flange of the main shaft (the weight springs attach to them) are loose and can actually be pulled out of the flange. I am wondering how hard is it to pull the main shaft out of the distributor? Do I need a puller of some sort to remove bearing or other?
Thanks in advance.

[Timothy B.]

Re: C-2 Vacuum Break

Larry,

The main shaft job is a easy job that you can do at home. If the pins are loose I would move that to the top of the list and check the advance curve and wear in the housing where the tach gear rides.

The choke vacuum break is a diaphram that pulls the choke open slightly when the car is started cold so it does not flood. Vacuum is always present there.

[Jim T.]

Re: C-2 Vacuum Break

Larry I recommend an pretty easy fix for your distributor advance pins. From what you have described about the loose pins I would try securing them with JB-Weld. Would not let any JB-Weld be present on the topplate area around the pins so the advance weights will have smooth non interference movement. You don't have to remove the distributor to try this fix, just give it ample time to set up.
Long ago I remember going into an auto parts store and seeing two engine valves with the vavle faces connected with JB-Weld. Good advertisement for the product.
I used J-B Weld to attach a dead bolt latch knob on a storm door. There was not a lot of contact area to attach the knob with JB-Weld. This was over 10 years ago and it is still functioning. Every time I use the knob to turn the dead bolt I wonder if it will separate because it took such a small amount of JB-Weld to attach.

[Larry S.]

Re: C-2 Vacuum Break

Thanks Tim and Jim . I always appreciate the resposnse tha t I get from the NCRS members.

[Duke W.]

Re: C-2 Vacuum Break

I would also recommend removing and disassembling the dist. and using JB weld to secure the pins - or take it to a competent welder who can braze the pins. The dist. R&I procedure is in the CSM and the disassembly procedure is either in the CSM or COM.

If your choke vacuum break question is in relation to converting the ported to full time vacuum advance, the reason to tee into the choke vacuum break line is that it has full manifold vacuum under all operating conditions, including idle.

Ported vacuum advance is an emission control technique that raises EGT to promoted oxidation of HC and CO in the exhaust system, but this higher EGT transfers considerably more heat to the cooling jacket than with full time vacuum advance, which is the primary reason SHP big blocks tend to run hot, and ported vacuum advance will typically deliver 10 percent better in town fuel economy. The higher the EGT the less of the fuel's energy is used to overcome internal engine friction and produce power at the crankshaft. The higher the EGT the more energy is being thrown away through the exhaust system.

Exhaust gas dilution slows flame propagation speed at idle, which is why total idle advance needs to be much more than initial timing - in the range of 24-34 degrees for medium to high overlap cams. By adding 12-16 degrees typical of full vacuum advance to initial timing, the total idle timing gets into the proper range to idle with the least fuel flow and lowest EGT.

Duke