69 distributor rebuild to achieve correct 26 degrees centrifugal advance

[Don W.]

I have a 69 427/390HP, THM400 auto. I have the correct distributor but it was rebuilt in the past with a problem identified as the cam base was welded to the top of the main shaft upside down thus allowing zero centrifugal advance. A replacement main shaft was purchased by a shop in the Seattle area, installed and run on a Sun Distributor machine.

Current status: I have been working on optimum timing with Duke, and we have found that my distributor only has 20 degrees total centrifugal advance vs the 26 degrees called out for distributor number 1111926.

What are the suggested way to correct this situation and achieve a full 26 degrees of advance? Do I try and purchase a new main shaft, purchase a whole ebay 1111929 donor distributor, or is there another way to achieve the 26 degrees? I did find a NOS main shaft here: http://store.bigcitycorvettes.com/pr...1967-1969.html and that shows P/N 1968530 as a proper main shaft.

Thank you for your assistance.
Don

[Timothy B.]

Re: 69 distributor rebuild to achieve correct 26 degrees centrifugal advance

Don,

If the vacuum advance is ported I would set the initial timing at 16* and leave the centrifugal advance alone at 20*. If you have full manifold vacuum advance at idle then you will need to lower the initial timing setting and you can file the advance slot to allow the centrifugal advance to move further.

[Don W.]

Re: 69 distributor rebuild to achieve correct 26 degrees centrifugal advance

According to Duke (Duke Williams) the Chevrolet Power Manuals states is the best power high rev advance for the mark IV big block is 36 to 40 degrees total advance at WOT for maximum power and torque. I have my idle set at 11 BTDC at 600 rpm and can only get 11+20=31 degrees total advance which comes in at 2800 rpm. I tried 15 degrees at idle, but I get mid range detonation and had to back down to 11 degrees. Therefore, I'm currently 5 to 10 degrees short of optimum WOT advance and I was thinking that I should start addressing this by getting a distributor that is at least capable of achieving the original 26 degree OEM specs.

[Richard M.]

Re: 69 distributor rebuild to achieve correct 26 degrees centrifugal advance

Contact Don Baker (815) 498-9522

[Joe R.]

Re: 69 distributor rebuild to achieve correct 26 degrees centrifugal advance

According to Duke (Duke Williams) the Chevrolet Power Manuals states is the best power high rev advance for the mark IV big block is 36 to 40 degrees total advance at WOT for maximum power and torque. I have my idle set at 11 BTDC at 600 rpm and can only get 11+20=31 degrees total advance which comes in at 2800 rpm. I tried 15 degrees at idle, but I get mid range detonation and had to back down to 11 degrees. Therefore, I'm currently 5 to 10 degrees short of optimum WOT advance and I was thinking that I should start addressing this by getting a distributor that is at least capable of achieving the original 26 degree OEM specs.

The thing that limits the maximum centrifugal advance is a pin in the main shaft that moves in a slot in the moveable plate to which the rotor cap attaches. So, to provide for more mechanical advance, this is the place to work on. Most GM distributors had a rubber sleeve on the pin, and simply removing the sleeve will allow the pin to move farther in the slot, but possibly farther than what you want. Another approach is to simply use a file to extend the length of the slot. This is best done by removing a little bit of material and then checking the total mechanical advance on a distributor machine or, if you are patient, on the engine itself. Needless to say, it is easier to make the slot longer than shorter, so proceed carefully to home in on the exact slot length you need.