Re: SBC Part Throttle Stumble/Surging
Going by your full manifold experience, it just sounds like the can is either wrong or just bad. It should only provide 10 degrees of advance but your numbers show you were getting 24.
There are a few things you can do in what would be my order if I had to do it.....
(1)-Call MSD and tell them and they may just send you a good one.
(2)-Since they use a standard GM points dizzy can, you can look up that article that lists all the GM vac cans and specs, then buy one with 10-12 degrees advance that will work with your vacuum level at NAPA or somewhere and install it. There are also a couple of places that sell an adjustable can.
(3)-With the engine idling and timing light going, use a hand vacuum pump on the vac advance and note at what vacuum level you get 10-12 degrees advance. Then pop off the cap and rotor, use the pump again on the vac can to apply that previously noted amount of vacuum, and note how far the canisters actuator rod moves. Make a stop to limit the actuator at this point.
Regarding the light stumble you've got now, you can most likely do what you already mentioned, and I'd try either timing or fuel first....keep your current advance, but just swap out one of the springs for the next heavier one to slow the curve down a little bit. You can try this without doing the jets. Less advance will run better with a rich mixture. If you were to change the jets but leave the timing you might find it runs the same since a lean mix needs a little more timing.
***Edit. I found Lars's timing papers and it looks like there is not a GM can that provides only 10-12 degrees advance. I think I remember reading somewhere that MSD just uses on of the GM 16 degree (8 distributor degree) cans and just puts a bushing around the arm so it bottoms out on the bracket.
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Last edited by Steiner; Aug 14th, 14 at 04:47 AM.