Have a '66 Buick Skylark with the 300 (Wildcat 310) 2bbl motor. I was playing around with my timing tonight (just got the car a couple of weeks ago), and it appears that the distributor uses manifold vacuum to the vacuum advance. Now, I've *heard* of cars that do this, but I've never owned one. For normal V8 engines, I know they like 36deg initial + mechanical at 3000 rpms. I always go to a smaller vacuum can (like a 10hg or 12hg) to keep I was planning to change out the weigths and springs to bring in that advance curve, but now I'm worried what the effect will be when running full manifold vacuum. Right now, with timing set per stock (2 1/2 deg at 550 RPM with the vac removed), the car is running over 20deg advance at idle. Now, the same effect applies at WOT (vac goes to 0 so you are removing the advance), but am I going to hurt anything but setting the timing this way?
If you want to bring in the advance sooner with a spring change, just shorten the amount of degrees in the vacuum cannister. With stock springs, full mechanical advance doesn't come in until 4600 RPM. As a result, only part of your mechanical advance is in at typical cruising RPM. The rest of your advance is supplied by the vacuum cannister. If you bring in all the advance at cruising speed, you will have too much with the vacuum advance added in. I would limit it to 8-10* vacuum advance, and hook it to manifold like stock. Initial timing on that engine is 2 1/2*, so it has about 34* mechanical advance. If I was you, I would use a big bushing on the advance stop pin in the distributor. This will reduce the amount of mechanical advance. Then you can run more initial advance, still have the optimal 36* total. If you shorten the vacuum degrees, you can still idle at 20* Automatic transmission cars like 20+ degrees at idle.
Here's an interesting side note. The carb I'm running today is a 2GC that I picked up at a local Buick-only wrecking yard. It was a nice core that I rebuilt and it cleaned up nicely. Put it on and it runs great. I went back and checked the ORIGINAL carb that came with the car. Externally the carbs physically look the same, but the *base plates* are actually drilled differently. Turns out that the *original* base plate has the port for vacuum drilled to a hole above the throttle plates, so it's actually a ported vacuum source. The NEW base plate, that same hole is drilled to go *below* the base plates, so it's manifold vacuum. So, the car originally was sending ported vacuum to the distributor. For giggles, I put the old base plate on the new carb body, and it works fine and now provides ported vacuum. So here are my questions. 1. Why would 2 identical carbs have different vacuum sources for the distributor? 2. Is there an advantage to running ported over manifold vac to the dizzy? Thanks!
Are the carb numbers the same? Use ported or manifold vacuum, whichever works best for you. It's just not that big a deal to switch from one to the other.
Manifold vacuum will give you the added timing at idle. This will make throttle response quicker, and the engine will run cooler. With ported vacuum, the engine will idle at 2 1/2*. Most Buicks engines like at least 12* That's why I suggested shortening the amount of vacuum, and mechanical advance when you use lighter springs in the distributor. If you could reduce the mechanical advance from 34* to say 20*, you could run 14* initial, and add 8-10* vacuum advance at idle. The engine would idle at 22-24* Then make sure all your mechanical advance came in below 2500 RPM, with a spring change. I'm sure you would notice a difference in bottom end response.
