My GS455 (just received) has a Mallory Unilite distributor and electronic module inside. While playing with total timing today, I find that the initial advance is about 10 degrees, centrifugal is 20 degrees for a total of about 30 degrees total. (with vacuum advance disconnected and plugged.) The vacuum canister is connected to the thermal control valve on the intake which is only allowing ported vacuum advance. Here's the crazy part.................I wanted to see what vacuum advance is at idle so I can consider using manifold vacuum for cruising and cooler running. So, I monitored timing at idle (10 deg) and hooked vacuum advance to manifold vacuum. Timing jumped to about 40 degrees!! Since idle increased I turned it down thinking some of the 40 degrees was centrifugal advance. But, even with the idle turned down below 500 RPM, advance was still 40 degrees! So, the vacuum advance canister is adding 30 degrees of timing (40 less initial advance of 10 = 30). There is an allen screw adjustment inside the canister port that I haven't played with yet but I've never heard of a 30 degree vacuum canister before. What gives? The allen screw is adjusted all the way clockwise so I should be able to reduce vacuum advance by backing off the screw. But, I have always thought the adjustment screw only changed the rate of vacuum advance, not the total? Is that right? Call me mystified.
It depends on the canister. I believe the Accel ones only have the screw. Have a look at the instructions, https://www.andysautosport.com/additional_product_info/accel_2_31034.pdf Looks like it maxes out at 30. Turn it CCW.
Even some GM vacuum advance canisters had 25 degrees of advance built-into them. This is one of them...after the excess advance was "fixed". Photo 1 (if it doesn't show up, you can view it at http://hbassociates.us/HEI_VacuumAdvance1asm.jpg ) Photo 2 ( http://hbassociates.us/HEI_VacuumAdvance2sm.jpg ) Photo 3 ( http://hbassociates.us/HEI_VacuumAdvance3sm.jpg ) Crane sells a "Limiter plate" that does essentially the same thing. Problem is, they tell you to install it on the WRONG SIDE of the movable rod. To do it right, you'd need to drill and tap a hole in the Vacuum Advance, and secure the limiter plate with a matching machine screw--short enough to not interfere with the rod directly underneath.
I've had that one before and prefer the one I just learned about that's on my '70 GS. Much better to be able to adjust the vacuum advance without removing the distributor cap.
What are you adjusting? There's TWO adjustments that need to be made. The ones I've seen have this "inside-the-distributor" adjustment for total amount of travel, with an "inside the vacuum canister" adjustment for the amount of vacuum needed to begin the advance. Is your "inside the canister" adjustment different from that?
Yes, the Accel canisters just have one adjustment through the nipple. https://www.andysautosport.com/additional_product_info/accel_2_31034.pdf
Thanks for that. Wow, what a mess. I wonder how the canister is designed, so that one screw affects both travel and spring tension. It's way better to have the two adjustments separate from each other.
Take a look at Larry's post #2 and note that the max advance is controlled by the internal screw through the vacuum port. I consider this method very satisfactory as the vacuum advance tapers off very quickly when engine vacuum drops from 10 Hg to 5 Hg. It works well on my engine and I'm currently working with 34 deg of total timing with vacuum advance at cruise of 10 deg. I'm adding one turn which is about 2 deg of vacuum advance every day and will stop when and if I get detonation or surge at cruise or medium throttle acceleration. I expect to be able to add about 12-14 deg of vacuum advance but we'll see.
