I see a lot of threads that talk about getting the vacuum advance limited on HEI and other distributors. Most involve or suggest welding and filing the slot. After poking around on the internet some, I came up with this one. It wasn't my idea but I did implement it on my Buick 455 and liked it enough that I thought I'd share it. There are a lot of threads on setting the timing on your engines. Most involve modifying the weights and springs and then setting the mechanical advance at some "full in" rpm. I won't try to tell you how to do that. The second part of the modification, for a street driven car, is to dial back the amount of vacuum advance that is built in. Everyone seems to agree that there is too much and usually you only need 10-15 degrees of additional vacuum advance. Crane makes a kit that includes springs, new vacuum canister and an adjustable limiter. p/n CRN-99600-1 from Summit Racing - $38.20. You can also buy just the adjustable limiter plate, pn 99619-1 for $3.05. The Crane instruction say to install the limiter cam so as to compress the spring in the vacuum canister and limit travel. This makes the spring very stiff as you compress it and the canister that comes with the kit already has an adjustable spring. Plus, it's harder to install and adjust the cam against the compressed spring. What I did was tap a 8-32 hole in the bracket and install the adjustable limiter to stop the advance in the normal direction of movement. (Caution. Install it with the adjustable teeth as shown in the picture or you will have interference problems.) Now, I can adjust the total amount of vacuum advance to some reasonable number and hook it to the manifold vacuum port and get a nice smooth idle with about 30 degrees at idle. I also have 30 degrees at full mechanical but about 45 degrees for cruising. The engine seems happy. I'm going to do this mod to my other vehicles as I get time. It's easy and it works well.