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I was having "Chugging / Surging" problems and through a lot of reading of posts on here I found that it was a timing / Vacuum issue.
After researching which replacement Advance to go with, from a B9 and a B26, the one on the car was a B18. I went with the Adjustable Advance from Crane part # 99601-1
Solved the problem! We got the car dialed in on timing/vacuum within an hour. Total 36. Tight where she is supposed to be. No detonation, no surging and lots of power!
I read on here guy saying to avoid the adjustable vacuum, but I can't understand why, it was really simple if you follow the instructions.
My first one took about 20 minutes.. the last one took about 5. I think I had to take out about 6 degrees on both.. 3 full turns or something like that. I like'em.
I've only used them on other, heavier cars, in order to tune out a part-throttle detonation problem, and the Crane unit provides a simple, systematic tuning process to optimize the advance curve. The only three things you end up adjusting are initial advance, rate of vacuum advance, and the limit to the total amount of vacuum advance, so it's not all that complicated. You could also fine-tune the mechanical advance-springs in there, if you really wanted to fine-tune everything all in one shot.