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Old 12-09-2011, 11:08 PM
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When using a vac gage inside the truck you can find the sweet spots for engine load VS engine vacuum. You can use these values to adjust your vacuum advance can. If you have a vacuum pump this can be used on the bench or under the hood to tweak the range of the vacuum advance can when it is connected to the distributor pickup. You can also use these vacuum levels to help choose and setup carb power valves or vacuum secondaries or choose step-up spring ratings for those types of carbs.

Vacuum advance has two things going for it. One is at idle the other at cruise. I use the maximum vacuum advance all in at cruise but just barely. Once you get into light throttle tip in you want that vacuum advance to start dropping out pronto. The adjustable can can help you do that as long as you are within the adjustable range of the can (typically from 5 to 11 "hg). This is all considering we are timed for max power. The engine is most likely to ping at highway light throttle low load tip in especially if the tranny does not kick down with little throttle and the engine vacuum only drops a few "hg.

Just to add, in my example the adjustable vacuum advance canister does not limit the total amount of vacuum advance, you need the limiter plate for that.

There are claims that some cans can actually change the range of when the advance is added as well as limit the total amount added, my crane adj can does not limit the total amount added it only allows one to change the spring range.

All in all with timing it is more important to have you initial and mechanical advance set up correctly, you can then set your vacuum advance up but it is more important to limit the vacuum advance than it is to adjust the diaphragm spring range as with the adjustable type.
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