You generally don't find vacuum advance on tractor and industrial engines for the same reason you don't find it on racing engines: It isn't needed.
There are two reasons you need spark advance: First, the faster the engine turns, the shorter the time for the flame in the cylinder to propagate, and you want the maximum cylinder pressure to occur roughly when the cylinder velocity is greatest, in the middle of the stroke. So the faster the engine turns the earlier the spark needs to occur. Centrifugal advance handles that nicely.
The second reason to advance the spark is that under low-load conditions the spark can be further advanced for better economy without risking detonation. That's important with passenger vehicles that typically are under low load most of the time, so vacuum advance is added to give extra advance under low-load conditions. But in the case of tractors and industrial engines, they typically are under fairly high load. And when they're not, the fuel economy gained by vacuum advance isn't that significant. So they often don't have vacuum advance.
Of course, modern automotive engines with computerized spark and fuel control consider a large number of conditions, including engine speed and manifold air pressure (the reciprocal of vacuum) to determine spark timing.
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Today's Featured Article - The Great Escape - by Dave Hollrah. It all began Monday with a little baling of second crop on the lake shore field, and as I drove out past my sister and her hubby's place, this small calling sound could be heard from the general direction of their manure pile. Out of the yard, over the cows and bale piles, through the dozer piles, poplars, and brush, out onto the ditch grade road, past the noisy 6.2 diesel engine pulling my well traveled Suburban along. Well it sure didn't take me long to figure out what it was because I alrea
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