The base timing should be about 4 to 8 degrees before top dead center. On a stock 250, it's not critical to get it exactly on the factory spec. The vacuum advance should provide about 12 to 18 degrees more advance just by sucking on the hose. There is an assembly inside the distributor called a mechanical advance, and it gives about 12 to 14 degrees more as you rev the motor up. You should check the mechanical advance with the vacuum disconnected, so you can see how much it makes the timing marks move. All you have to do is rev the motor up to about 2500 rpm, maybe 3000, and the timing mark should be done moving.
If the mechanical advance doesn't work, the distributor has to be removed and completely disassembled, cleaned and lubricated. They usually stop working because the grease gets nasty-hard and nothing can slide like it's designed to do. The option is to replace it with a rebuilt unit, but there's usually no reason for that other than money or time. I prefer to take them apart and fix them. Sometimes there are worn-out parts inside; if that's the case, then get a rebuilt.
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David
newbie #4153
