I have found that timing with a vacuum gauge works best if you use a tachometer along with the vacuum gauge, as follows:
1. Vacuum gauge and tach connected (vacuum gauge connected to manifold vacuum, not ported vacuum, which is the distributor source), engine at normal idle speed--500-600 RPM.
2. Advance the distributor until either the RPM or the vacuum reaches a peak- it doesn't matter which reaches a peak first. Stop advancing the distributor at that point.
3. Retard the distributor until the vacuum drops 1"Hg. You're done, but don't forget to tighten the distributor clamp.
If the engine speed is considerably higher after the adjustment, adjust the idle speed and do the timing procedure again.
Its been my observation that setting the timing at the highest vacuum more often than not leaves the timing several degrees advanced, which may result in detonation, depending on the engine and all the variables that affect detonation.
Also, I would not run the engine at 2000 RPM as suggested previously, because ported vacuum (distributor vacuum) will be applied to the vacuum advance at that throttle position unless you have the distributor vacuum line disconnected. Also, the mechanical advance mechanism is likely to be operating at that engine speed (1000 RPM for the distributor).
If memory is correct, the triangle setting mentioned previously was used on 1959 and later 235 engines to provide 5° advanced timing from TDC. The ball represents TDC, which I think was a carry-over from the hand-crank days, when you didn't want any spark advance while you were pulling on a hand crank. (You pulled upward on the crank and avoided pushing downward.)
Another point of history, but immaterial to the original question, the crank jaw was first removed from the 1959 235 engines, even though the jaw hadn't been accesable for several years prior. I don't remember when the passenger cars could no longer be hand-cranked, 1948 and later, I believe, but the AD trucks though 1953, and maybe through the first series 1955 models could be hand-cranked- if you kept your wits about you and you wished to do so...
Hope this helps.
Harvester
