The Stovebolt Site Forums The Shop Area Engine Shop 261 timing

Every Saturday in good weather i ride a 1960 Matchless motorcycle about 20 miles on my errands. It has a magneto ignition with a manual-only spark advance/retard. One doesn't forget to retard the spark on shut down lest it kick back unmercifly when kicked over next time. I have found that the timing doesn't matter much when running down the road. Holding the throttle steady and advancing or retarding the spark doesn't change things performance wise. What does matter is that it will begin to buck at idle and off-idle when riding through town unless i retard the spark about half way. Timing on this engine is a very inexact process involving a machinist scale stuck down the spark plug hole looking for 3/8 inch before top dead center. In the past when I got the timing a little too advanced I found that performance was good and more or less unchanging as I advanced the spark -- up to a point after that power fell off and the engine ran "hard." This is all to say I agree with Hot Rod's contention that too much advance is way worse than too little.

In many engines the advance can be constant over a wide RPM band because of such phenomona as squish and swirl in the combustion chamber that accelerate burning. Perhaps the hemispherical combustion chambers of the Matchless single is one of these. I suspect that the combustion chambers of a Stovebolt Six are not. These chambers are a sort of modified "bathtub" chamber that was used way back when because the design tolerated low octane fuel. I would guess that swirl is almost nonexistant so they require significant advance as RPM increases. Perhaps someone who has studied combustion chamber design more than I has some further comments. I'd like to know about bathtub chambers vs wedge chambers vs hemi chambers. What did GMC use? they're much better than Chevrolet of this era or so I'm told.