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Al Gietzen wrote:
When I got my 20B with a 12A front housing from Atkins, the relationship of the pointer to the marks on the pulley made no sense. I used my simple method to determine a TDC mark, and worked from there. On the dyno we put a pressure transducer into a trailing plug hole and adjusted timing a bit while operating (I don’t recall conditions – I think 22” MAP) to get the pressure trace to look right. We chose a point 2 steps retarded on the EC2, - 3.75^o .
Subsequently I did some variation at cruise conditions to see effect on performance, and again ended up at two steps of retarding, although I found performance change almost undetectable over a 3-4 step range (~7^o ) to either side. So that setting is pretty consistent with Lynn’s data that the method leads to a TDC that was a bit BTDC; but is of no consequence.
That’s my story, and I’ sticking to itJ.
Al
Let me offer this idea. If you know where the tip seals are, then you can derive TDC. Level the base of the engine, and the tip seals should line up verically. Pull the trailing plug, rotate the shaft until the tip seal shows up in the first chamber, then make a mark on the starter ring gear. Rotate the engine until the next seal shows up, and make another mark. Count teeth to get the center one, or make the two marked teeth line up vertically. This would eliminate the importance of the spark plug hole location.
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