BME..
A Herbrandson is a big tractor, made to run at 7000 all day long..We use .022 from the piston to the top of the chamber, don't know what the compression ratio is...
Tested many 3w engines...Had to remove the braid from one plug lead to put the inductive lead from the light to make it work...
minus 10 to plus 20 is way more than I have ever seen...At minus 10 it would start backwards, and plus 20 would not be enough..I have two 3w ignitions here, neither shows any advance or retard at all..They're both old, maybe the electronic advance function no longer works...When I set a 3w with the light I use 28 degrees, seems to work best....I have "power timed" some 3Ws, when checked later it was close to 28....