Like I said before many times on here. The two marks that are close together, the first one coming up when turning the engine in the normal direction of rotation is top dead center for number one cyl and number 4 cyl. The next mark is 5 degrees after top dead center. As the book indicates a 40 degree spark lag in magneto and you want a 35 degree of advance when engine is running, you set the magneto to trip the impulse at that 5 degree after top dead center mark. Now you will have the 35 degrees advance when the engine runs. The spark lag, or impulse lag holds the shaft that has the lobes for opening the points from turning until impulse trips. By that time the piston is on it's way down so it does not kick back. There are no factory marks indicating the total advance while running. There is one other factory mark all by it's self on the pulley of an H that is for lining up the pilot bearing grease fitting in the flywheel. Over the years, marks may have been added to the pulley by someone. I have added a lot of them when I install battery ignition units or when installing fire crater pistons. Different compression ratios require different total advance and the critical timing is when engine is running full throttle, therefore to check the advance with battery ignition units, thus the added markings. With a magneto you have no control over spark advance. Once the impulse kicks off the spark is fully advanced. That is the chief disadvantage of a magneto. On battery ign the advance is gradual with increase of engine speed and the maximum amount of advance is controlled with the slot in the distributor shaft and the rate is controlled with the springs. You can always add marks to any degree you want on the pulley by calculating the circumference of the pulley if you want to actually check to see how far the spark is actually advancing. Who knows what impulse coupling has been installed in that magneto over the years. Different ones have different lag.
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Today's Featured Article - The Cletrac General GG and the BF Avery A - A Bit of History - by Mike Ballash. This article is a summary of what I have gathered up from various sources on the Gletrac General GG and the B. F. Avery model A tractors. I am quite sure that most of it is accurate. The General GG was made by the Cleveland Tractor Company (Cletrac) of Cleveland, Ohio. Originally the company was called the Cleveland Motor Plow Company which began in 1912, then the Cleveland Tractor Company (1917) and finally Cletrac.
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