1940 Sport Scout Ignition Advance degrees?

[Bill Huth]

I have the 18 degree arm on my bike now (marked M18). But picked up one that appears to be a 25 degree arm (the slot is wider, and is marked 25). I have looked through the specs, but I do not see the advance degrees anywhere. Does anyone have a reference to the correct degrees for this bike? Also, I am not stock. I am running Shunk cams and Bonneville followers. Also stroked to 52" if any of this affects my question.

[Yellow53Chief]

Hi,
A good place to start is Jim Mosher's site: performanceindian.com On that site you will find a chart that converts flywheel degrees to piston distance Before Top Dead Center (BTDC). For a Scout, the typical piston distance BTDC when the points open is about 3/8" (0.325"). Jim's chart gives the degrees for a 45" engine and 57" engine. For a 52" engine, the degrees would be somewhere in the middle and it works out to be about 30 degrees advanced. Jim does discuss the advance arm; mainly to limit the advance when running a Keihin carburetor. The Keihin does not need as much timing advance as the Linkert. Since your engine is hot-rodded, the best advance for your engine will probably be found by trial-and-error. If your pistons are on the loose side, the motor will 'rattle' a little easier as you approach maximum advance. Some of the Chiefs will run as much as 40 degrees advanced. On my 80" Chief, I have limited my advance arm to around 25 degrees.
If your Scout has the timing hole under the carburetor brace, you will have a much easier time checking your timing. For starting ease, at full retard my engine is exactly at TDC when the points open. The engine can't kick back, and my motor starts quite well. The 841 shaft-drive Scout had automatic advance, and for that engine it was timed at TDC. The automatic advance took care of advancing the spark as the engine speed increased.
A longer slot in the advance/retard arm will only be helpful if you really need it. On my Chief I had to shorten my slot so that I can't over-advance or over-retard the spark. When shifting into first gear, I go to full retard which for me is TDC. The engine runs quite slow enough so that there is very little gear grinding shifting into first. Once my bike is in gear I go to full advance (about 25 degrees as I'm running a Keihin carburetor). If my motor should 'rattle' (pulling a hill or a load) I retard the spark some until the 'rattle' goes away. Then I can usually go back to full advance once the going is easier. I think you will find Jim Mosher's site helpful and very interesting. It is one of the better performance oriented sites out there - with the focus on Indian engines.

[Bill Huth]

Thanks! I knew I had seen this somewhere. The flywheels are marked in 1/8 increments. I have the points open at 5/8" ADV, like the Bonne timing. No big timing hole, I use the pipe plug on the bottom. Full retard at TDC sounds like the logical start point. I will dig into Jim's site and go from there.