I have a cross-drive with a Bosch DU-4 and being the Speedster is RHD I have no way manually to retard or advance the spark. I was told I need a Magneto Impulse coupling. Can anyone help? Thanks, Bill

Bill, even with a impluse coupling, you will still need to retard or advance the spark. You need to design a linkage arrangement to make it work.

Bill, This shows two differents examples of what a mounting looks like with the advance retard system installed for a magneto which runs on the right side of the engine along side of the block. You just need to make up some bell cranks and levers and you will have it. The impulse just aids starting when hand cranking at a slow speed when the magneto will not work as well.

As you can see the front crank is moved by the spark lever on the steering column and the rod travels back and moves the lever on the actual magneto. You can whittle one out of the junk you find in the parts drawer.

I looked at an Exelcior motorcycle from the teens the other day. They engineers at Excelcior didn't like - or didn't know about bowden cables, so the connection from LH handlebar to the magneto advance and RH handlebar to the throttle was done by bell cranks and levers in five steps. Looked like an impressive rube goldbergish tecnical challenge in bright nickle finish, so it can be done

Bill when I sold you the DU4 set-up it had that first pc included that Mr Harris shows

Excelsior would be the correct spelling.
Found a picture on the net of a 1919 with the throttle likage visible. (I think Indian started using cables by 1917)