Stroke affects the relationship between mm BTDC and degrees BTDC. I'll look in my A. Grahm Bell book when I get home and give you the formula. I just found out that header length and cone dimensions can make a huge difference in the power band, so it could be the pipes. If you think it's the timing, why don't you try retarding it a little and see what happens? More advance should improve mid range and less should allow the engine to over rev and gain on top end.