Ignition timing

Started by Russ, March 01, 2024, 01:43:00 PM

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Hi Russ,

you can estimate it by yourself by considering the angular connection between crank rotation and linear stroke. A full motor revolution of 360 degrees transports the piston from one TDC to the next. I set the zero point of the necessary sinus function at half stroke (49mm, 90° BTDC). That means, the crank rotation has its upper maximum (TDC) at 90°.

55° BTDC are then 35° from zero of the sinus. sin 35° is about 0.57 of the 49mm half stroke, hence at 28mm from zero. That is far away from 49mm – 14.3mm (9/16") = 34.7mm. So, either 9/16" or 55° are wrong. My 1937 Model 9 runs perfectly well with 9/16" BTDC (as given in the user manual). The upper calculation then results in 43° BTDC.

BTW: When for any reason an adjustment on the road is needed you can just take a 9/16 spanner as a measure.

Cheers, Thomas
1946 BSA C11
1937 Sunbeam Model 9
... and a scratched Hyundai
(MSCR member)


ok chaps so I'm after ignition timing  for my 1927 model 9 I've seen the chart here and know it's 55 deg's BTDC but does anyone know what that is in inches so I can stick a measuring stick down the plug hole.....I've read in one of the old Sunbeam manuals it gives it as 9/16" BTDC does that sound right?