I read the manual on re-timing the ignition on the big twins (non-Norton). Here is my mild re-write with some extra thoughts.
To re-time the ignition – Twins (Magneto Models). Before proceeding to time the ignition, check the breaker point gap, which should be from .012” to .015”. Ensure that this is done first, as it will affect the advance point if you do it after.
Loosen the nut from the magneto driving gear, and release the gear from the tapered end of the magneto shaft with the correct extractor (part number 015374 – also used for the small crankshaft gear if that ever needs to be pulled).
Remove the inlet rocker cover and spark plug from the off-side cylinder (that’s the right-side of the bike). Insert a rod into the spark plug hole, or if you have it, a dial indicator gauge (if you’re really cool), or a spark plug timing tool as is available on eBay or other outlets (see pic).
Turn the engine in its normal direction until you have found top dead centre after the inlet valve has closed.
Hold the rod in the plug hole, and mark the rod where you can see it register, or mark your dial indicator, adjust the tool, etc… Take out the rod if you’re using it, and mark it higher up 3/8in (9.5mm) for the 500/600cc twins, or 11/32in (8.7mm) for the 650cc models. Put the rod back in (you DID keep it at TDC, right?) and rotate the engine backwards until the higher mark is at your registration point.
Put the ignition control lever on the handlebars in the fully advanced position.
DO NOT Disturb or move the engine. Turn the magneto in a clockwise direction (looking at the contact breaker side, not the shaft side. Keep turning until the contact breaker points are just about to separate as the fibre block moves against the cam.
You can use a piece of cigarette paper or similar super-thin paper between the points to do this, or a test lamp. When it separates, you’re at the exact right position. Press the magneto driving gear back in and lightly tighten the securing nut.
Recheck the timing by repositioning the piston and then tighten everything down. You can check with a degree wheel if you have the engine far enough apart.
3/8” = 39º 11/32” = 35º
And Bob’s your Uncle.
