CJ750 toolbox
Bart Sanders on how to set M1 ignition timing...    the smart way

Ignition advance specifications and how to measure them

The M1 engine features a traditional ignition advance setting system... by hand that is!  Using the lever on the left handlebar one can pull the ignition moment to the latest or earliest moment or even somewhere in between, depending on what the engine needs at that moment. Fine, but how do we know if full advance or full late are within factory specifications? And what are these official specifications anyway?   First of all, I checked all available documentation with the CJ750 M1 to find out that the latest moment is specified as...

Late setting: piston is at 0.5-1mm before top dead centre (BTDC). Contact plate is in full late position, contacts should just start to open.   Also, the contact point opening is specified as 0.4-045mm.   But no information could be found on the full advance setting! And as one knows, that setting is much more important than the full late setting.   So I dug out some old motorcycle literature and there I found a table with all kinds of German motorcycle engine setting information including the needed full-advance setting information! For the BMW R71 grandfather, this book (1955, Trzebiatowsky, Motorräder Motorroller Mopeds) specifies full advance setting: piston is at 6-8mm BTDC.   Okay, so I had all the data. But because I do not want to remove the cylinder heads each time I check the timing I contemplated for a way to measure these two moments without having to strip the cylinder heads. Luckily Trzebiatowsky shows an elegant method to convert the specified piston positions to crankshaft/camshaft rotation angles. Using this method (which basically projects the longitudinal piston movement on the circular crankshaft movement, taking the engine hub and connecting rod length into account)  I came up with the angular ignition values...

Late setting: 6-12°  (nominal 6°)     Early setting: 29-33° (nominal 30°)    

As you can see there is quite some tolerance in these original specs but then again the M1 (R71) engine is producing very low power values compared to its size.   Anyway, I now had the angular values. Knowing that the M1 design does not allow for looking at the flywheel to find and check little angular marks, I had to find another way.   The only axis that's accessible for this task is the camshaft. So using the end of the camshaft for doing some angular movement measuring proved to be the way to go.

Adjustment Process by Photos:

The tool modeled after the high voltage distributor rotor. The quarter circle plate shows TDC, late and early angle values.
The camshaft end ready to pick up the tool. The multimeter clips are measuring the contact opening resistance. Pull lever to full late position.
Fix the tool on the camshaft. Now comes an important step—with rear wheel off the ground, shift to fourth gear, remove spark plugs and use a bright small light to look inside the cylinder via the sparkplug hole. Rotate the engine using the rearwheel until it is at exact TDC. This can be done very precisely. Take care that you work for the correct cylinder!  
At this TDC point, mount the tool's quarter circle angle plate and align its TDC line exactly with a fixed point.
I have used a small pointer fixed to the Bowden cable pickup.
Live operation for late setting. The exact late setting is now easily obtained by turning the black M4 slotted bolt in or out until the multimeter stops beeping (ie, the contacts open) within the short black angular area. Late is now at nominal 6°. Same operation for early advance. Release the advance lever fully and check that the beep stops within the long black angular area. This moment can be adjusted by the eccentric screw located next to the contacts. Repeat this process for the other cylinder. Usually you have the same settings if your camshaft is okay. Adjust the Bowden cable for 0.5mm play when in full released position (ie, full advance.)