ignition timing a Matchless G9

[Seeker]

My friend has a Matchless G9 (and a G11). Both bikes originally were magneto so the engine's rotation provides the spark and the contact breakers position provides timing.

Back in the days when this was built (I think his is a 1954) it was a case of putting in a piece of cigarette paper to determine when the points had just opened and thus when a spark was produced. The advance and retard is manual through a handlebar control.


He has changed this to a Thorspark which uses a Hall effect sensor with a magnet fitted in place of the contact breaker cam. The sensor fires a coil so the magneto is only used as a housing.


It's been reliable but he broke down at Cadwell Park (as he was passing) because it blew and kept blowing ignition fuses. The Hall effect sensor sits on a metal plate which is electrically isolated from the body of the magneto if, as in his issue, they contact the fuse blows.

We found the problem but it required the bike to have its timing reset. Haphazard is a word that springs to mind. There is a mark on the Hall sensor which is supposed to indicate the "precise" trigger point for the sensor - it was done with a broadish Sharpie pen (by Thorspark). By the time he had fitted a degree disk to the crank, located TDC, there were so many inaccuracies creeping in, it's surprising that old bikes could ever run.

I thought I could hook a meter on the output of the sensor to get a reliable trigger point but that didn't work. I looked up Hall effect sensors - they are a semiconductor device that rely on a passing magnet to divert current flow. The only way we could get this to work was by having the magnet move past the sensor quickly if the magnet sat over the sensor it didn't produce an output. There are no strobe marks on the engine, I should add.

We ended up using the broad Sharpie mark on the sensor which aligned when the engine was set to the correct angle (39° btdc if I recall).

We (I) forget that these bikes were meant for daily use and to be roadside fixable (which was not what the RAC guy thought), so a bit of cigarette paper dropping out of a contact breaker was good enough for the bike to run. The G9 is a 500 cc twin producing 28 bhp and, when running, sounds really good.

[garynortheast]

Ah, this brings back memories of doing the timing on my old Norton 650ss Dommie.

I'm not sorry to be riding the curvy!

[Craig380]

Heh, takes me back to setting the points & timing on my Suzuki triples Suzis were less fussy than Yamaha twins or Kawa triples when it came to timing, but it was still worth taking the time to get it right. I used a dial gauge to establish the correct timing marks then would use a strobe to set the timing while running, doing the timing statically could introduce errors if you weren't very careful because the points cam only had a gentle ramp, and I preferred to set the timing as it would be with the engine running.

Magnetos were horrible things, often chain-driven which introduced further variability in the timing as the chain slackened!