I borrowed one of the devices that Francois Grosset makes to allows the engine to be run while a timing disc is mounted in the timing side mainshaft. Francois is very clever and makes some excellent equipment including his electric starter motor kit for twins. I could not get the borrowed mounting device to work satisfactorily. The owner of the device didn't have much success either.
I tried putting marks on the magneto fibre gear and viewed them through a perspex window fixed over the hole in the timing cover. This was not too satisfactory as oil splashes tend to obscure the view and as the gear is small and rotating at half engine speed the resolution of marks is poor.
Convinced that driving from the magneto gear was the way to go I decided to make something to bolt in place of the ET136 timing cover window so that it would:
- Seal in the oil splashing around the timing gears.
- Extend far enough out from the timing case to: a) clear the exhaust pipe and b) allow a full size timing disc to be fitted so that resolution of the marks was easy to read.
Using the best Heath-Robinson principles I looked through the scrap under the bench and found some tube. On one end I welded a flange and in the other end I put a piece of nylon to serve as a bearing. I cut one end off an old 1/2" drive socket extension. A socket fits on one end to take the drive from the magneto gear fixing nut and the timing disc is fixed to a flange on the outer end.
The contraption seems to work very well. (Does this mean that I will be inducted into the Heath-Robinson hall of fame/shame? ;-))
I bolted on a pointer for the timing disc and set it to TDC on the rear cylinder. I have a BT-H magneto fitted. I slackened the three fixings on the flange a little so that I was able to just turn the magneto with the engine running. With strobe pick-up fitted to the rear cylinder plug lead and the engine running fast enough to get full advance I observed the marks on the degree disc and moved the magneto to get the timing to the figure I required. (Remember that it reads half engine speed. e.g. If you want 34 degrees advance, set it to 17 degrees.)
If you are using a wasted spark ignition system e.g. Boyer, Pazon, etc you will probably see two marks, one for the rear cylinder and another the cylinder angle out of phase as the trigger passes the front cylinder pick-up.
Sorry for the long post. The pictures below may make things more clear.
Has anyone used a strobe light on a twin?
I can only think of marking the "mag" pinion, then using a perspex cover with a scale on it.
My next problem is, if this works what will the wasted spark do to the position of the strobe flash(I'm using Pazon)? Will I see the timing mark in 4 places?