Any Magneto magicians?

[750rideruk]

Trying to sort my Bsa b31 1953 plunger. Can you time the points using a bulb/battery or a mutimeter to check points opening?
My points block shows no insulatin between points and backplate when on the bike. Off the bike all is well so it seems the centre bolt is causing the short. The centre of the points block has insulation in it so I assume all is well. Am I missing some insulation underneath the points block/ Can anyone help with this. I still have a spark at the plug with the short but it is not blue but will jump about 10mm.
Please can any one help

[idie]

A multimeter will not work on the magas there is always a small resistance through the windings. Use a 11/2 thou feeler gauge or one of the buzzers that you can get. The long centre bolt has to be there as it pulls back the taper behind the points.

[cdsdorset]

I always set mine up using good old fag papers.

[idie]

The trouble with using fag papers is that the oily hands that we tend to work with ruin the papers. That's why I use a thin feeler gauge.

[cdsdorset]

I don't smoke 

[Pete Gill]

Consider buying one of these gadgets sold by the Magneto guys.

It’s an electronic ‘light’ that can be used on a magneto in the same way as a lamp and wires for a points set up.
Not cheap but it works for me and seems very accurate

https://www.themagnetoguys.co.uk/magneto-static-timing-light

[Derek996]

A multimeter will not work on the magas there is always a small resistance through the windings. Use a 11/2 thou feeler gauge or one of the buzzers that you can get. The long centre bolt has to be there as it pulls back the taper behind the points.

I beg to differ. A multimeter connected across the points and set to a low Ohms range will read 0Ω (or very nearly) when the points are closed. When the points open that reading will increase to something between 20Ω and 100Ω depending on the magneto. The thing to watch for is a sudden increase in the resistance. This is the point  where the contacts are opening.

[Calum]

Does the short go away if you back the centre bolt off ever so slightly? It screws into a nut which I believe is encased in plastic/resin to insulate it from the mag body. If this screw is overtightened I believe it can crack the plastic and short across. I only ask about backing it off slightly as a test as I found I had a similar issue on my mag, although with the locktab folded over it did run fine. I couldn't get my head around what was touching where.
I bought one of those timing lights but off someone else on eBay, a little cheaper than the one from 'the magneto guys' but both would be a worthy investment. Very helpful in my diagnosis work on my bike.