B31 Thorspark Electronic Ignition - placement photos

[Fraz]

Today, I fitted (my friend fitted)    the electronic ignition and gpmjatmp switch to my B31, as an option to a magneto rebuild.

I thought I'd share some photos of placement in case it helps anyone else considering doing the same.

Thanks

[neil1964]

Looks good although not too sure that the switch terminals sitting up like that near the seat nose is ideal, as it is bolted to the seat nose bracket how does your seat fit?

[Rog1]

I put my Thorspark coil under the seat, but my regulator isn't in the usual place under the seat and on the back mudguard. I had to make a clamp for the frame tube.

To convert from distance BTDC to degrees BTDC I just drew a diagram at 3 X life size: how did you do it? I drew it at 3 X life size to reduce errors, and it's within the reach of an old school type drawing compass. 41.63 degrees is the calculated measurement when working it out with hard maths (thanks to my brother who worked it out). By drawing I got 41.5 degrees; close enough when the timing shakes around over a 10 degree arc when you run the bike.

[Derek996]

Ignition timing for the ZB32 Gold Star (same bore and stroke as the B31) is quoted as 7/16" or 38º BTDC.

[Rog1]

Ignition timing for the ZB32 Gold Star (same bore and stroke as the B31) is quoted as 7/16" or 38º BTDC.

Most online sources I could find just give a measurement, not degrees. But for what it's worth the Haynes manual states 15/32" or 39º for the DB32GS. Can anybody enlighten us?

[Derek996]

What I'm saying is that the ignition timing for a B31 is 7/16" BTDC which equates to 38º.

[Dan_B33]

7/16 inch before TDC it is for both B31 and B33?

[Derek996]

7/16 inch before TDC it is for both B31 and B33?

No, the B31 and B32 are given as 7/16" but the B33 is given as 3/8" BTDC fully advanced.

[Dan_B33]

Thanks Derek!
Might come handy! I have manual where it is empty. Just B31 setup given!
Let's see one day I might get to the point when the ignition needs to be set ...and bike kicked:)

[V500]

All those are only starting points, given that the engines have been in use and rebuilt several times for probably 70 years, and fuel is different now to when the bikes were designed.
The ZB32's etc were sports bikes so a degree setting was appropriate, but the iron-engined ride-to-work plodders needed nothing more sophisticated  than a pencil down the plug hole. More than good enough, especially with manual A/R.

[Derek996]

All those are only starting points, given that the engines have been in use and rebuilt several times for probably 70 years, and fuel is different now to when the bikes were designed.
The ZB32's etc were sports bikes so a degree setting was appropriate, but the iron-engined ride-to-work plodders needed nothing more sophisticated  than a pencil down the plug hole. More than good enough, especially with manual A/R.

Not all ZB32s were Gold Stars. The ZB32 Competition originally had an iron engine, same as the B31. The alloy option came later. For 1950 they were offered with an iron engine, an alloy ZB32A engine or a ZB32 GS engine. The same was offered for the 500cc ZB34 Competition.

[Kernowpete2]

So getting back to the original topic  -
I have a thorspark system on my B31 plunger. It originally had the same switch as the one in Fraz's photos. I've changed it for a Lucas key switch which should give a little bit of security although it would only take a minute or two to find it and short it out.
I think it's good to have the switch accessible whilst riding especially if you continue to have running problems as it will make plug chops easier. I've been doing this on mine recently.