Author Topic: EW magneto  (Read 519 times)

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Offline tck

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EW magneto
« on: 26 Apr 2023 at 15:33 »
I am putting a EW engine together with new guides pistons etc
cant believe I am asking this but the points on my EW BTH mag only open once per revolution of the mag ! (fag paper test)
As the mag is half engine speed it follows that it has to spark twice per mag revolution and the pickup band  for the HT confirms this
a cursory glance at the cam ring cannot confirm two dips/humps but there should be two or am I going blind/mad?

Offline cardan

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Re: EW magneto
« Reply #1 on: 26 Apr 2023 at 22:12 »
Blind/mad or a worn out/messed with magneto!

Yep there should be two ramps on the ring, but in old magnetos stuff happens. Often with wear the lift is not the same for each ramp, so it's possible that the fibre rubbing block is only hitting one ramp (and opening the points), but missing the other. If so you could try adjusting it to hit both ramps - the mag will still work OK if the points opening is a bit different on each ramp.

Another possibility is that someone has substituted a cam ring from a single-cylinder magneto with only one ramp - if so check they haven't gone the whole way and substituted a single-cylinder slip ring as well. (On a single-cylinder slip ring the brass contact is continuous, on a twin it is segmented into two parts.)

The fall back position is sending it off to the magneto expert.

(Presumably the mag is a BTH M2 W2 or similar?)