HOME and how to join Forum Open Area General Scott topics Magnetoes Re: Re: Magnetoes

Roger Moss

The higher the voltage, the more easily it breaks down suspect insulation. I had a couple of BTH compromise mags rebuilt once by John Cooper, now sadly no longer with us. I had a couple of Scotts with mags rotating in opposite directions. I asked John if he could rebuild them so I could use them in either rotation only by changing the contact breaker. He told me he could do this, but to do so he would have to set the contact breakers so neither were breaking at their optimum flux. He said that although the spark would be of slightly lower voltage, it should work OK. One lasted 5 years racing and the other 6 years racing. I then had them rewound by another rebuilder as John had died and I believe they were wound with extra windings to give extra voltage. Unfortunately, when this is being considered, it should be remembered that the mag on the Scott runs at engine speed and if you are racing and using 5000 rpm, then this is equivalent to expecting the mag on your four stroke to rev reliably at 10000 rpm. One mag lasted 3 laps of Cadwell and the other 5 laps. Do remember that the voltage required to jump a spark plug gap is directly proportional to the gap. If you have a weak mag, the first thing to try is to drop the gap down to about 10 thou. It usually starts much easier and you will be hard pressed to notice any difference in the performance. Of course I take for granted that unless you are a masochist, you are using a decent plug. I never economise on plugs, but then I never compromise on anything if I can avoid it. I use NGK Iridium point plugs BPR5EIX which folks will tell you are soft, but they suit the Scott as it runs unusually cool. Given I have always pushed the boundaries, I made a test rig to check out the accuracy of the mags I had. I could not get a good BTH mag to go beyond 3500 rpm before the cam started floating. Sure there was a spark, but the position started floating about and it became obvious that what I had thought was engine power / gas flow drop off was in fact the spark losing timing. I made my own electronic mag but because of component sizes, I had to run it at twice engine speed. It worked OK and Richard is still using it on his bike. When old racing friend Tony Harris bought the BTH tradename and started to make his electronic mags, I had a prototype. Tony has retired now and the company has a new owner so I can say without any fear of favoritism that I have never had a moments problem with my new type electronic BTH mag. So the golden rule if you want a bike to enjoy is get a good mag, make sure your carb is in good condition with special attention to needle jet and needle (as the jets wear very rapidly) and if you are using a four stroke type silencer, fit a bush in the tailpipe with a 7/8″ bore to give a bit of backpressure. If a customer requires an original mag rebuilt, I send them to Sean Hawker who is a kindred spirit. Same as everything else in life, you get out in proportion to what you put in! Kindest Regards Roger