HOME and how to join Forum Open Area General Scott topics Two sparks… one to many? Re: Wasted Spark

Roger Moss

Just a note to add to Richard’s entry.
Some years ago I was trying to increase power on the racer. The engine felt like it was just running out of steam, but it was doing it too abruptly to convince me it was a normal condition. I built a mag test rig and found that the mag points were bouncing at over 3500 revs. (Like valve bounce) The loss of spark timing accuracy and efficiency was resulting in engine roughness and power loss. I rebuilt the BTH mag as best as I could, but could not eliminate all the problems. In normal running on a road bike, you would not notice and just think that you had reached the limit of what the engine had to offer. I bought some PVL electronic ignition components from Rex Caunt and manufactured a “One off” conversion into a traditional BTH mag body. The one drawback was that for space considerations, I could not get two pick up coils in and so it was run at twice engine speed with sparks at both top and bottom. This worked fine and far better than the traditional mag, but it had one penalty. Whatever the backlash / lost movement in the drive system, causing timing firing point variation, then as the rotational speed of the mag was doubled, then the range of possible error was doubled.
Other than that, this wasted spark system worked just fine on the racer for years helping to give an output well above that normally attained with a Scott, so don’t worry, it will work ok.
When my friend Tony Harris carried on development alone, that he had started with Rex Caunt, he produced the new BTH electronic magneto. He wanted to test this under the most rigorous conditions and the Scott engine speed mag application has long been recognised as demanding. We had a prototype BTH mag on our racer and it has given perfect results and reliability for several seasons now. I recommend the new BTH mag as the very best, if you can afford it.
Kind Regards