Square 4 magneto problem

[malcolm.johnson]

I have now done about 350 miles on my rebuilt 1947 4G finished some months ago. Most of the mileage has been done on steady runs of about 20 miles to bed in the newly built engine. On the last three runs the engine has begun to misfire badly after about 15 miles, especially at lower revs, leading to my worrying about arriving back home on a trailer. So far that has not been the case, but ......? The project came to me with a beautifully FULLY restored magdyno unit, which had been done by Stuart Towner in 1995 - so it was unused for virtually 18 years after rebuild. Although the problem has not occurred for about 300 miles, I am suspecting the magneto as the culprit now, since the engine runs fine when cooler, but would welcome alternative suggestions to likely faults. Are there any tests I could carry out to establish that the magneto is at fault before dismantling the unit off the bike.

[robjameson]

If it is misfiring when hot it might be the magneto condenser, these don't need to be well used to fail at temperature. I have no idea how to check though!

If you are anywhere near birmingham it is worth calling tony cooper who will be happy to test it for you.