Blackburne prep for the Ixion Run

This is where I always go wrong…

 Common sense says, if you have a bike that works perfectly well and there’s a run coming up this weekend, you should leave it alone. But there’s a problem in that for me. Often I have some new part or job lined up that I haven’t had time to attend to and needing the bike for an event is the perfect excuse to put it to the top of the queue.

When I bought the 1915 Blackburne it came with a fairly period Bosch ZA1 magneto that was a bit suspect. I replaced it with BTH. Up until the Great War, the world relied rather heavily on German Bosch magnetos. When the unpleasantness kicked off everybody on the Allied side had to start making their own. Hence a BTH is not really appropriate on a pre (great)war bike. Anyway the BTH has worked perfectly ever since but having finally got around to rebuilding the Bosch, I decided with three days to go before the Sunbeam Club Ixion Run I would change it. Since this Blog is supposed to be about repairs and workshoppery I thought I’d show how to time a vintage mag. 

  Here’s the rebuilt mag. I’ve fitted a rewound armature and remagnetised the body and it sparks like a good ‘un. Hopefully it will continue to do so on the run on Sunday…

First step is to remove the chain cover and disconnect the magneto chain. There may not be a link, in which case it will be necessary to slacken the chain adjustment and roll it off the sprocket. After that use a soft drift and strike a tooth to jar it off the taper. Aim at the root of the tooth, the strongest part, to avoid chipping. Having released the sprocket, replace it and the chain and nut, leaving it loose on the taper for setting.

Then check and if necessary set the points to the correct gap – generally 0.012″ (twelve thou). Varying the points gap alters the timing so it’s important to have a consistent setting. To adjust, slacken the locknut on the stationary point and screw in or out,

Next remove the compression tap, usually found on the top of a vintage sidevalve cylinder head. You need something to measure TDC. I use a stick of plumber’s solder as it is soft enough not to scratch anything inside the engine and also flexible enough that it won’t jam or snap if you accidentally turn over the engine. Having marked the TDC position, mark the timing point below. On a Blackburne engine that’s 11mm before TDC.

Turn the engine over until the inlet valve closes, that will put you onto the compression stroke. Continue turning over until the piston reaches the timing mark, before top dead centre.

Now set the ignitnion lever to fully advanced and then turn the magneto shaft in the correct direction until the points break. Note that on a magneto the fibre heel of the points needs to be trailing, so the points shown go anti-clockwise, seen from this side. I’m using a points break indicator from The Magneto Guys 01323 840203 (see Classic Bike April 2014). Such precision isn’t strictly necessary on a vintage sidevalve but it’s a handy tool that lights up when the points break.

Now,  using a socket to avoid the protruding armature spindle, tap the sprocket onto its taper. It should grip firmly enough to allow you to check the setting by rotating the engine back to the mark on the depth gauge and ensuring it matches the points break. If so replace the sprocket bolt or screw and tighten up.

So there we are all ready for Sunday’s run…Hopefully. I’ll let you know how I get on.

R