Sunday, August 5, 2012

Veteran Motorcycle Adventures (Part 2): Breathing fire back into the beast!

As Scooby Doo would say:  "Ruh roh, Raggy!"
Months back, I had mentioned that magneto failure was the biggest cause of problems during the pre-1916 Cannonball. Most of this was due to the extreme heat along the route, namely in Texas. For a bit of background, magnetos built during the early 1900s, especially Bosch and Lucas, utilised a non-conductive resin to encase the coils. This material, known as bakelite, was literally melting in the Texas heat and forced riders to do multiple rebuilds along the way. Proper magneto rebuilds do not use bakelite these days, rather other materials not prone to melting. Knowing that my Lucas magneto had been rebuilt in England before the bike was re-assembled, I wasn't sure what kind of internal material was used and thought it could have melted. Had this been the case, it would have been "bad ju-ju" to say the least.

Instead, the cause of my power loss was actually quite simple to diagnose. Removing the timing cover, I found my timing chain laying slack and the exhaust cam sprocket almost completely off of its shaft! It turned out that both of the machine screws that hold the timing chest to the motor, as well as the sprocket nut hadn't been loc-tited and therefore backed right out. Where most applications like this would use sprockets with keyways, both the exhaust cam and mag shaft sprockets are actually sitting on tapered shafts with no key ways, making it relatively easy for them to pop off in a situation like this. It was relatively simple fix--except that it would require re-timing the motor, which is something I had never done. (Insert comment saying "What kind of mechanic doesn't know how to time a motor?")
This tiny little machine screw was the cause for all my failures!
 A quick call to my friend and fellow C'baller Jim Crain provided some great advice on how to go about this. Not to be discouraged, I was determined to teach myself the right way to fix the problem, and would use the "roadside" method as if I was broken down on the roadside during the 'Ball. What makes this motor a bit more difficult to time is the fact that she's a four-stroke. For my non-motor fans, the way that Elizabeth's motor works involves four strokes of the piston.

  • Stroke 1: Intake-- intake valve opens and fuel air mixture from the carburettor enters the cylinder.
  • Stroke 2: Compression-- both intake valve and exhaust valve remain closed as the mixture is pushed towards the top of cylinder, to the sparking plug.
  • Stroke 3: Combustion-- a correctly timed spark from the magneto travels through the spark plug and detonates the fuel/air mix. This detonation must occur a few degrees before the piston is at the very top of its stroke (known as Top Dead Center), and forces the piston back down.
  • Stroke 4: Exhaust-- hot exhaust gasses from the detonation are expelled from the motor as the exhaust valve opens. This process then repeats.
Poor quality picture, but shown here is the spark
plug, nickel-plated valve caps, and compression tap in the background.

The trick to correctly timing the motor involves finding TDC by removing the compression tap and sparking plug screwed into the top of the motor, inserting a rod inside the cylinder. Because the valves are on the side of the motor, as you rotate the engine, the different strokes can be determined easily. As the piston comes to the top of the compression stroke, the rod will rise up and pause momentarily before moving back down. This is known as TDC. Next, you must remove the points cover on the magneto. Elizabeth has one set of platinum points, and the gap between them is .0013mm--otherwise known as not much. The exhaust cam sprocket needs to be tightened, and the magneto sprocket loose. With the motor just before TDC, you rotate the mag shaft and examine the points spining inside the casing. Just as the contact is about to break, you tighten the mag shaft back up and make sure the timing chain is tight. Rotating the engine, you should note the position of the points and make sure they break at the same point. Re-installing the compression tap and sparking plug, the motor should fire to life. Well, three days and two sleepless nights, hundreds of curse words, several thrown wrenches, and a lot of sweat later, I figured it all out and Liz roared back to life.

I rewarded myself and the BSA by going on a threadlocking binge that lasted three days. Now, just about every single nut and bolt has been removed, cleaned, threadlocked, and re-installed. She's a happy girl.

All tidied up and re-assembled


Buck Carson
Confirmed Cannonballer #3
Carson Classic Motors Race Team

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