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John Glimmerveen

2-Stroke Ignition Timing

By July 12, 2011

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I'll never forget watching a motorcycle reverse into a car at a stop light! The poor rider's bike had stalled; he went through the normal start procedure (check fuel, ignition on, out of gear) and gave the kick start lever a hefty kick. The bike fired up and sounded normal. As the lights changed he selected first gear and, much to his (and the car driver's) surprise, went backwards!

The cause of this problem with 2-strokes is the ignition timing. If the timing is close to TDC (top-dead-center) it is possible to catch the piston just at the wrong time with the result that the engine runs backwards.

This problem can only happen on a 2-stroke because there are no valves to be operated in a set sequence, as in a 4-stroke. Typically, this problem would happen when the contact points became worn, or more precisely, when the contact point's heel became worn. The net effect of a worn contact point heel is the ignition timing progressively becomes retarded.

Checking the ignition timing on an early motorcycle is best done on a more regular basis than their modern counterparts; monthly if the bike is ridden daily such as a commuter bike. Not only will the potential for running backwards be avoided, but the engine's entire performance range will be optimized too.

Comments
July 17, 2011 at 8:27 am
(1) Pete says:

Another good one John!
I had that happen once! There was 5 or 6 of us half way a long ugly hill and my 360 Yamaha stalled. It fired up and sounded a bit off, but I figured it was hot – which it indeed was.
The next sequence of events was
1 – Pin it
2 – Drop the clutch
3 – Eat the handle bars, followed by the front fender.
I wish I coulda seen the look on the face of my buddies lined up behind me!
As I recall that one hurt too…
I have a TDC gauge in my tool box…it just screws in to the plug hole (works best if the plug is dead center on top of the piston). If timing is supposed to be 3.5MM BTDC it’s very easy to find.

September 15, 2013 at 2:31 pm
(2) Lew Black says:

I had this happen in 1970 with a Bultaco Matralla road racer. I put it in gear-and went backwards. I just had an email exchange with a friend I used to ride with, and after telling him that story decided to do a search-to make sure I hadn’t invented this memory. Thanks for validating the event.

September 15, 2013 at 9:47 pm
(3) classicmotorcycles says:

You’re welcome Lew.

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