There are several issues with sprigs. I am speaking now only of Lantern or AVL bikes. Most sprigs fail because of a kickback of some sort. A much smaller number self destruct because of the loose bushing in the inner primary case. So from most remote to most common:
1. If you change a sprig check the fit of the bushing for the idler gear into the inner chaincase and the the fit of the idler gear into the bush. Replace if needed. Sometimes a Sp rag failure can crack the inner case and go unnoticed, so inspect youres carefully if repairing a damaged sprag.
2. Far more common - The Brits did a lot of research on this. Some of it led to blind alleys. For a while they thought that low battery voltage may change the timing. This rumor has taken on a life of its own. After lots of testing in many conditions they determined that it did not - dry hole number 1.
Then they discovered that most of their sprig failures occurred right after the bike had been restarted when hot. They also discovered that you could get the engine to kick over once with the key turned off when hot. That led to a lot of work on hot spots etc. Another dry hole.
The first answer was a delay circuit. This would not allow a spark until the engine was cranking some, thereby eliminating any tendency for a spark to make the engine kick. Good idea, but in practice was only partially helpful. The issue was mainly due to getting the circuit right in a manufacturing setting. Too much variance. We gave away a lot of these and it helped though.
Finally after years of work with a scope, they determined that the starting motor caused inductive coupling with the ignition system. Said another way, the starter motor interfered electrically with the ignition system causing it to spark at inappropriate times. The first answer was to fit a resistor across the two leads of the ignition system. this helped make it less vulnerable to the electrical noise from the starter motor. The resistor has worked well, but not at 100%.
Most recently the factory has sent us some modified ignition systems that are not at all vulnerable to the extra electrial impulse created by the starter. So far these have worked 100% in the UK. We are now sending them out with all new AVLS' as part of a test program. If you have an AVL engine we would be happy to send you one, just let us know. They are easy to install.
Topic: *&^%!!@#**!! Sprag unit! (Read 12600 times)