Originally Posted by pecanflyboy
#3 If you have a weak battery, poorly grounded engine or starter, or corrosion in the starter relay or cable terminals, you can get a backfire during the start. It has nothing to do with the accuracy of the SIM timing during start, and will not be corrected by timing several degrees after TDC. There fix is usually to check the health of your electrical system (weak battery in your case).
It has to do with the system voltage dropping below the required voltage (8.5VDC) for the SIM to operate. I believe the SIM charges the coils as the engine approaches TDC on compression. If the system voltage drops below the operating voltage of the SIM, it will shut down and it is possible for the field to collapse and a spark to be released before TDC. You will see this as the starter struggles to get the blade over compression, and the engine kicks back. Check the voltage at the starter teminal when the starter is engaged to check the health of your electrical system.
Earlier in this thread, starting timing is covered nicely IMO, so I'm not going to rehash that. However, the low voltage- lets take that one head on. I hope this is wrong. If you right, that is a major design failure. A good design doesn't risk grenading the system when a failure mode that has a 100% chance of occurring occurs. Cold happens, weak batteries happen, masters get left on, etc. Please tell me a spark doesn't get triggered by an event (low voltage) unrelated to piston position.
** another thought. If for some reason power dips when you are 50 degrees BTDC, and you are WOT, full power. (you win the bad luck lottery)
Update- I've heard back from Bill. He insists a low voltage will not cause a firing event. But also says that if the prop stalls (before TDC) the pre-charged coil pack can't hold it forever. It could fire substantially before TDC. In this edge case I prefer the impulse coupling. Preignition event at WOT? Clearly, one can shut off the SIM (ground the P-Lead), WOT, full power and the code manages to shut down each cylinder appropriately. It has been done many times. However, what is the timing behavior if voltage is removed, WOT, full power? Is it a lottery or a safe shutdown?