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Kickback problems in 4 strokes with Powerdynamo systems
We occasionally get reports from customers that 4stroke engines they
equipped with our systems tend to have violent kickbacks during engine
start. Once running however, the engine mostly performs flawlessly.
This article assumes that initial timing of the system is correct as wrong setting also may produce kickbacks which surely can be cured by applying correct setting. The article does not apply to systems without advance control (2strokes)
|It has to be said first that in those old bikes with some unknown history of service and change since production and customerns with different perceptions and physical abilities (kickspeed, long or short legs, age etc) situation differs from case to case and would need some (careful) experimenting with timing and starting procedure.
There is however some underlying problem, - which happens mostly in larger single cylinder engines with high compression (experience says single engines above 350cc and 9:1 compression can bring such problems):
The system (the advance control unit) calculates the moment of ignition by first reading engine speed (in systems with external sensor by taking the time the rotor protrusion takes to pass the sensor, in internal trigger systems the time the magnetic hole needs to pass the 2 synchronisation coils). This is a calculation of time, not of position, as once the sensor has passed the trigger there is no way of reading crankshaft position in terms of angle.
As soon as this engine speed is found, the inbuilt table of ignition characteristics is consulted and the value for the needed advance at this speed is read and activated. This happens extremely quickly and in the same crankshaft revolution, not only one revolution later as in microprocessor controlled systems which our systems not are.
Now comes the problem:
The speed with which the kickstarter turns the crank is not stable, it changes during kicking. Near TDC it often slows down because of the resisting compression. This however happens after the system had calculated the moment of ignition in terms of at what time after reading the signal it should happen.
The result: spark happens at the calculated time, but the crank is late and has not yet reached the position it was calculated to reach at this moment. The engine backfires.
Unfortunately this can not be helped with the advance control units we use. Everything is fine for as long as starting speed of crankshaft is relatively stable. This in smaller or less compressing engines mostly is the case, but in larger cc engines, notably those with high compression and bad kickstarter ratio it tends to go wrong during start.
For this reason we have already taken a few systems where we repeatedly
got such kickback reports off our offer and in future we will refrain from
making systems possibly prone to kickbacks.