Some of the motorcycle owners always ignore small parts when doing the maintenance for

their motorcycle, and then it will leave some problems or quality issues for the motorcycle

accessories even after repaired and affect the service life.

Here we will discuss about some problem which is easy to be ignored in the process of

motorcycle maintenance, like circuit, oil, gas or mechanical parts.

1. Minimum ignition speed

In general, magneto ignition system has minimum ignition speed and it can ensure that the

ignition system can realize high voltage ignition at low engine speed. However, most

repairman will usually remove the spark plug on the fault engine and do the arcing tests on

high-pressure damping cap when there is a ignition failure. These methods are simple and

easy and have been widely adopted by them. But some engine failure like difficult to start, is

due to ignition coil and the electronic igniter with a high ignition speed.

Such as Honda CH125 water-cooled scooter, its minimum ignition speed is 300 r/min. If the

minimum ignition performance reduces, the minimum ignition speed increase, and it

sometimes even need to reach more than 400 r/min to ignite. If the repairman still use simple

arcing tests as above, then they may be fooled by some false appearance and even mislead.

This is because there is no compression resistance in the cylinder when the piston runs up

and down without spark plug in the engine; the start speed is above 400 r/min, which is much

higher than with speed with the spark plug. At surface, the spark plug does have a spark at this

moment, but after putting it on engine and then apply electric start or pedal starts, then, there

is certain pressure inside the cylinder and the running resistance will increase, start speed will

be only about 300 r/min. If the battery voltage is low, then the start speed is even lower, which

can’t meet the minimum ignition speed, of course, the motorcycle will not start.

Therefore, when using the spark plug or ignition coil to ignite, make sure to install a spark plug

in the engine, or it is even better to use electronic tachometer to measure start speed. If using

the old battery around 12 V, the start speed is around 300 r/min to 350 r/min, the spark is weak,

while using the new battery above 13.5 V, the start speed is more than 400 r/min, the spark is

blue and white, then we can get a preliminary conclusion: the minimum ignition speed is too

high. Then, you need to test the performance of each electric equipment of ignition system,

and replace the ignition coil and electronic igniter for contrast test, which can eliminate the

failure.

2. The ignition triggers gap or the air gap

The ignition system of motorcycle engine generally adopts the flywheel rotor type, while

magnet and flywheel are united into one and form as a part of the engine flywheel, and

permanent magnets rotate together with the flywheel and form a magnetic field, then there will

be induced current in the fixed coil, at the same time play a role of mechanical energy storage

and release. Four-stroke engine use non-contact ignition system, the trigger coil of magneto is

at the external of the flywheel rotor, where there is a permanent magnets, with the rotor

rotating, trigger coil will induce pulse current and provide trigger signal to electronic ignition

device (CDI).

In order to ensure that the ignition system has a strong ignition trigger signal, while it will not

interfere trigger poles and flywheel rotor, so there is trigger gap around 0.40 mm to 0.80 mm

between the trigger pole and the flywheel rotor. In general, the gap can be adjusted by the

mounting hole or oval hole on trigger pole. Due to the error at manufacturing, or reassemble

after repair, or trigger pole wear and tear, the ignition trigger gap may get bigger, thus affecting

engine ignition performance in different degrees. Besides, there is also air gap between

cylindrical stator and flywheel magnetic, under normal circumstances, unilateral air gap should

be no greater than 0.75 mm, in order to ensure the magneto ignition and charging system work

normally.

For a 125 type motorcycle, driving more than 43000 kilometers, after the stator component

was changed for the magneto fault and then electric start can’t work while the pedal start is

normal. Then, repairman will check both static and dynamic of battery voltage, which turns to

be normal, overrunning clutch, start circuit and relay are also normal, then check

the starter circuit, carbon brush and so on, they are all normal. Remove the spark plug, put it

on the high-pressure damping cap and do electric start check, the spark is dark red, and after

changing a spark plug, electric start is still not working. Then you need to replace the stator coil,

and remove the ventilation cover, check the trigger gap between flywheel and trigger pole,

there are unexpectedly about 2.0 mm. Realigns the trigger to standard 0.50 mm, reinstall the

parts, then electric start can work successful now.

Another 125 scooter, due to loss of excitation with the magneto flywheel rotor, vehicle cannot

be started even after replacing new parts. After several check, the problem turns to be the

flywheel rotor wall, whose diameter is too large and exceeds the specified value between the

stator cylindrical. Replace the magneto flywheel, check the gap between stator cylinder and

the flywheel rotor wall, make sure it is less than 0.50 mm, assemble the dismantled parts, start

the engine, then the failure is solved.

You can be seen from the examples above, repairman didn’t adjust the trigger gap according

to the rule when changing the stator component, the ignition trigger gap will be too large. When

applying the pedal start, because recoil inertia is big, it can barely achieve start speed; and

when electric start, because starter speed is relatively low, the large gap will reduce the pulse

signal, the spark is not strong enough to start. And the other motorcycle can’t be started

because the air gap is too big between cylindrical stator and the flywheel rotor magnetic

cylinder wall, which will seriously affect the ignition performance of magneto.