Motorcycle Mojo – July 2019

60 MOTORCYCLE MOJO JULY 2019

MOJO GARAGE

tech by Costa Mouzouris

Servicing

Points

f you own a bike that’s more than

four decades old, chances are it has

ignition points. Electronic ignition

systems replaced mechanical

points in the early 1980s, and with

that change came relief. Ignition

points need constant servicing for

trouble-free running. Neglecting points

causes problems, from a hard-starting

engine to an engine quitting in the

rain. Servicing the points isn’t very

complicated; taking a little time to do

it once a year will prevent a roadside

breakdown.

How does a points ignition system

work? For simplicity, I’ll describe a

single-cylinder system. The main com-

ponents in a points ignition system are

the spark plug, ignition coil, condenser

and points. A coil is what generates the

high-voltage electricity needed to jump

the gap at the spark plug and ignite the

fuel mixture in the cylinder. There are

primary and secondary windings of

wire within the coil that wrap around

a metal core. When current passes

through the outer, primary winding,

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generate a spark, power must be cut off

at this primary winding, thus causing

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it passes through the inner winding,

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Acurately setting

the points is critical

for proper engine

performance

DIFFICULTY LEVEL:

which generates a brief high-voltage jolt

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spark plug.

Timing this cutting-off of the current

to the primary winding of the coil is

critical, because this is what sets the

ignition timing. This is where the points

come in. When the points are closed,

they complete the electrical circuit to the

primary winding of the coil, allowing

electricity, which comes from the battery,

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to the primary winding as soon as the

points begin to open. The points operate

via a lobe that pushes them open and

a spring that shuts them. That lobe is

either located the end of the crankshaft

or, as in the case of the Honda ST90 used

here, on the end of the camshaft.

Timed properly, the lobe pushes

the points open as soon as the timing

mark on the crankshaft aligns with its

mating mark on the crankcase. These

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top dead centre [TDC], which is when

the piston is at the top of its stroke)

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the time the spark should take place in

the cylinder). The F mark will always

precede the T mark by several degrees,

since combustion must begin before the

piston is at TDC.

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is to inspect the contact surfaces for

pitting and to see if they are burnt; in

either case, they must be replaced. Also

replace the condenser, because its job

is to protect the points from surges in

voltage when they open; otherwise,

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400-grit sandpaper, although the points

must then be cleaned thoroughly.

Whether new or freshly resurfaced,

the next step is to set the gap when the

points are open. To do this, you must

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a wrench on the end of the crankshaft)

until they are fully open. These specs

are found in the service manual; on the

ST90 the gap is .012 to .016 inch. Setting

An access plug on the

crankcase reveals the

timing marks on this

particular engine.

Don’t forget about

the timing advance

mechanism that

usually resides

below the points

mounting plate.

The timing advance

must be clean,

slightly lubricated

and have properly

tensioned springs.