need help on ignition system...

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bobochan - 02 Nov 2007 04:02 GMT

Hi guys,
Im converting a gasoline engine so that it can use alcohol as its main fuel.
I want to ask more about motorcycle ignition system especially on a suzuki
gs125.From the manual, it uses a CDI system, where it picks up signals from a
pulser coil in the magneto assembly and the capacitor in the CDI block will
discharge 150-300volts into the primary coil(part of the ignition coil) .This
will create an output of about 20000volts from the secondary coil to power
the spark plug.
Questions:
1) Does anyone know for this type of bike, what sort of system does it use to
control ignition advance/retard? (ATU/transistor/digital) From most of what
ive read, is it correct to assumed that contact breaker is usually couple
with ATU but on the other hand CDI would be paired with transistor or digital
control system?
2) Is it possible to modify the ignition timing for CDI system?i need to
change its ignition timing simply because of the fact that alcohol burns
slowly compared with petrol.

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Albrecht - 02 Nov 2007 12:32 GMT

>Questions:
>1) Does anyone know for this type of bike, what sort of system does it use to
>control ignition advance/retard? (ATU/transistor/digital) From most of what
>ive read, is it correct to assumed that contact breaker is usually couple
>with ATU but on the other hand CDI would be paired with transistor or digital
>control system?

The ignition retarding circuitry is built into the CDI unit. It's probably a
bunch of coils resistors and capacitors (LRC circuit) that increase the
amount of time that the low voltage pulse from the pulser coil takes to get
to the switching transistor that controls the ignition coil primary voltage.

The pulser coil itself is set at the full advance position of perhaps 32
degrees BTDC. The LRC circuit would retard it to about 10 degrees BTDC.

Some more powerful Suzuki motorcycles have a tendancy to wheelstand in first
gear. So a resistor is added to the ignition retard circuit to keep the front
wheel down when the transmission is in first gear.

This site talks about using a 15K resistor for a "timing retard eliminator".

http://homepage.hispeed.ch/sv.650/

The SV650 probably has a special position switch on the transmission.

>2) Is it possible to modify the ignition timing for CDI system?i need to
>change its ignition timing simply because of the fact that alcohol burns
>slowly compared with petrol.

If you need to advance the ignition, but don't need to change the retarding
curve just take a swiss file and make the pulser coil mounting plate holes
oval-shaped.

If you do need to modify the retarding curve, learn about LRC circuits.

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Motorcycle Forum / General / Technical / November 2007

need help on ignition system...