change Ignition timing, how to do that

[Addy Leung]

I read my service manual of my GS1100, the ignition timing is 15 degree BTDC when engine below 2500rpm, and it is 35 degree BTDC when engine higher than 2500rpm.

My first question is how the bike can automatically to control that change this timing control without a computer IC unit or some timing control unit. So I think it must be controlled by mechanically. I guess the magic unit is the advancer unit behind the pick-up coils. There is two springs to hold the rotor. When engine it spine faster, it will increase the length of the springs and make the timing retard. Do I think it right?

My second question is why it has to retard (hope I understand it right) the timing to 35 degree before the top dead centre (BTDC). I see a lot of after market ignition unit and advance unit is let the bike to have 3 to 5 degree to advance to the ignition to get better performance.

My last question is, if it is fixed the springs length, would it get better performance to the engine after 2500rpm?

Addy

 

[RacingJake]

We call it advance BTDC. As the engine rpm pickups the spark needs more advance to lite the fuel/air for that big bang to push down on the piston. If the spark is too late then you loose power cause the piston may be half way down the cylinder at high rpm. If it's to early then you get pinging.

The mech advance has weights so as the speed increases they are force out turing the rotor, advancing the timing. Works pretty good for low tech.

To much advance at low speed, 2 up riding, or on hot days cause's pinging and the bike may be hard to start as the starter cranks the piston up too much advance cause's a spark way before tdc so the piston is driven back down the cylinder and you hear this funny sound.

I run good fuel and set my timing with and extra 2 degrees of timing for a total of 36 degrees. Just get a timing light and you can do it too. Just my rotating the plate alittle.

 

[duaneage]

I would not mess around with the timing on a regular bike. The engineers that designed the bike took into account the compression, head design, cylinder bore diameter, temperature, and speed of the flame to spec out the starting point and the advance point.

If you radically altered the head, used longer duration cams, ran more compression and were using 104 octane racing fuel in a race only situation then the timing would be open for change.

Altering it on a mostly stock bike ( pods and pipes are not reason enough) is really not necessary and may cause problems like engine damage or burn valves.