Topic: Dynamic timing question

[Knottyandgrain]

First time ever using a timing light. I've always done static timing but I finally brokedown and bought a timing light.

First question, why does my idle jump around? 1100 to 1400 range?

Second question, when I'm using my timing light and my rpms increase/decrease the tick marks are moving with the rpms, normal? I can post a video if need be.

Thanks guys!

[Bodi]

The strobe fires when the sparkplug fires. At low RPM the advancer is at minimum advance, if you had it on a smooth running electrical motor the tick would stay dead still but it's on a gasoline engine with 2 power pulses per revolution, so the crank is being accelerated by them and then coasting down a bit. That uneven RPM jerks the advancer around making the spark timing also jump around a bit.
Increasing RPM makes the centrifugal advancer advance the timing so the spark is more before TDC. This is to give more power, the fuel is ignited early so that it reaches high pressure when the piston starts down on power stroke. The air-fuel mixture takes some time to get burning well.

It's best to set the timing at the full advance mark around 3000RPM or whatever RPM is high enough for the timing mark to have moved till it stops and sits steady - the advance mark is before the F and T marks, just a small unlabelled line on the advancer plate. Two reasons: the F mark, as you have seen, dances around on almost all engines at idle RPM which makes setting difficult. Second, you will be at full advance for almost all your riding and advance has a large effect on power so you want it set correctly there. Also, too much advance can cause preignition which can melt your pistons.
Too much advance at idle causes poor idle and kickback when starting, too little advance at high RPM reduces power output quite a lot (and may burn exhaust valves). All auto/bike gasoline engines use an advancer system, balancing decent idle against high RPM power without an advancer isn't practical except for relatively low RPM and low horsepower engines as used on lawnmowers and the like.

Note that some problems with the advancer assembly or points plate can cause also the timing to dance around.

[pjlogue]

Bodi's reply is spot on and quite eloquent description of the advance.  I would add that you need to make sure your advance mechanism is working properly with free movement before setting the timing using the full advance markings.  You should also check the small diameter shaft that the advance rotor mounts on to be sure it isn't bent.  A few thou out and it can cause your timing to bounce around if you have points. 

-P.

[evinrude7]

First time ever using a timing light. I've always done static timing but I finally brokedown and bought a timing light.

First question, why does my idle jump around? 1100 to 1400 range?

Second question, when I'm using my timing light and my rpms increase/decrease the tick marks are moving with the rpms, normal? I can post a video if need be.

Thanks guys!

two reasons the idle jumps around could be vacuum leak at carb boots or carbs not synced. 

[HondaMan]

The timing jumps around a LOT when the shaft for the spark advancer is bent. I outline in my book how to fix that.
It gets bent when the engine is turned CCW direction, using the big nut by the points, with sparkplugs still installed. Don't do that...

[PeWe]

Feed the timing light by a separate battery is more convenient. Works better.

I use an old battery that did not have amps enough to start my bike. Work fine for timing light.