timing

I'm new to mopeds and I've heard about timing on mopeds. I know it has something to do with ignition but what exactly is it ? In the manual it says the timing should be 2.0 mm BTCD. What does this mean ?

RE: timing

...Timing refers to when the spark plug fires in relation to where the piston is at that time..... If your moped says the ignition timing is supposed to be set at 2.0mm BTDC..... then the points inside the flywheel are supposed to open (causing a spark at the sparkplug) when the piston is 2.0mm (2 millimeters) Before Top Dead Center (BTDC).... Top dead center is when the piston reaches the closest to the sparkplug before it starts back down. .... The spark plug is supposed to fire BEFORE the piston reaches the top so that the fuel mixture has a little time to start burning before it reaches the top so that it is ready to push hard on the piston once it has reached the top and started back down.......OK..?

RE: timing

Wayne Broderick /

Could you make some comparisons to a cars timing??

I don't understand how an engine with only an "on" or an "off" (meaning that it is firing or not) can lose it's timing...

With a 4 stroke it makes sense to me because there are 2 cylinders and so it can become out of synch..

I understand how a cars distributor works and I see why screwing up the plug wires cause bad misfires or no running at all---

Simplify all this into a moped and a basic timing system-

Thanks (I learn by comparison and analogies...)

RE: timing

....what I just explained is as simple as it gets.... but an ignition's timing can get "off" on a moped because most of them have mechanical "points"....which wear over time.... there are 2 things on points that can wear, the point surfaces themselves, and their 'rubbing block' that rides on the cam that opens them... and slowly... as it runs... the rubbing block wears down and the timing gets retarded a small amount (in other words, it now fires at 1.9mmBTDC, instead of 2mmBTDC)..... Electronic ignitions don't have points... so there is nothing to wear, so the timing can't "go off" and you never have to mess with them (like on modern motorcycles).... but cars with distributors have wear in the distributor cap and rotor.... hope that helps your understanding.

RE: timing

The ignition timing can best be compared to pushing someone one a swing. If you push too late it'll have very little effect and if you push too early you're going to be in trouble.

Some mopeds have speed limiters in the ignition timing, on the flywheel of some peds with a "retarding" ignition timing are one or two swingarms held back by a spring. When the revs rise the swingarms will move outwards because of the centrifugal forces, just like the blocks in an automatic clutch do. The outward position of the swingarms will move the cam of the ignition timing backwards causing the contactpoints to open a bit later and "retard" the spark. This will cause a dramatic powerloss at high revs and therefor will limit the revs. On a fourstroke engine this kind of speedlimiters are a real bitch because the exhaust valve will start to open while the ignition is still going, this will cause the exhaust valve and valveseat to burn or even melt.

If you have a retarding mechanism on your moped (2-stroke and 4-stroke) you really should bolt the swingarms down. This way the swingarms can't move outwards and the ignition won't be retarded. This will allow much higher revs, about 15% more.

You could also replace the retarding flywheel for a lightweight advancing flywheel, at high revs this flywheel will advance the ignition in stead of retarding it. At high revs the mixture hasn't got enough time to fully ignite, by advancing the spark there will be more time for the ignition to take place and will therefor increase power at high revs dramaticly, this will also allow much higher revs, about 40% higher than with a retarded ignition.

The trick to making your moped run faster is having more power at high revs, power at low revs will only speed up the takeoff and wear down the clutch much faster.

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