2 Stroke Timing - Confused Again

I figured I would turn this issue into its own post in the proper subforum. I'm working on the Suzuki FA50 scooter. It has a magneto/flywheel driven ignition system with a CDI. Spark is good, coil and CDI are working fine and have been tested on a running model.

I put a timing light on tonight, as well as a dial gauge to confirm TDC. The flywheel is keyed. The CDI has a curve as pictured below. The ignition is a lost spark setup with two sparks per revolution of the crank.



According to the manual the timing mark on the flywheel should line up with the cast mark on the case at 4000r/min. The line on the left on the flywheel (marked TDC) is where the flywheel lands when the piston is at TDC - just for reference, I don't believe this is really relevant). The blue mark on the case is irrelevant but can be used for reference to the timing mark on the engine case on the second picture.



When rotating the crank with a drill (obviously not 4k rpm), the plug is firing at the blue mark on the case, which is about 90 degrees from the cast timing mark.



It just seems as though the plug fire is just happening WAY too late after TDC to ignite the mixture. Even at lower RPMs, the plug should be firing much closer to TDC. As of right now, it is firing halfway to BDC.

I rewound the primary coil on the stator, and I now believe that the new winding has somehow completely altered the point at which the charge is sent from the stator coil to the ignition system in relation to TDC.

My thinking is that I need to rotate the stator plate counter clockwise by making a backing plate for the stator itself, since the way its mounted only allows for small advancement/retardation of timing.

Is it possible that either the rotor or stator came from a different model? are we sure that the marks line up when they are supposed to?

I'd pull teh woodruff key out and then time it up and bolt the rotor on without a key in roughly the right place and see if it fires up.

You have a timing wheel and a depth gauge?

Most two strokes have the advantage of having the plug hole right above the piston, so you can easily work out TDC and then use the wheel to set the timing.

A screwdriver works too. It works on most bikes.


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irk said:

A screwdriver works too. It works on most bikes.

Click to expand...

Check in with Ichiban on how to determine if you have the correct metric screwdriver.

I think it's the green handled ones? I could be getting that confused with the zip ties or assembly tape, though.

Haha. Very funny. A long tipped screwdriver works on pretty much any motorcycle. I've done it on 2 different CB750's, my XL250, my XL175, 2 CB360s, an XR80, an XT250 and a host of V8s. All you're doing is looking for the piston at TDC. It's isn't fucking rocket science and it sure as the fuck doesn't require some precisely calibrated instruments. Assholes. If you were around during Barber, you would've seen me change a valve seal without ever taking the head off the motorcycle. Imagine that.

That's a REALLY long way off, so it ought to be something fairly glaring. Wrong part per teazer? I can't see where winding the coil would have this effect under any condition. Since you have spark it sort of has to be ok - the windings have no effect on the actual timing - just the quality of the output. The spark happens when the circuit to the coil opens either by opening the points or an open transistor in an electronic gadget triggered by a pickup coil (another transistor). So as long as the coil is in the right area to supply max voltage when the circuit opens it should be fine. Did you make any changes to the flywheel (aside from swapping it for a different application) or trigger/points location? Any chance you are spinning the thing backwards with the drill (sorry - that far off has to be something major! - don't ask me why this would occur to me!).

How are you checking for TDC? If you are using the "screwdriver method", your TDC mark seems reasonably close to the timing mark, at least compared to where it fires. Ignition timing is massively more important on a 2 stroke than typically on a 4 stroke - at least for engine safety, (even more so on a bity 50cc engine as the poor thing will only ever know two speeds - idle and WFO) so if your parts do not provide the right reference, you do need a very precise determination of top center so you can determine for yourself where the correct timing point is. If you get an accurate determination of TDC, and your factory timing marks line up in the twentyish degrees before you get there rotating in the normal running direction you can be reasonably certain all is well and you can focus on why the trigger is out of sync. How is it that you have lost spark on a single cylinder two stroke?

It turns out that when I rewound the stator, I somehow mixed up the beginning and end of the wire. Therefore the whole ignition system was out of phase. I swapped the two wires from the primary winding going into the CDI and a shot of ether had it firing.

I am going to fully reassemble everything and double check the timing with a light - as per manual the timing mark should line up with the cast mark at full advance (4000 RPM).

Thanks for the input gents. Looks like I had myself running around in circles for a while.


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