So I work in this lab with a small dyno for a two wheeler engine hooked onto it. The engine hasn't been used for years and I had to correct few things in the wiring. I had to remove the flywheel rotor to check the pick up and the condition of the coils and while putting it back, I am pretty sure I did not put it back the way it was. I think I know the answer but I am going to try. Will the ignition timing change when the flywheel rotor is put back together differently?

I have attached an image for reference.

A generic Image

Thank you!


  • Don't forget ... we have some ladies who frequent here as well :o) Jul 21, 2016 at 22:00
  • Hahaha. Noted :)
    – Vish_evo
    Jul 23, 2016 at 5:55

2 Answers 2


As Steve Matthews says, get your flywheel in a proper position, it shouldn't be able to fit it wrong way. Unless you have lost a part number 6 "keyway". To me looks like

  • coil 3 is a high voltage coil for your ignition,
  • condenser 5
  • trigger 4
  • flywheel 7
  • keyway 6
  • All of that should be enough to get a spark in a proper time. Condenser (5) screwed to earth, the wire of a condenser goes to a trigger moving part, it might be the end of a spring, which is bolted to a trigger plate (4) through plastic washers to keep away from earth. The coil (3), I believe, the wire with a ring terminal at the end, should be connected to the same trigger where the condenser connected. If that wire gives you 150 - 180 volts, then leave the other wire loose. If not, put on earth. Make sure that trigger "lever" touches the plate or any other earthed part only through terminals. The attach a extra wire to the same trigger, the 3rd wire :D and send it to a induction coil, which will give you enough for a spark.
  • The idea is - when the trigger is OPEN, the coil 3 generates about 160v, and condenser boosts the power, sends it to a induction coil, which will hit the spark plug. When the trigger CLOSES, it is closed loop, nothing happens, no spark. For a start make a trigger gap about 1mm, and then go smaller while adjusting. The moment when the trigger opens is the ignition timing. Not when it closed.
  • I might be wrong somewhere, you also might have a induction coil in there, but I don't see a place where to attach a Spark Plug Lead..

If anything not right let us know, we will get it running :)

  • So we got it running... for a while. The oil pump gear's sprockets got sheared and the oil was not being supplied to the engine. The engine got seized. But I did figure out the timing with your method. Thank you!
    – Vish_evo
    Aug 4, 2016 at 3:04

I would say that, almost certainly, the orientation of the flywheel relative to the position of the engine will be important. Looking at the diagram, I'd guess you physically removed part number 7 to inspect what was inside. Part numbers 3 and 2 look like some sort of electromagnetically wound induction coil and I assume that the wiring coming out of part number 2 is the low tension side for the ignition system. Part number 4 looks like some kind of "points" system which is probably responsible for triggering the ignition event.

I'd also guess that the screws (part 9) fit through the elongated holes of part 7 which gives them some scope for fine adjustment. You are almost certainly going to need to align the ignition timing events from the parts shown here to the engine position.

You'd usually expect components like this to have either a "keyway" (which forced them to be fitted in the correct orientation) or that there would be some manufacturers marks somewhere in the body of the parts to line up. If you can't find these or any other data regarding starting positions, I'd be tempted to try and time the ignition event to somewhere near Top Dead Centre (TDC).

On a four stroke engine, this is the point in the rotation of the engine that the inlet valve has closed and the piston has then traveled as far up the bore as it can go. Once past TDC, the next valve to open will be the exhaust valve but this will not happen until after the piston has traveled down the bore. On a two stoke engine, it's easier to establish TDC as this is the point that the piston is at the top of the bore and will go no further.

This can be established by removing the spark plug and using something soft (that will not break off, I've used a drinks straw and a pencil in the past) to work out the piston position. Once you've established TDC, make a small paint mark somewhere on the rotating parts of the engine so you can line it up.

Next you need to establish when the ignition event fires. You need to be very careful here as you are dealing with high voltage and there is a shock hazard. The preferred method is to use a timing light but assuming one is not available, you can hold the spark plug WITH INSULATED PLIERS against the engine block. With the ignition on, tuning the flywheel (i.e. part 7) with retaining nut (part 16) either loose or removed so that only part 7 and not the shaft to which it is attached moves, should cause the plug to spark at some point in the rotation.

If you can't rotate part 7 without moving the shaft, this is why it's important to mark TDC somewhere. Once you know the point at which the plug fires, you should be able to either refit part 7 in the orientation so that this takes place at around TDC or adjust parts 2 and 3 by loosening screws 9 and moving them.

If you get no spark at all, check that the points are opening and closing as you are turning things over. It's the closing / opening of the points that usually triggers the ignition event in a system like this.

Provided the engine is getting fuel, it should sound like it is at least trying to start with the ignition set at TDC. You can adjust it a couple of degrees forwards or backwards in small steps upto around 10 degrees either side of TDC. If the engine still won't run in these conditions, it's likely not the ignition timing.

The lesson here has to be, take photos or make paint marks (preferably both) before taking something apart.

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