Honda crm problem

Sounds unlikely - normally the 3 yellow wires are the alternator output. The ignition pulse is normally one or two wires coming out next to the yellow ones, and going to the CDI.

There's a wiring diagram here which backs this up. Looks like the wires should be Blue/Yellow and Red/Yellow.

Have a look at the diagram, and see what you have / haven't got, and come back if you don't get anywhere / want to know how to test stuff.

Not quite sure what you mean by "engage the pick up sensor manually" but:

Some systems use a pulse coil on the alternator to actually generate the energy for the spark, it builds up in the capacitor in the CDI and then gets sent to the HT coil. If it's one of these then the alternator would need to be spinning fairly quickly to generate a spark.

Other systems  just use the pulses as a trigger, with the spark energy actually coming from the battery.

In terms of what to check:

CDIs are generally very reliable

Coils can fail, but even failed coils will generally produce at least SOME spark with the plug out of the engine.

You think the stator's been replaced, and if you've got a good ohm-meter you can do  some basic checks to see if that's got a problem

The rotor could've de-magnetised - particularly if the engine's had a knock so hard that the casing shattered. No simple reliable way to check that, it'd be a case of picking up a new / good 2nd hand item.

MOST likely - wiring's all to pot. Clearly you'll be checking the the wiring on the supply side, and on the feed to the HT coil, but quite often the CDI is earthed when the ignition switch is "off" so if some of the other wiring on the CDI is wrong / dodgy, that could be a problem too.

Now stop calling me Tabby

By "rotor" I mean magnetic flywheel.

180ohms sounds reasonable for a pulse coil. At a fast kick-over I'd be looking for about 50V from the pulse coil.

First off -  you're clearly checking resistance with the pulse coil disconnected from everything else. When you check for voltage, is the pulse coil still disconnected? Are you measuring across the pulse coil wires (correct), or from one of them to the bike's chassis (wrong).

Assuming you've got a digitial multimeter you might struggle to get any kind of reading from a pulse coil because it's so brief as to not register. If you can find an analogue meter or even an oscilloscope you'd be able to see what was going on. Alternatively if you know anyone with starting rollers you could use those on the back wheel to spin the engine constantly and you might start to get readings on a digital multimeter. This would also allow you to measure the alternator output (3 yellow wires) which might start to shed more light.

I appreciate that's not a lot of help but this is the down-side to kick-start singles - I had similar problems with an 600 single and got quite fit!

If all the wiring's correct, and the HT coil resistances are correct (I'd expect ~ 0.7ohms primary, & maybe 20Kohm secondary) then I'd take of the alternator cover and have a look.

Does the stator coil look like it's been replaced? If that looks new, well fitted etc then you might have a rotor problem. Does the rotor look damaged? Does it still have all its magnets? They can detach or break up. If it looks OK then it could still be de-magnetised in an impact but as I say I'm not sure how to easily test it - most of the experiments I can imagine would be full of potential errors and probably inconclusive.

You'll need the correct tools to pull it off the crank end - a legged puller or similar won't work. Undo the bolt securing it and there should be a female thread inside the rotor. You need to find a corresponding male thread, and use this to pull the rotor off (being careful not to damage the crank end). Clearly the engine / transmission will have to be locked in top gear and the rear brake applied.

I do have an oscilloscope so if you can get it to Bristol we could have a look, but it might still be a bit inconclusive as we don't have a healthy system to compare it to.

ive got the same problem mine sparks some times and runs but stops then wont kick up as theres no spark

Are you sure? What have you checked / ruled out so far? If you haven't narrowed it down to the same stuff as Stevb then you haven't got the same problem, and you've barely got the same symptoms.

How long have you had the problem? When did it start happening? What have you checked / tested / ruled out so far, and how? And what bike are you talking?

Does anyone know what ignition switch would fit a crm 125, was thinking the vfr because it looks the same but dont want to buy it and then it doesnt fit? 

anyone know?? 

Do you have a copy of the wiring diagram?