THE strangest spark situation with a 250

[tri again]

I would Not believe it if I didn't see it with my own eyes.

Plug out, touching head, good spark.
Plug in, no fire.

Here's my favorite part.
Plug out touching head, good spark.
Leave it there and screw another spark plug into the head and there's spark until the
plug #2 is tightened.

Seems compression related, like compression kills the spark in the external (testing) plug.

If the dummy plug is loose slightly in the head, the external test plug will throw spark.

This is all with the electric starter spinning the engine over.
I've had them act differently between E start and kick but never this bizarre.

[ytm200steve]

sounds like electrical issues i hate electrical issues good luck

[STiForester]

Weird. I can't think of what it could be. Sorry I can't help. Good luck buddy.

[Macs]

Maybe porcelien is cracked. When it compresses, the crack gets worse causing it to grouned out?

Just a thought.

I purchased a Hodaka several years ago that had a freshly rebuilt engine. The PO couldnt get it to run afterward, so i got it for a song and a dance. Anyway, i was having a similiar issue. It seems the PO owner had smeared antisieze on everything... Im talking about the bolts, gaskets, spark plug threads, everything. Eventually i pulled the topend off and cleaned it all up and put it back on. Ran like a champ.... I guess the antiseize was causing a grounding issue or there was another problem i fixed in the process.

[Big Mike]

When you have the plug tight in the head, does it seem like the motor is slowing down with the starter spinning it over? Could be like you said a compression issue and that when the plug is in and tight, it is slowing the stator down just enough to kill spark.

[atc500x]

Change the coil,compression need a lot more KV for the coil than the spark outside the engine.When the KV is too high for the coil,the coil simply shut down.Try to change the coil.
I had a similar problem with my xr80,always run fine.Except when I accelerate hard,fire simply cut,change the coil and the problem solved

[ktmbk]

Had the same problem with my KTM 350mxc. Great spark with the plug pulled, but nothing when the plug was tightened in the head. Turned out to be the ignition output wasn't strong enough to spark with the engine compression. Check toe coils, pickup, stator ect...

[tri again]

I like to thank you all personally for Great suggestions.

They all have extremely valid points.

Latest news is the spark is getting worse.
Now it sparks once per kick or per 5 second starter spin. dummy plug creating compression or not
so at least That's different.

New coil and many new ngk's. cdi swap with known working unit.
Stator coil ohms out per spec. but that is most suspect at this moment
for no particular reason.

Guess it time to shelve some known working parts by replacing stuff that isn't bad.
but we promise to do it one component at a time so we know which one is misbehaving.

It will sure be great to post a definite answer to this problem so we are willing to sacrifice some $.

Thanks again.

[sblt500r]

sounds like the stator. had a ktm65sx that would do this. spark great, then stop, then spark again after a few kicks. i was told it would build up energy then when it was high enough release and create a spark. and the stator ohmed out within spec.

[atc007]

Deep breath,,back to basics. : ) It is weak ignition. Nothing else. You need WAY more spark under compression. If we're talking 250 ES,SX,,my moneys solid on stator. If we're talking 250R,stator,CDi or coil,in that order. Get an ohm meter and have at it! It is weak spark,nothing else.Don't mean to come off cocky,,just wanted to be straight to the point,no fluff! lol

[dougspcs]

Change the coil,compression need a lot more KV for the coil than the spark outside the engine.When the KV is too high for the coil,the coil simply shut down.Try to change the coil.
I had a similar problem with my xr80,always run fine.Except when I accelerate hard,fire simply cut,change the coil and the problem solved

Agreed..this is a phenomenon that has been seen in the past. Cracked plug ceramic as another meantioned also..forces the spark 'underground' under compression.

Double check your coil grounds and spark plug lead wire/boot also..

Electricity is lazy..it will take the path of least resistance to complete the circuit.