what am I doing wrong 74 kx250

[211]

Help!
I have a 74 KX250, ground up resto and I just cant get a decent spark. While the bike runs it runs like crap and I think its due to really weak spark.
So far I have swapped out all electrical from a bike that runs well - including the carby just to be sure. Problem remains
Could it be that the rotor has de-magnetised? Its the only bit that I haven't changed out. Spent half a day yesterday trying to figure it out - any pearls of wisdom out there - what have I missed?

DT

[bazza]

Dont you love these problems Dave?
Kill switch fault?
Clean earth on frame?
Try another plug?

[Slakewell]

Run a extra earth wire from the stator to the one on the coil. Maybe an air leak, Get a can of start you barstard , get the bike running and spray around the engine, if the revs pick up you have found your air leak.

[bigk]

It's not an electrical issue. Buy a new carby (cheap & easy to source), throw the old carby at someone you don't like.
K

[211]

I had thought about an air leak but as I cant get a good spark in the first place I have sort of parked that idea. MH I took the carb from my 76 that is working fine problem followed.
Yes to a good earth, no kill switch involvement and multiple plugs - thanks Baz
will add another earth just to see

[OverTheHill]

If it starts & runs [i think] the problem lies elsewhere. CDI spark never looks 'that healthy' but would make ya hair stand on end--or fall out. No chance of something 'accidentally' left under the seat restricting air cleaner air flow. Sounds like restricted exhaust as another option, or main jet fell out or slide needle still sitting on the bench. Sorry if that sounds a bit off but more or less just letting the cat out of the bag to all the cock ups i've made over the years lol. & just when i think i've covered the field of cock ups i go & 'discover' a new one. If it's definitely electrically missing its bum off then ok--i give in & ignore all my rambling. Just tend to think if it starts--it's ok--unless they have a high speed winding separate from the initial source coil but think you said swapped stators so that ideas out the window. Cheers [or tears]. ps, i road one of those for a few meets back when new & was a very enjoyable time too--well till a rear brake shoe spring broke & locked the wheel miles from the finish of a long distance race. Very willing motor.

[John Orchard]

l had a problem one day with a Suz RGV250 that had a weak spark and would not run clean, read somewhere that lifting the plug-cap off to about a 10mm gap resulted in an increase in voltage due the increased resistance.  Yep sure did, ran clean, put the caps back down and the bike started blubbering.

l guess these findings would prove an exciter coil not creating enough power? Maybe from a weak magnet?

[211]

John - that's were I'm at, 3 different coils and stators, couple of CDI just in case and no improvement. need to get a puller now and try that I guess - I do have a spare
appreciate the help one and all
DT

[Mick D]

Ok Dave, a weak spark will certainly make for difficult starting and poor running.  Shows up when opening the throttle, especially under load.

You have swapped out a lot, except for magnetic rotor?

These days I test the magnetism of every rotor that pass through my hands.
I have a jar of m6 nuts.
I lightly hand engrave the result of how many on each tested rotor(then highlight with paint marker or texta). All other info on rotors is factory stamped so there is no mix up with a hand engraved figure.

I have noticed massive variations of magnetism on identical to look at OLD ROTORS.

It is no problem if you don't have a puller yet and want to compare or confirm your suspicions.
You can see how many nails or whatever while it is still mounted on a steel crank end.
I have noticed little to no difference whether testing its capacity while mounted on a crank or removed.

I also include an earth wire between all ignition components when I make harnesses.

Hope this helps a bit?
Have fun   

[pokey]

Sounds like a lot of guess work going on and thats okay some of the time but when it doesnt work its time to make sure Break out the multimeter and test ALL compoents and make sure they are within spec.. Cpil cdi, primery and exciter...doesnt take long.
 Do a leak down test for the bottom end seals .
 pull the carb and and doable check your float and jets are correct.

 also check your plug cap. make sure its the right type for your bike as some dont like resister caps.



 as mentioned, its a new build so earths may be not as good as they should be with paint pr PC. Make sure you have a decent earth or even run an extra wire .
 

[LWC82PE]

Weak flywheel magnets could definitely be an issue.

[211]

Thanks all - great tips.
going to pull the rotor this weekend and see if that makes the difference
when the thing runs it runs like crap, I thought about the main seals and still consider this as a possibility but usually the bike will be at least rideable down low and breakdown up top - this thing is crap across the range.
I think that once I can get a decent spark I have a base to work from, comparing my 76 and 78 KX250s the difference is obvious, big blue sparks compared to almost nothing from the 74. The only thing I haven't changed is the rotor!
Full report next week
Thanks again
DT

[John Orchard]

Can flywheel rotors be re magnified?

[Mick D]

Can flywheel rotors be re magnified?

Yes

[matcho mick]

takes a pretty big magnifier to do rotors,usually glass ones  ,