88 YZ250 Weak Spark - Need Ideas

[craigeyz250]

I have an 88 YZ250 and it has a weak spark. She's almost impossible to kick start, and I have to clutch start it each time, usually by being towed. Once running, she goes great, runs a little lean but has plenty of power and plenty of speed, however she doesn't want to idle. Looking at the plug, it has a really weak spark and I have to kick the engine pretty fast to produce one. Even if I remove the plug cap the spark is still weak, only jumping about 1-2mm. I have changed the spark plug but it didn't make a difference. The spark plug cap has 5k resistance, so it's not that. I have checked the resistance in the stator coil and the ignition coil, and they all have a reading of under 10% above whats in the manual, a little hight but not enough to cause concern, and no closed or open circuits. All wiring is good and connections are clean. Today I pulled apart the carbie and cleaned her out but it didn't make a difference (was hoping that maybe a blocked jet could be the problem). The only thing left is the CDI unit. Is there a way to test these, or should I take the plunge and buy a new one and hope for the best? Also what about timing? The bike runs great when runing so I'm guessing the timing is OK. I'm pretty much out of things to test and out of ideas, apart from throwing money at new parts and hoping for the best. Any suggestions would be great. Thanks in advance.

Oh also (slightly off topic) I've been told to run the bike at a 40:1 mix, but the manual says 25:1. What do you guys think???

 

[bruno670]

Most likely the problem is a weak source coil. I think that Ricky Stator in So. Cal makes a replacement. As far as mix. The oil mix is what ever you want to run it at. Just make sure the jetting is set for the mix that you want to run. If your plug is a chocolate brown then run it at that mix. Just remember that the lean /rich ratio is gas to air not gas and oil only so much fuel mixture will go through the jet. The more oil means less gas and the air is a constant. So adding more oil means less gas and you will run it lean. Possibly burning a hole in your piston.

 

[chicoman]

I second that, I've never seen a bad CDI box but I've seen lots of bad Stator windings which test fine with an ohm-meter.