Sudden rough running

[JZ05220r]

Been awhile since I made a post here. My 220 has been running excellent for the past year or so. Suddenly, it started to run rough. I I had just got fresh gas and cleaned and oiled the air filter. I’ve already tried different gas and a different filter in the problem still happens.

Checked the operation of the KIPS
Checked silencer packing
Tried a new spark plug
Borod cylinder. Great shape
Checked cleaned carb, jet block o ring is sealed

Still starts up in one to to two kicks, but it’s hesitating under light throttle and just overall running rough. The bike seemed off that day, but I had confirmed there was something definitely up while riding at high rpm WOT and feeling the sputtering/ or misfiring. When I got back and tried a different filter to verify whether I overoiled, it was sputtering worse at wot. The next day, after trying the other things in my list, it stopped doing It high rpms, but it was just overall running rough with a weak idle and lack of smoothness, crispness. So right now it’s intermittently missing everywhere under light throttle/acceleration and not performing up to par on harder acceleration. It has been running the best it ever had prior to this. I’m definitely not getting any air leak vibes or anything major mechanical. I’m wondering if this is some type of electrical issue I feel like I felt this before and I thought I fixed it by going down on the jets, but I wonder if that was just a coincidence if there’s some type of intermittent fault the bike shows from time to time. Another interesting thing I found is when I turned the headlight off the idle jumps up I turn it off. The bike idle is low enough to the point it wants to die. Typically the extra load on the engine with the headlight on doesn’t make a very noticeable difference as I’ve studied in the past. But like I said, it’s been going strong for a year now. I have about 40 hours. Any ideas? Thanks

[Chuck78]

I'm sorry to hear of your current running problems, but I'm sure you will get it figured out. After swapping in a good-known spark plug after testing it for resistance to see that it is around 5,000 k after scratching the surface to get a good continuity connection (I've been finding a lot of bad NGK spark plugs in the past 2 years, their quality has gone downhill severely), I'd ohm out the ignition stator source coil and the high tension ignition coil. The high tension spark plug coils don't readily seem to fail on these at all, that would be the last thing I would suspect but it is possible.

Swap CDI from another KDX if all else fails.

I'd be highly suspicious of the ignition system health, spark plug and stator coil would be the first suspects. The CDI boxes are aging electronic components that are only good for 20 or 30 years typically, the capacitors at least, so there may be an issue in that as well, the output transistor/capacitor? They're not easily serviceable, as you must dissolve the epoxy potting just to access the circuit board componentry as I have learned through Jaguar (Michael Forrest).

I hope you get this one solved quickly, best of luck

[JZ05220r]

Thanks Charlie, I definitely won’t stop until I have it figured out, that’s for sure. I’m just not savvy when it comes to electronics. Jeff Fredette said to replace the ignition coil. I will test the spark plug as you described. Do you know the solid replacement for the source coil? I know a longtime member on here Slick Nick rewound his at home and had excellent results.