Running choppy at high RPM...

[reepicheep]

I've rejetted my 95 KDX-200 (with 225 big bore) and think the jetting is dialed in pretty well.

The problem I am trying to chase down now is that the thing runs choppy at high RPM. It's making lots of power, when its firing, but is running pretty rough.

I also noticed on my last ride that after powering through a big puddle, it got worse for a minute or so, then got better again.

So that's got me thinking bad connector or intermittent coil or something. Any suggestions before I start tearing things apart?

Thanks!

 

[sr5bidder]

you can try to clean and tighten the grounding piont for the coil right in front of the tank and inspect the wire really good from the coil to the plug cap...

I'm not sure these have rev limiters I always shift when I feel that I have exhuasted the current gear / useable power, and never really pinned it to see how much more there was.

 

[reepicheep]

I thought the way mine was is "just how a two stroke is", but I'm helping a buddy jet his 200, and while we still have him bogging a little WOT off idle, he pulls smooth through the power band. Like you say, you shift because it's not pulling as hard, not because it's breaking up.

Mine pulls pretty hard, but you shift because it's breaking up. It sounds and feels like the "skip spark" soft rev limiter on my Buell, but doesn't cut in abrubtly like the Buell one does, it's kinda always there but getting worse.

It's a really old and beat up spark plug wire... that sounds like a really good place to start. It'll be easier because I have another bike here and I can do the part swapping for just he price of labor...

 

[sr5bidder]

I have read your other jetting post and it looks like your at 42/160/3rd clip pos.
if its not post what you have now.

I would suggest you lower the neddle to the second groove from the top and see what effect that has, it is possible that your loading up in the middle I think you should try it out on your buddy's 200 as well
I was amazed at the difference it made threwout the entire range and the responsiveness...

I rarely ever go from idle to WOT in any riding situation.. best way to test is be in a gear higher (2nd or 3rd) so you don't flip the bike and make certain that the bike is at full temp otherwize your not really getting true findings

 

[reepicheep]

I got my new jets in, so I am not at 45/155/3rd clip.

I'm pretty sure its electrical, not jetting, but that doesn't mean my jetting can't get better. And the friends 200 (stock) does sound like it might benefit.

And you are right about idle to WOT. The only time I use it is when I am doing wheelies in the driveway to test my new jetting. ;)

 

[sr5bidder]

I got my new jets in, so I am not at 45/155/3rd clip.

I'm pretty sure its electrical, not jetting, but that doesn't mean my jetting can't get better. And the friends 200 (stock) does sound like it might benefit.

And you are right about idle to WOT. The only time I use it is when I am doing wheelies in the driveway to test my new jetting. ;)

testing ...Only after running it down the road to get it fully warmed up and cleared out....right.....

I need to run my kdx around a bit today to test my vapor computer out../.always evolving KDX

 

[reepicheep]

Didn't have much time to dig into it yet, but it appears my voltage regulator is baked (based on the ohm test in the service manual)... I wondered why my tail light was so bright.

Hopefully that didn't fry my CDI...

 

[sr5bidder]

CDI does not run in the regulated circiut...
Also did you use the "Special Tool" Hand Tester s7001-983 as results may vary. :laugh:

what is the output while the bike is running? (between ground and yellow at the headlight switch) measured in a/c

 

[reepicheep]

LOL. I caught that. I am currently measuring like .5 megaohms, same in both directions, so I suspect the regulator is fried.

Thanks for the information on the CDI, that helps. That'll help debug... I should keep the schematic out in front of me when I think about it.

I have two other KDX's in the garage right now (long story), so I can find out what the *real* measurements are supposed to be for a good VR and post them here. Measured with a good DMM, not s7001-983. :)

My headlight was removed by one of several possible previous owners... I'll have to go hunting the remaining circuitry.

I suspect the voltage is very high, as my taillight (that came with my replacement UFO fender, the only light now on the bike) is freaking bright. Probably too bright...

I may have time to look at it tonight, I'll post what I find for the archives.

 

[julien_d]

I would not suspect the VR to have any effect on the bikes running, good or bad. It's basically only there for the lights anyway...

 

[dirt bike dave]

IMO, you could be jetted a little too fat on the main jet.

Your jetting might be the best all around compromise, but you might be able to clean up the choppy running at high rpm with a slightly smaller main.

 

[reepicheep]

It's choppy with a main anywhere from a 150 (way lean) to a 160 (rich). 155 was the best pulling still choppy jet, which is right where Fredette and FMF said it would be.

I pulled the coil. The primary coil (the little wire) checked fine, the secondary coil (the big wire to the spark plug) read infinity. How the heck the thing even ran is beyond me. It must have been a diesel ;).

I even tried the secondary with the spark plug wire off. Still infinity between there and the top bolt attachment point... so it's not just the spark plug wire.

So off to scare up a new (err... used) coil. Sigh.

 

[reepicheep]

Wow. Double checked the coil again last night, and sure enough the secondary coil is infinite resistance. But the bike ran when I pulled it (but choppy at high RPM's). Go figure.

I have a used KDX coil on the way, but I went to the "great big pile of parts" and pulled out a coil from an 85 KLR-250. Unlike the KDX coil, it checks out fine.

I noticed that while the KLR coil is bigger, it has the same bolt pattern and would actually bolt right up. The electrical connectors are different, but could be easily adapted.

Out of curiosity, Are coils fussy about exact matches? Obviously, a coil that is too small will be too small, but as long as you are "big enough", would any coil work? If so, I may keep that KLR coil around as a spare, I won't get squat for it selling it on ebay...

 

[reepicheep]

Update.... New (used) coil that tested good is in... no difference in running. So the old coil had an open, but it must have been on the secondary and it must have been small enough that it jumps with no problem. Who knows how long it would last though... anyway, a good coil is in now.

I continued playing with the jetting, and was all the way down to a 152 main, and the choppy got less and less each time I dropped a size. Instead of going down to a 150, I found a new FMF Turbine core silencer on ebay, and bought that (I already had the gnarly expansion chamber). It went on tonight, and that seems to have cleared up almost all the rest of the chop. I didn't warm it all the way up to keep from bugging the neighbors.

Amazing the difference it makes, the thing does pure throttle wheelies in third gear now... it feels like my neighbors 450. Fun stuff for sure.

The other thing I did was fix my float height. They were *WAY* off. That explains why the bike drooled gas all over the place every time it was leaned over even a moderate amount. No idea if that changed the runnability as well, or if all the improvement was the FMF silencer.

I also adjusted the float level in my sons KX-60. But now it bogs at WOT (it didn't used to). It doesn't drool gas like it used to, but the bog has to go. I'm not sure if I kinked the fuel line, or over did it with the float height, or if fixing the float height got it "right" and threw the existing jetting out of whack. The thing starts first kick with no choke on an 18 degree day (literally), so I think it is REALLY rich. Ill pull it back apart and see what jet is in there, back off the floats a few mm (I probaly moved the float height 7mm just to get it in spec) and pick up a few leaner slow and main jets and tune it on the trail.

Anyway, when you have the carbs out, check your float height. Mine was *way* off, and fixing it made things much better (no more drooling, and running better).