KDX Runs Poorly at WOT at Low Engine Speeds

[RDHKDX]

I have a 1992 KDX 200. The bike runs great at WOT at high engine speeds. Thus I assume that the main jet size is very close to perfect (the spark plug confirms this.) However, the bike runs poorly at WOT at low engine speeds, when vacuum is low. It used to run fine, then somewhat suddenly started pooping out, which points to something suddenly happening.

It runs poorly as soon as the bike is started which indicates it isn't necessarily a heat related problem. The problem might indicate a radical error in fuel/air ratio under these conditions: air leak, bad reeds, float too high or too low. Reeds would seem to be ruled out since I replaced them recently with a new set from the dealer.

I recently disassembled the carb and soaked it overnight. The next day I cleaned it with new gasoline and used carb cleaner to make sure all of the orfices/lines weren't plugged. I also checked and adjusted the float level slightly.

I am running non-stock expansion chamber, silencer, air box(top) and reed valve.

In summary the tune-up items that I recently performed are the following:
new reeds
carb soaked and cleaned
new plug
repacked the silencer pipe
cleaned the air cleaner

Suggestions?

 

[jaguar]

suggestion: don't run it WOT at low engine speeds.
why would you want to do that anyway? Even on a MX track I am never Wide Open Throttle except at top rpm's.

 

[wibby]

Just wondering how do you do WOT at low engine speeds? :think:

Seems to me if you have it WOT, the motor is turning pretty darn fast! :eek:


Well, now that I've thought it over, I guess you could be WOT in too tall of a gear, thus turning very few rpm's. I dunno... think I need another beer!

 

[john_bilbrey]

Maybe you tweaked something on the reeds during install and they are failing now?

 

[RDHKDX]

I need to explain what I mean when I say WOT at low engine speeds.
I meant to describe it as more of a bogging. For example, if I'm climbing a hill and the rpms are relatively low and I give it more gas, the bike will not respond (this is even the case when I'm in 1st gear). It feels like the carb is either getting too much fuel or too little. This situation has gotten worse and can even be experienced on relatively flat ground if the rpms are low and gas is cranked hard. If the rpms are high and the throttle is cranked, the bike will respond.
As far as the reeds are concerned I replaced them recently hoping that they would help cure the problem.
A "friend" last night suggested that I look at my power valve. He suggested that it might be opening too early. I will try to disable the power valve temporarily and see if that changes the symptons.

 

[jaguar]

OK I understand now.
you need to go leaner on the main jet.
I am always diddling with my engine and changing jetting and that is what I experience when I go to rich on the main jet.

 

[levert]

If the bike was running fine and then all of a sudden developed the problem it probably isn't jetting. Usually jetting changes are a result of air temp, pressure or a change of your intake exhaust system or possibly a change in your fuel oil ratio.Unless you've changed one of these I'd go elsewhere.
Does your idle wander?
Youv'e checked for air leaks?
Checked your tranny oil, if the main seals are leaking your oil will get discoloured real fast, usually black?
If the KIPS isn't funtioning its usually the top end that suffers.
When is the last time you've replaced the piston and rings?

After saying all this the problem your having sounds like rich jetting, but since it ran before it is confusing its not working now.
If you have a fresh top end, the Kips is clean, clean filter, good spark, fresh gas start at basic carb settings and work from there.
Check the boot from the carb to the reed block and the boot from the carb to the airbox for cracks.

Very interested to know what the problem is.

 

[RDHKDX]

Thanks for the interesting points in the reply from Levert.

I will try to verify PV operation at this point. I think the Halmhotz Resonator chamber has an external cover (left side?) that could be removed and checked for blockage (which would render it useless) and might provide a view into the valve to check for sticking. There should be a hole in the PV from the HR into the main exhaust port at anything below 6000 RPM (including engine off.) Even if it is stuck in this position, I would think that the bottom power would be okay.

I will check for air leaks by getting a steady idle and spraying a fluid around the manifold joints.

I'm not sure of a good way to check main seals, but I would think you would get blow-by through the trans vent and/or disappearing tranny fluid if there was a leak here.

I'm not sure of a good way to check spark, but that would be something to try - maybe only visually.

 

[fuzzy]

Those pointing to jetting are correct, but going the wrong way. I'm going to take a wild guess and say that it was warmer out when the bike ran well. Your pilot is now too lean. WOT creating the bog = more air and not enough fuel. Lean bogs are very bad. Try adjusting your air screw IN until it's almost shut and see if it helps. I'd guess it will help, but not cure it meaning yuo'll need a richer pilot(and probably a leaner main for when the temp warms up). I've got a jetting chart for every 10-15 deg temp change for both of my bikes. It really only takes a handfull of brass to be 'close' most of the time. I could easliy get away with a winter, and summer jet-set but I'm anal, and like my bikes to wheelie over logs without using the clutch.

FWIW, I run WOT at low speeds a lot. I commonly run a gear higher then 'normal' when on my WR as it can get out of hand very easily, but I've got it setup for low-mid and I just lug through everything--often climbing hills easliy while chugging up them in 3rd, under 10mph, WOT, just cause the bike will. Just keep two fingers on the clutch!

 

[levert]

Fuzzy could be right as well.
It would be good just to ensure the motor is all spec and proceed with the jetting.
I too run my bike a gear higher.I have a KTM 300 with a Steahly flywheel weight.I have come to realize reluctanctly my skill level with the throttle sucks, running a gear higher I don't loose control.
Even after 27 years of riding I still get a rush watching the experts wizz by me in total control doing things I never thought possible.
Now if I could have one day , just one day with that sort of skill level!!!

 

[fuzzy]

I get em from time to time as I used to be fast.....Now one little wash, etc and I'm a wuss the rest of the day---This keeps getting better, but I spent 12 years w/o riding---WORST mistake of my life!!