1999 kdx stumbles at lower revs?

Robinkdx · Aug 3, 2012

Hi guys i am new here and i hope you can help me with this problem i am experiencing with my bike. What happens is that when ever i play around at lower revs it stumbles as if it doesnt fuel right or something. At full throttle then there is nothing wrong with it and goes like a bat out of hell. Also what happens is when i play a lot at half throttle for a long period of time the engine cuts off then i wont get it going again. Any suggestions will appreciated. Thanks in advance.

dirt bike dave · Aug 4, 2012

If your bike is 100% stock KDX, a smaller pilot jet, larger diameter needle and a new spark plug (perhaps one step hotter) will probably cure your all of those problems.

But at least change your spark plug for a new one.

EYY · Aug 5, 2012

I agree on the jetting. You might even find that the pilot jet just needs cleaning out which may help a lot.

Robinkdx · Aug 5, 2012

Thanks for the replies guys. Jetting is good and clean. I bought the bike about 6 months ago and it was running perfect. I did change to a hotter spark plug (BPR8ES) about a month ago and it did improve allot but i still experiencing the same thing. At least it doesnt die that much as what it did before. The exhaust is a Pro Circuit. What i also did was i cleaned the power valves to see if there is any improvement but nothing. Something tells me its the needle and seat bypassing while i drive causing a over reach fueling. The reason i say that is if i dont close the stop **** then engine will flood with fuel sometimes. Anyone know a cheap place or affordable place where i can order carburetor spares from? My local bike shop charges me R350($41.60) for a new float valve. I would also like to get the needle and new jets and the float bowl. Or if anybody know of someone that want to get rid of his old carb? I would gladly like to buy it from you.

dirt bike dave · Aug 5, 2012

If there is a piece of dirt keeping the float valve from seating, it will do as you say with the overflow and flooding.

Remove the carb, take the float bowl off and give the bottom side of the carb a thorough cleaning. If it is just a piece of grit in there, that will usually help.

Robinkdx · Aug 5, 2012

dirt bike dave said:
If there is a piece of dirt keeping the float valve from seating, it will do as you say with the overflow and flooding.

Remove the carb, take the float bowl off and give the bottom side of the carb a thorough cleaning. If it is just a piece of grit in there, that will usually help.

No dirt, i think it is worn out a bit. I opened it up a lot of times and dont find any dirt in the carb. I also fitted a new petrol filter.

EYY · Aug 5, 2012

I thought b8es was the right plug? I might be wrong though.. I'd suggest checking the float height and cleaning the carb one more time with compressed air before buying new parts of it was running fine 6 months ago. A clogged silencer will also make for poor running at low rpm, it's happened to me before.

Robinkdx · Aug 6, 2012

EYY said:
I thought b8es was the right plug? I might be wrong though.. I'd suggest checking the float height and cleaning the carb one more time with compressed air before buying new parts of it was running fine 6 months ago. A clogged silencer will also make for poor running at low rpm, it's happened to me before.

Thanks for the feedback. The silencer is still good i checked that not too long ago. The exhaust system is also fairly new.(pro circuit). It had a b9es plug in before but now its been replaced with a bpr8es. I will change the float valve and see if there is any improvement. Will keep you guys updated once i have replaced the float valve.

EYY · Aug 6, 2012

I'd get the right plug in there, a b8es. It might not hurt anything, but it's better to be safe than sorry. The bpr8es will be longer due to its protruding core, and sits further into the combustion chamber which may cause clearance issues.

Did you put the pipe on it after you got it?

Robinkdx · Aug 8, 2012

The old one i took out and the new one is the same length. All "ES" plugs are the same length. I am going to fit the new float valve and see what it does.

EYY · Aug 8, 2012

The bp8es is definitely longer. The thread is the same length, but the core is projected.

Robinkdx · Aug 9, 2012

EYY said:
The bp8es is definitely longer. The thread is the same length, but the core is projected.

Thanks, i will look into that.

Robinkdx · Aug 9, 2012

Update. OK i stripped the carb today and checked the jet sizes etc. The main=155 and the pilot=48. At what setting can i put the needle? Currently the circlip sits on 4th groove from above. Another thing that bothers me is that the overflow pipe in the bowl was broken off and that the bike shop blocked it off. Could that interfere with the fueling?

EYY · Aug 9, 2012

So the pipe inside the bowl has been broken off, not on the outside? If that's the case id be getting another bowl. The fuel needs somewhere to go if too much gets in due to a leaky needle and seat or incorrectly set float height in which case the bike is likely to flood

Robinkdx · Aug 10, 2012

OK i put the carb back today and i also adjusted the needle height to the second groove. The bike was pulling much better and much more responsive. What i also picked up is when i forgot to open the petcock and just before the bike run out of fuel then it goes really well. When i took off the front wheel lifted in 1st and 2nd gear without me even trying. I also adjusted the float level a little lower. What is the correct level BTW?

dirt bike dave · Aug 10, 2012

Robinkdx said:
What i also picked up is when i forgot to open the petcock and just before the bike run out of fuel then it goes really well.

This is an indication your jetting is still too rich.

Not sure on the float level, but IMO you should put in a smaller pilot and put your needle clip in the top groove.

Are you adjusting your air screw? This is the little brass screw on the lower rear, left side of the carb body, just above the float bowl. See how many turns out it is, by turning it all the way in. Turn it out two full turns and give it a test. As a general rule, if the bike runs best with it two or more turns out, you should put in a smaller pilot jet. If the air screw is already turned all the way in, this will cause the bike to run richer on the pilot.

Robinkdx · Aug 10, 2012

dirt bike dave said:
This is an indication your jetting is still too rich.

Not sure on the float level, but IMO you should put in a smaller pilot and put your needle clip in the top groove.

Are you adjusting your air screw? This is the little brass screw on the lower rear, left side of the carb body, just above the float bowl. See how many turns out it is, by turning it all the way in. Turn it out two full turns and give it a test. As a general rule, if the bike runs best with it two or more turns out, you should put in a smaller pilot jet. If the air screw is already turned all the way in, this will cause the bike to run richer on the pilot.

Thanks Dave. The pilot jet is 48 and the main is 155. The air screw is turned out by one and a half turns. What size jetting do you advice?

dirt bike dave · Aug 10, 2012

Personally, I would focus on the low speed jetting and leave the main alone for now.

Jeff Fredette says 45 or 48 on the pilot. He's in Illinois, low elevation. If you are at elevation, I'd order a 45 and a 42 and play around with them.

http://www.frpoffroad.com/aboutus.htm

His article does not mention the needle, but a fatter needle (blocks more of the jet so the bike runs leaner) might also help.

Go 2.5 turns out with your 48 pilot in there, and see if it runs better than it does at 1.5 turns out. If so, put in a 45.

Keep in mind weather and elevation make a difference, too. Hot, humid weather, low barometric pressure or high elevation = less oxygen to burn, so optimum jetting would be leaner than what you might run in winter.

Besides making sure the float level is correct, there are a few other things to rule out. For example, an over-oiled air filter will also cause a bike to run too rich.

Never had a '99, but every KDX I've ever had (3 of them) was jetted way too rich for my riding conditions (4,000 - 9,000 feet). The 250 was the worst - it needed a hotter plug, new slide, needle, pilot and main to get the most out of it. I had a 38 pilot in that one.

1999 kdx stumbles at lower revs?

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