My DX falls on its face at 1/4 throttle

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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 11:08 pm Aug 28 2013

Hello all, Im new round here. I just bought an 86 KDX 200 and am fixin er up to do some trail ridin with. Biggest problem Im having right now is that it bogs badly at anything above about 1/4 throttle. It also will not run well at all unless the choke is on. It starts well (1 st kick even afte rsitting for several days), and idles ok, and when Im light on the throttle it seems to be good. I pulled the carb and cleaned it well, but I get no fuel out of the float bowl when I pull the plug out of the bottom. if I pull the float bowl off there is fuel in it. The petcock seems to be flowing enough fuel but it wont shut off flow much in the off position. I also changed the plug and plug wire with no change. The loss of power is very abrupt, which makes me think its probably not fuel related. As soon as I back off the throttle to the right spot she picks right back up again. Any ideas out there?
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Re: My DX falls on its face at 1/4 throttle

Postby Julien D » 06:17 am Aug 29 2013

It's either going lean or flooding out. Probably flooding out. Does your bike have a PWK carb?
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Re: My DX falls on its face at 1/4 throttle

Postby bufftester » 12:24 pm Aug 29 2013

You're going to hear it over and over and over on this site when it comes to the KDX...pull the carb, tear it apart, clean it.....then clean it again. Reassemble, jet, and ride like a banshee.
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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 08:26 am Aug 30 2013

I don't know what a PWK carb is. Its a Mikuni of some sort, beyond that im not too sure what it is. Ive had the carb apart and cleaned er up a couple times now with no real changes. could a cracked reed boot cause it to run like this? mine has some weather checking.
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Re: My DX falls on its face at 1/4 throttle

Postby 4x4fab » 08:58 am Aug 30 2013

Ford Junky wrote:I don't know what a PWK carb is. Its a Mikuni of some sort, beyond that im not too sure what it is. Ive had the carb apart and cleaned er up a couple times now with no real changes. could a cracked reed boot cause it to run like this? mine has some weather checking.

Yes! It could be that anything over 1/4 throttle is enough vacuum to draw air through a crack and cause a very lean condition. The reason the choke helps it run is because with the choke on it is making it run richer.
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Re: My DX falls on its face at 1/4 throttle

Postby Roadhazardguy » 09:12 am Aug 30 2013

Pull the Carb off again and check the float level, put a piece of line on the fuel inlet to the Carb and blow through it, adjust the tab until the line on the float is parallel with the bottom edge of the carb body (at that point sealing off any airflow). Try that first and see what happens.


It will do a lot of surging and not want to idle down if the boot is cracked.
Tony
1988 KDX 200
1980 KD 80
1989 ATK 406
1984 RM 500
1980 KZ 1300
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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 11:03 pm Aug 30 2013

I pulled the carb apart again this morning and on closer lookin found the needle jet is damaged. The one in there is a 521, is that the right size? My local dealer doesn't think it is, but not sure. I haven't been able to source another one yet. What would be a good starting point for main jet? I would be riding mostly between 3 and 4000 feet.
The bike seems to idle clean and low rpm seems crisp, but as mentioned, lean.
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Re: My DX falls on its face at 1/4 throttle

Postby Julien D » 08:31 am Aug 31 2013

I will have to check on the needle jet. 42-45/150-152 is probably a good safe starting point at that altitude. There are no shortcuts in jetting your bike though. Taking recommendations on jets is like asking someone to blindly tell you what size shoes you need. :)

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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 09:16 am Aug 31 2013

I realize I'll havta set it up, I'm just looking for a rough n dirty startin point, and thanks for that. Im pretty sure that someone has been into this carb that probably shouldn't have been so who knows how far off the jetting is.
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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 11:13 am Aug 31 2013

And no, I don't have a PWK, its a Mikuni VM20.
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Re: My DX falls on its face at 1/4 throttle

Postby Roadhazardguy » 11:19 am Aug 31 2013

Yeah 88 was the first year for the PWK
Tony
1988 KDX 200
1980 KD 80
1989 ATK 406
1984 RM 500
1980 KZ 1300
2008 SM450R
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Re: My DX falls on its face at 1/4 throttle

Postby Goofaroo » 12:29 pm Aug 31 2013

I have an 87 model which I assume has the same Mikuni as your 86 model. I live at about 1150 ft. and stock jetting is perfect and it has seamless power delivery. You should be able to get it running pretty damned good at the factory settings and then maybe make a minor jetting change from there.

If your intake boot and airbox boot are cracked (as mine were), they can be restored using aerosol Plasti-Dip. Just do a search on the forum and you will find some info but I just scuffed my boots with a wire brush and sprayed several coats of Plasti-Dip on them and they are good as new.

How is your needle damaged? Is it actually broken or is it just bent? Have you COMPLETELY disassembled the carb and thoroughly cleaned every part and every passage? Does your float bowl drain work now? Is the float valve working? Do your floats float? The carb on your bike is a ridiculously simple design and it sounds like you're just missing something when you're cleaning it. Here's a pic of mine disassembled if that helps at all.
209

As for the petcock not sealing in the closed position, you just need to get online or stop at your dealer and get new rubber and rebuild it.
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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 08:40 am Sep 05 2013

Can anyone tell me what stock jetting is for this carb on this bike? I need to find a new needle jet ( not the needle itself, but the part it slides into) but it seems the one that is wrecked was the wrong size any way. Im hopin to find stock sizing for needle and needle jet, as well as main and pilot jet. I've done some searchin but come up empty handed so far. If I can git this thing back to stock I should be in the ball park.
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Re: My DX falls on its face at 1/4 throttle

Postby Roadhazardguy » 11:11 am Sep 05 2013

That's your main jet, depending on where you live and if you have any mods to the bike like exhaust it can be anywhere from 300-340. http://www.partzilla.com/parts/search/K ... parts.html
Tony
1988 KDX 200
1980 KD 80
1989 ATK 406
1984 RM 500
1980 KZ 1300
2008 SM450R
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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 12:14 pm Sep 05 2013

The main jet threads into the needle jet. My bike is stock.
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Re: My DX falls on its face at 1/4 throttle

Postby Ford Junky » 12:15 pm Sep 05 2013

Thanks fer the link though. Preciate it.
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Re: My DX falls on its face at 1/4 throttle

Postby Roadhazardguy » 12:20 pm Sep 05 2013

The needle is your metering rod, its not a jet.
Tony
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1980 KZ 1300
2008 SM450R
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My DX falls on its face at 1/4 throttle

Postby Ford Junky » 12:47 pm Sep 05 2013

Part #16017 /A-D on the diagram you linked to. Its what the needle slides into. The needle and the needle jet work in conjunction to di the metering. Main jet threads in the bottom of the needle jet to meter fuel when the needle is pulled out of the needle jet as far as it will go (WOT).
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Re: My DX falls on its face at 1/4 throttle

Postby bufftester » 03:22 pm Sep 05 2013

Likely if the needle jet was damaged then you should replace the needle as well to be safe. On road bikes the needle jet is often also called an emulsion tube. On the Keheins Pilot/air screw takes care of everything below 1/8 throttle, needle/needle jet+main+pilot take care of 1/8-3/4 throttle (with the needle being the greatest contributor...thats why you see so many different lengths/profiles/clip positions), and 3/4 to WOT is primarily handled by main jet (though the needle and pilot are also in use), Mikunis are similar - this link has some good info, http://www.carbparts.com/mikuni/mikuni_tuning_info.htm
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Re: My DX falls on its face at 1/4 throttle

Postby Ford Junky » 08:31 am Sep 06 2013

It looks like the needle jet was damaged by someone trying to turn it out of the carb body instead of pushing it out. If the needle isnt stock I will change it out for a stock one, just gotta try n find what stock sizing is for all this stuff. If I can even find a setup that's working well that I can use as a starting point. From what I can tell, my bike is stock, the only factor I can see that may effect my jetting is the silencer seems to have lost most of the packing. That n the elevation here is around 3000 feet.
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