Mikuni pumper carb symtpom

Boit

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#1
My Stroker built KLX has the Mikuni 33mm pumper cabr and up until now, this thing has been perfect. Suddenly, without warning, the engine keeps a high idle when the throttle is chopped. Sometimes, the idle is quite high, other times it's much lower, and even othet times it's normal. Does this sound like the slide bore is worn and the slide is getting cocked to the side and not seating squarely at the bottom of the bore? I've searched for air leaks and have none. While in neutral, I can blip the throttle several times and it might go to normal idle 50% of the time. The rest is all over the place. If it idles high, I blip the throttle, and it returns to normal.
 

penguin

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#2
have you checked your cable and throttle tube for binding? Are you running handgurds could the throttle be rubbing on the handguard? Unless you have extremely high mileage on your KLX, the slide being that worn is doubtful. The all of a sudden appearance of the problem leads me to believe that it is something in the cable, throttle housing or linkage, not the carb itself. I have had 3 KLX's, with every carburetor made for the bike, and they have all been stone reliable. I have had problems with the cable, throttle and linkage binding.
 

Boit

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#3
The cable was fraying so I thought that this was the basis of my problem. I installed a new Motion Pro on it, lubed it, and carefully routed. It doesn't appear to bind as I've checked with the bars turned fully in both directions....but, I'll check it more carefully. I inspected and used graphite to lube the throttle tube when I changed the cable. When I twist the throttle and release it, it snaps closed quickly and I can hear the slide bottoming out. Of course, on the track, I can't release the throttle like that. I DID remove the slide and very lightly greased the bore which helped for about a half hour before the idle started acting up again. This is why I suspect wear in the bore/slide. I use the bike for MX so I have no handguards. Rocks and stones are not a problem in this area.
 
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#4
Had the exact same problem with my old XR 250. And yes, it probably is the slide, or more accurately, it's probably the carb housing. There's only one fix. You know what it is,so I won't be the one to speak the bad news. Sorry 'bout that. Bummer, dude.

Slow Jack
 

Boit

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#5
Jack. I know what you are hinting at. However, I spoke with someone at an aftermarket shop and he claims he can plate the bore so that the wear will be non-existent once he's done. I'm a bit leery but this might be a viable alternative to replacing the carb. I've been doing a little research and I have found evidence that some Mikuni pumper carbs DO experience unusually high wear in the slide bore. This is a very fun machine to ride and I want to keep it running in top shape for a few more years at least. I've got to do some top-end work soon so I'll tackle this problem at that time. Thanks for the info and ideas.
 
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#6
I would just check your slide for wear, If the slide has a little wear, or a grove worn into it, just replace the slide,and also check the needle. I have seen the clip grove wear out. The needle may rock a little and end up hanging on the side of the needle jet.. More than likely the slide has notch or rough spot on it were it drags on the body, combine this with engine vacum and it will stick. Also double check your fuel screw and make sure that it didnt back out.
I may have an extra slide and needle for that carb. Let me know if you want to try it.
Cya
 

scar tissue

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#8
Boit,

Definately sounds like the slide is worn. It's kinda common. As an FYI White Bros. carries it as does Stroker. As long as you know the part #. You can call and compare pricing.
 
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#10
webbrace;
If boit doesn't want that slide let me know how much and where to send it 2. I have the same problem.
 

Boit

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#11
Webbrace: Although you offered me that slide first, it's OK if you want to go ahead and send it to Captdirt. I have some downtime and he may be able to use it now.

Captdirt: When this problem surfaced with you, did it happen suddenly and worsen steadily from that point on? I inspected the slide last Saturday and can easily see the wear. It's pretty clear now how Webbrace is correct about the roughness causing the slide to hang up when the vacuum is high. That's probably why Kehin uses bearing(rollers) for the FCR slide.
 
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#12
Boit,

You have a slide that is worn. Mikuni puts a very fine layer of
teflon on the slide and given the clearance in the slide bore the
slides wear quickly. You can get a new one from Sudco, I just
got my replacement, I believe it was ~ $28 to my door.

http://www.sudco.com/

I made a slight modification to my existing 33 Mikuni, that puts
more preload on the spring, which keeps the slide from hanging
up. I am going to hold off replacing my slide, until this mod no
longer works. I posted a write up on Thumper Talk iin the KLX
Forum, in case it may be of interest.

http://www.thumpertalk.com/ubb/Forum27/HTML/000004.html

BTW this is fun carburetor to disassemble, you might want to make
notes, use a digital camera, or a camcorder and watch out for that
pumper cylinder and spring. They both like to fly out, real easy.

Here is a URL with a diagram of the 33mm Mikuni Pumper Carb;

http://carbparts.com/mikuni/tm_expl_view.gif

Hope this is of some help MC !!!
 

Boit

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#13
Thanks for the info Michael. I have one of the first Mikuni's offered for the KLX that has the venturi bored by an aftermarket shop. They ruined it. This carb would be an excellent example of a shop that should NOT be doing this type of mod, plus what the consumer should be aware of. I was naive and assumed that this shop knew what they were doing. Oh well, that was years ago and this is now. I was just a few minutes ago inspecting the slide in the ruined carb and it shows very visible wear so it's probably not a good candidate as a replacement for my present carb. That leaves me with an option or two. If I stick with the Mikuni, this slide problem will resurface in the future. Another option is to fit an FCR carb. I have been extremely pleased with the low-end response that the Mikuni provided so I might have to sacrifice that asset for more high-end with the FCR. I prefer the Mikuni style of power because it complements the 6 speed tranny. I spent a lot of time dialing in this setup so it's pretty frustrating to encounter this problem. By the way, Yes, I'm very familiar with how the accelerator pump works. The plunger seized inside of the float bowl circuit on the present carb so I cannibalized from my bad one. The lesson here is to never leave fuel in a carb for extended periods of time. I should have known.
 
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#14
Hi Boit, kinda curious as to who did the mod? I am only aware that
Cycle Gear used to bore out the 33mm to a 35mm and they had
problems with that mod. I had always had considered doing the
Pro Action swirl cut in the venturi.

Thx MC !!!
 
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#15
Good morning. Upon further inspection I have found that the carb body along with the slide is scored so I guess I will have to replace the whole thing. It was a used carb I picked up from a DR350 so I can always use it as a parts carb. Guess I'll buy one from Sudco since I already did all the other mods and have the right jets etc. no sense paying the extra $120 to Stroker for stuff I already did.