New Carb Hesitation

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bmfouts65

Member
Jul 27, 2005
21
0
New Carb Hesitation - Updated

1992 RMX 250.

Was having serious issues with excessive fuel and plugs fouling out, so after multiple fixes, I bought a new carb. It's obviously not the same carb, but is Mikuni and is very similar to the old one. Hard to get it to fit. But, got it installed, kicked it over a few times and yeeha, it started. Idled great, throttle response good. Went to take off on a test run and it just cut out, then a couple of seconds later, it took off. Did the same in second.

So, it has a serious dead spot at low speed but will pick back up. Acts like it's almost bogging down. Either way, what is the adjustment that I need to make on the carb to address this issue?

Thanks!
 
Last edited:
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RACER557

Member
Mar 11, 2007
31
0
check float level and make sure it has the right needle and seat for your bike. good luck!!
 
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bmfouts65

Member
Jul 27, 2005
21
0
Does it matter if it's not the original carb or is aftermarket when it comes to jetting, needles, and seat?
 
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mox69

Member
Mar 26, 2007
236
0
Well besides the fact that you might need different needles/ jets than what you currently have, probably not.


I believe carbs are measured by the size of their output hole? Someone correct me if I'm wrong here.

As long as the old and new carb are the same size, the jetting will be very close.


I sounds to me like your float level is off as well. Not enough gas is sitting in the bowl and it runs out of gas for a second when you hit it.
 
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bmfouts65

Member
Jul 27, 2005
21
0
Yes, the intake/outtake are the same size. I will take a look at the float level and needle setting then. Also, will run the bike and check plug. May have to size up the jet as it has aftermarket pipes and may be running lean.
 
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bmfouts65

Member
Jul 27, 2005
21
0
I checked out the floats and they were correct. So, what may be the issue now? I adjusted the needle higher to increase the fuel at 1/4 throttle and it seemed to have a positive affect on the response, but it still has a dead spot if I give it full throttle while going slow. It doesn't cut out completely now, but just bogs down real bad then suddenly takes off. Any further advice?
 
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mox69

Member
Mar 26, 2007
236
0
Does the throttle respond the same way while in idle?

If you hit the throttle while in neutral will it misfire or rev up perfectly fine?
 
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bmfouts65

Member
Jul 27, 2005
21
0
No, while idling in neutral, the throttle responsds as it should, perfect. It's only while under load (in gear) that it bogs down/cuts out.
 
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bmfouts65

Member
Jul 27, 2005
21
0
Floats are stopping flow of fuel and compression is good. It's amazing how well the bike runs after initial bogdown, but I really need to adjust this. It's the 1/4 throttle that needs the adjustment. Starts and runs great otherwise.
 
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mandark1967

Member
Mar 12, 2007
246
0
bmfouts65 said:
Floats are stopping flow of fuel and compression is good. It's amazing how well the bike runs after initial bogdown, but I really need to adjust this. It's the 1/4 throttle that needs the adjustment. Starts and runs great otherwise.
Click to expand...

I replaced the PWK33mm on my 1998 KDX220 with a PWK35mm and ran into the same problems you did...here are some of the things I researched and changed to correct my problems.

I am assuming you've cleaned your carb very well, and have made sure your float levels are adjusted correctly..

According to the graph at the just KDX site:

It looks like there could be 3 different causes for your bogging that are related to jetting.

Throttle Valve cutaway (the actual slide that goes up and down)

Needle Clip Position

Needle Itself (diameter as well as taper angle)

I'm assuming you've tried adjusting the clip to different locations on the needle so that eliminates one of the possibilities.

What's the needle size you're currently running? Remove it, look up the part number, and get the next leaner one with the same taper, then put your clip in the middle position and work from there.

If that doesn't correct the bogging, you should get the next leaner needle (as compared to your original, not the replacement I just mentioned above) that has a slightly larger diameter. Install with clip in middle position and adjust from there.

If neither of those work, you may actually need to go to a different throttle valve cutaway.

I went through this whole process myself recently (March 2007) and while it was very frustrating at first, I've learned a great deal thanks to the people here and the

http://www.justkdx.dirtrider.net

site. Although you do not have a KDX the sections on carb tuning, float bowl adjustment, and needle valves all contain great information that can help you solve your problem.
 
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bmfouts65

Member
Jul 27, 2005
21
0
Not a worn motor as I rode it prior to installing this carb and it ran fine except for excessive fuel was fouling the plugs. Carb was shot, replaced with current, but need to fine tune the new carb. Just have to get the parts...
 
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oilspot

Member
Sep 11, 2006
156
0
Are you sure that your not sucking oil through the crankshaft seal? What ratio premix are you using? Do you ride fairly aggressively or do you let the bike idle fairly often?
I've got a 92' rmx also. When I got it it was running really rich and had the wrong plug in it. I've dropped the main two sizes, and I'm running the correct plug now.
The bike was fouling the plug within 45mins when I got the bike. Now I'm not having any problems with fouling at all.
 
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