NX250 running issues - cuts out at ~70% throttle and unstable idle

[ze clasen]

Hi everyone,

I'm trying to diagnose some running problems on my NX250 and would really appreciate some advice before I start buying parts or taking the carb apart.
According to the Honda VIN/model information, my bike is a **1993 NX250P** (MD21).

The main problems I'm having are:

- At around **70% throttle and above**, the bike stops accelerating properly. If I open the throttle further, it either **doesn't go any faster or starts cutting power**. It feels almost as if the engine isn't able to get the fuel/air mixture it needs.
- The **idle is unstable**. The revs go up and down rather than staying steady and sometimes after a few time it shuts down. (see video)
- When I first got the bike, the idle problem was noticeably worse. After riding it for a while and adjusting the idle screw, it seemed to improve somewhat, although it is still not completely stable.
- The bike otherwise starts and runs, but the high-throttle problem is quite noticeable.

I'm guessing its something related to an air leak but I'm realy not sure.

I'm considering removing the carb, cleaning it thoroughly and replacing the relevant gaskets/O-rings. Before I do that, I'd like to make sure I'm heading in the right direction rather than replacing parts unnecessarily.

I also don't seem to be finding a specific carb rebuild kit for my specific model year, 1993. I was thinking of ordering this one that even tho its not for my model year looks about right. Whaat other alternatives should I consider? Buy gaskets and jets separately?

Has anyone experienced similar symptoms?

Where should I look into first?


Thanks in advance!

[scrambler]

Hello,
welcome to the club . Are these new issues, or has it "always" been this way? If the NX250 sits for a long time with fuel in the float bowl, deposits form that lead to all sorts of problems; the idle system, with its tiny passages, is usually the first to be affected viewtopic.php?t=1673 .

Another possibility is incorrect jets, jet needles, etc. The Keyster carb kits are designed for the Canadian '88 model; they don't fit other versions and can cause serious carb issues. Therefore, use only the gaskets from the Keyster kit—not the brass parts! For carb overhauls, I use only genuine jets and components. When ultrasonic cleaning, you should use a suitable cleaning agent and avoid running the cycle for too long—I clean and tune carb for forum members. With a few tricks, removing the carb can be done without swearing or destruction viewtopic.php?t=163

It looks like you have one of the rare 1993 MD21 models. For some unknown reason Honda produced a 249cc MD21 again in 1993 (alongside the MD25) viewtopic.php?t=361 (frame number MD21-54000xxx). It is identical to the 1988 MD21, so it uses the same PD6BA carb and has identical tuning specifications—for example, a size 138 main jet.

[ze clasen]

The main jet has lots of deposits and some holes clogged which i think explains the loss in power from 70% power.
As for the idling I'm not too sure because the jets overall seem clean. The diaphragm pump was disconnected, could it be related to the idling issues?
Because its my first time opening a carb I dont know if the gaskets and rubber bits are Ok, they look a bit hardened but might be good. I already ordered the kester kit to replace the gaskets just in case but out of curiosity wanted to know if it might have been an issue as well.

I'll give more updates when the parts arrive and the carb is rebuilt

Thanks for the help

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[scrambler]

The main jet has lots of deposits and some holes clogged which i think explains the loss in power from 70% power.
As for the idling I'm not too sure because the jets overall seem clean. The diaphragm pump was disconnected, could it be related to the idling issues?

I don't think so; there's very little chance for air leaks there. So, again it turns out gunk in the carb was causing the problems. Ideally, you should have the disassembled carb ultrasonically cleaned, as residues can remain in the tiny passages. At the very least, you should blow everything out with compressed air viewtopic.php?p=34553#p34553. You can check for obstructions —for instance, in the pilot jets —using a cleaning needle set designed for printer nozzles https://www.amazon.de/D%C3%BCsenreinigu ... 637&sr=8-6


The main jet is the original one, so it appears no Keyster parts have been installed here yet. I would replace the seals; they tend to get brittle after so many decades. Installing the new float bowl gasket and the O-ring on the accelerator pump nozzle isn't exactly easy, as they tend to fall out when you put the float bowl back on.