At the end if my wits… 250sx

[MKE.rob]

I have an ‘85 and ‘86 250sx in great condition. They ran okay when I got then but I figured a carb rebuild wouldn’t hurt. Ordered the Shindy rebuild kit and followed the excellent carb rebuild tutorial on this site very closely. After the rebuild the bike ran seemingly great. Fixed some other issues like the neutral/reverse light not working, and a new 250es ignition switch since I lost the key to mine. She ran awesome for about 10 minutes until I really got on her and drifted around the yard a bit. Suddenly, the headlight started cutting in and out, and she would just suddenly die. Would fire back up but then limp and die.

Let her sit for about a month. Here I am tonight finalizing some things and I tried taking her out. She would barely start, then only sit idling with full choke until suddenly RPMs would shoot up insanely high to which I’d turn the choke off—she would die instantly. Took her out of the garage and tried riding but she kept dying and the headlight would cut out intermittently.

What the heck is the issue here? I’ve spent so many hours on this thing and its worse than when I started!

Why would it run great, then suddenly once I get on her a “switch” flips and she doesn’t stay running?


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[ATC King]

I'd check the voltage. It may be the regulator or stator having issues.

Check at idle and as you rev it. If possible, strap the voltmeter somewhere where you can watch it as you ride.

[El Camexican]

Besides checking your electrical, empty the flow bowl of your carb and drain it as well as your tank, then put everything back up and try running fresh gas through it. Check your carb boot for cracks as well.

[MKE.rob]

Where would I hook a multimeter up to?


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[ATC King]

If you can, the battery would be good. Providing the leads are long enough to allow you to bungee the meter to the front of the seat or somewhere like that if you need to ride it. You may be able to find the problem with it just running in neutral though.

First check the state of the battery, which should be above 12.5V. As long as it's not flat, anything above 12V minimum should work, but that doesn't rule out a bad cell.

Check the voltage at idle to see if it has increased, but more importantly in your situation, if it's steady. Then continue to slowy rev the eninge higher and see if the voltage holds steady or if it's bouncing around by several volts. When regulators go out, one of the possible failure modes will cause the voltage to swing wildly at higher RPMs and the headlight to flash or blink.

Essentially, there'll be some type of intermitent grounding going on inside the regulator and it throws the whole system into a frenzy.


If you get frustrated, remember to not get tunnel vision, that it's process of elimination and one step at a time. Sometimes things aren't what they seem and you may have touched something, but it's something else unrelated that simply decided to turd out at the same time. When dealing with electrical remember that mechanical vibration can have an effect on bad connections and a chang in engine RPM or the movement of the vehicle will cause the intermitent issue.

[MKE.rob]

Just got in from testing…

Voltage checks out. Idle it sits between 12-13V, and at rev and high rev its well above 12. At no point did voltage dip under 12V.

The headlight issue was a barely connected ground in the wiring harness, so thats been fixed.

Replaced the spark plug since it seemed like it was misfiring, but old spark plug seemed fine. No issue fixed here.

For some reason though after a high rev, when it comes back down to idle it overshoots and idle too low and dies. The normal idle is a good purr though, so I don’t think cranking up the idle is the correct option.

New problem today is after a little run around the yard I brought her in and the carb was just pissing gas. Why would this be? Drain plug was securely in. Something (dirt?) suddenly stuck in the carb? Im really confused.


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[MrConcdid]

Hi,
at this point I think you have carb issues, the needle and seat and float are sticking causing it to overflow.
I think you need to go back through the carb step by step, you may have missed a step.

[MKE.rob]

Just got back in from another test, had to warm up the hands.

I think most if my issues were because of the gas tank being very close to empty. Filled up the tank and she ran pretty good. Did randomly come down from idle and die one, but all other attempts to make it happen again failed.

Not sure why the carb pissed everything out when the tank was empty, but I filled it up and drove around and nothing was leaking again. What the hell?

It does sound like its misfiring occasionally, or running a bit rich as she gives a little woof after shutting it off, like a small backfire.


Still sort of confused, lots of weird quirks.

I no way I missed a step in the carb rebuild, but clearly something is wrong. Argh….


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[El Camexican]

If you replaced the needle and seat in the carburetor and everything is in order and correctly positioned then you have junk in your gas. Did you flush your fuel tank at any point? Do you have a fuel filter? Find particles of rust, paint and whatever other crud gets into a fuel tank will give you Nonstop problems.

My best guess is that you have something intermittently plugging up your pilot jet which is causing the idle to fluctuate wildly and you probably have some junk stuck in the needle and seat.

Suggest you clean your fuel tank, petcock, install a fuel filter on the fuel line, clean the carburetor out especially the pilot jet and try again.

[MKE.rob]

Any ideas why the unit would seem to be running rich?


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[El Camexican]

Probably because there’s crud in your carburetor that’s keeping the float open and raising the fuel level to the point where you are drawing in more fuel than you should be.

[El Camexican]

If I understand your first post correctly the only thing you have messed with is your carburetor, correct? If that’s the case, and your electrical system has checked out I think it’s safe to say it’s a no-brainer, you have a fueling issue and it’s not something that an adjustment is going to fix. You’ll have to get the thing right before you worry about making adjustments