NC50 Express Neihin Carb

What causes the increase in idle speed when you inadvertantly have forgotten to turn the fuel valve on?

Re: NC50 Express Neihin Carb

Jonas Quimby /

You're running too rich, so the lack of pressure from the fuel being on means less fuel getting to the engine. Therefor the bike runs better for a moment.

If your bike tends to bog down when you give it gas (especially after it's heated up) then I'd suggest a good cleaning and re-adjusting.

Here's how, if you feel up to it.

Take a look at the diagram

That shows you the two main screws and the important bits.

Now, carefully screw the idle mix (Air Screw) and the throttle stop screw all the way in, and write down how many turns out they were. This will let you put everything back as it is now if something weird happens.

Then take your carb all the way apart and clean it.

Pay extra attention to the little passage ways. Also unscrew both screws all the way and clean everything with a good dose of solvent.

Take a look at the needle on the end of the throttle cable too. It's held in place by a little C-Clip. That C-Clip should be in the middle notch on the needle, otherwise your mixture will be thrown off.

Now once it's back together and a clean air filter is installed, turn the throttle screw out by about 1 and a half turns. and the idle mix screw out by 2 and 1/8th turns.

Start the bike up any way you can and get it good and warmed up. Try riding it for a few minutes without going down to idle much (the screws arnt right yet, so it might wanna die at an idle.)

Now put er up on the kickstand. Turn the Throttle Stop screw very slowly outward until the engine is just barly running. You want it to be running so low that you think it'll die at any moment. Making that "click click" sound from the starter even now and then. This is so it's low enough you can hear the changes to the engine.

Now you can very slowly turn the Idle Mixture screw in or out by up to 1/4th of a turn until the engine is running as fast as it can. 2 and 1/8th is pretty good usually, but see if there's a place it runs faster and better.

Now just turn the Throttle Stop screw up slowly until a hair before the point where the back wheel starts to turn.

Then rev up the engine a few times and let it back to an idle, see if it runs right or if the wheel spins or of it dies alltogether. Adjust the stop screw accordingly.

Go ahead and ride it at this point, see what it does.

If all goes well you have now overcome the Express's biggest weak point. A finicky, hard to adjust, easy to dirty carb.

Almost all Express owners have a variation of this problem. Bogging down on acceleration, hard starting, bad idle, dying at stoplights, etc etc etc. But do this and you shouldnt need to go through any of that.

Re: NC50 Express Neihin Carb


That sounds exactly like the problem I'm having. I will give it a try and see what happens. The step-by-step you describe, sounds very thorough.

I have yet to verify a clogged muffler, but I did try the blow test [I understand that may be inconclusive].

Thanks again........

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