Ninja 250 sputtering at high speeds?

Question

I just bought the ninja 250, 89', it runs great until i level off at 40 to 60 mph. when I level off at these speeds it sputters and is very jerkey. If i accelerate it stops but starts right back up wehn i level off. Ive dumped the tank and refilled but no help.

Answer 1

That's "too rich" in mid-range/steady cruise.

An indication that carb's needles need to be lowered one notch.

Indications of Too Lean in that mode are Surging,,,accompanied by a bad Bog/flat spot in throttle response upon acceleration.

The fact that the stuttering/burbling STOPS when you accelerate are positive confirmation of Richness.

If you had Fuel Starvation from ANY Cause,,,iw would act Dead Opposite of what you've described.

If You had an Ignition Fault,,such as bad/weak Plugs,Wires,Plug Caps,,,it would act Dead Opposite of the symptoms You are having.

take it to a shop,

get a buddy to help,

or Do it Yourself.

Whatever way you need to go to get it done.

But what needs doing 1st and foremost is to :

Remove the covers from the carbs>

*Take the Slides out>

*Remove the Needles from the slides>

*Raise the CLIP on the needle,,so that the Needle hangs Lower in the jet.

*Reassemble

If any of that behaviour remains,,,

Try closing the the Air Screws on the carbs1/4 Turn.

That will Lean it a little during the transition from Low speed circuit-to-Intermediate range

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Tricky Part is the Balance between the 2 stages,,,and managing the crossover point.

Example: You May not be able to Lean the Low Speed Air Screws without causing them to be Too Lean.

So You'd want to try Lowering the Needle 2 Notches rather than 1 clip position.

Which is a Large jump in the Leaner Direction,,a Big reduction of fuel mixture .

Which in turn sends you BACK to the air s***w adjustment to TRY richening it a little by opening the screws.

Depending on How Sharp you want it to throttle,,,

You may find  that 1 Needle position is Not enough,,,and 2 is Too much.

Go to a hardware store and get some small brass washers to slip on the needle as a "shim".

Usually 1 or 2 washers amounts to a "Half Clip Position"

It makes a finer adjustment by moving needle position in smaller increments.

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I should mention that You'll need to CLOSELY inspect the NEEDLES for any wear.

Use a manifying glass under a good light if neccessary

That Road Speed range  happens to be where Most street bikes Average Out spending most of their life,,,especially of longer term.

Any bike older than 3 or 4 years which has been ridden average,usual Mileage,,,is Always Suspect of having WORN  Needles.

When they Wear---they go Rich.

Its same effect as using a Smaller Needle>>passes more gas thru a given size needle Jet.

Soon as You lift or lower the Throttle,,,,Bike "Instantly" regains it's composure and runs correctly.

What You've done there,,,is to Immediately pull that wear-band on the needle Outa the Jet and are then back to a Normal sized needle.

The Transition is very sudden and quick,,,almost like flippin' a switch.

Tiny throttle movement changes Mixture from all screwed up to RIGHT.

All way more sudden that the needle's normal gradual taper.

It would not surprise me whatsoever if the needles DO need replacing.

So Look extra Close & careful when ya have them out.

If they're good,,,then all you can do is Lower them,,,and go see how it runs.

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A Dumb simple thing to "Test" it Before you put the tools to,,,

Remove the Airfilter and go run it ,,,see how it behaves at the "problem speed"

It's BOUND to run like c**p at very low speeds with the Filter removed,,,so Expect that.

But at 40~60mph niche,,,that will Lean it enough to show you confirmation of the need to go Leaner.

And,,,NO---it's not a clogged up airfilter.

If A/F was clogged,,,the faster You Go,,the Worse it gets.

If it's sputtering under Cruise conditions due to bad filter,,,opening the throttle & Increasing demand for more Air ,,,,it would NOT get Better.

It would REALLY turn to $#!+.

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One last thing..............

You can Trick worn needles,,to act a bit Richer/Leaner by moving them,,,and by using shim washers to move then in fractional steps.

But all You are doing is Moving the Throttle position at which they cause problems.

Example:

You can Lean out the Needle  a clip position or 2,,,and get it running OK at 60.

But open it up to run 75,,,and what You've done is pull the Worn Spot right back Up and into play.

Ya just "Move the Miss".

The Higher RPM tends to mitigate the severity of the prob,,

But it WILL be there.

If ya got WEAR,,the only Real Fix is New Needles.

All ya can do is Look and See,,,and  Try it.

Then you will know.

Answer 2

You need to pull the petcock from the tank, it may be plugged (mostly).  And if there is an inline-fuel filter, check, clean it or replace it. This would be an aftermarket filter not a factory part.  

If not there may be junk in the jets, so do the Seafoam thing. (a gallon of gas and a can of sea foam. run it for 30 minutes a day for a week, to let it remove all that varnish)

Then replace the plugs, which may be another cause of rough running. (check the wires to the plugs, a cable may be irregularly shorting out or too much resistance to the spark.)