Motorcycle Stuttering Problem?

Question

I bought a new bike a few months back, it is carby, 250cc. Anyway, when I go around corners and sometimes just going in a straight line I go to accelerate and the bike stutters for a second, then resumes accelerating. I have taken it back to the bike place but they think I am nuts. I tried it again but it still does it. Mainly when it's slightly colder but still, there is a definate problem. Any ideas appreciated.

Umm....it's not the normal lag or anything. When I am say half way through accelerating (even in the right gear) doing it nice and smoothly it will just cut out..like the motorbike looses power (stutters) for a little bit then resumes as normal. I think it is a little dangerous as a different bike I ride is perfectly smooth through the corner. Hope this makes sense...I'm only new to the bike scene! :)

Bike is a 2006 Kawasaki ZZR250.

Answer 1

Stutter/Burble/Blubbering/Sputtering,,,,anything that sounds like "rolling your R's while trying to speak Franch,Spanish,etc"
....Which is TRANSIENT when opening Throttle is a TOO RICH fuel Condition.

Too Much Fuel,,,failing to consistently combust.

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OPPOSITE of that is ,,,BOG,Lag,Hesitate,Surge,Blah,,etc
from Opposite Condition.
Too Lean,,not enough Fuel.

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How/Why:

The Fuel is held in the Carb's Bowl at a Specific Level.
It's SUPPOSED to be only "Vaccumed Out" as-needed Thru the various Metering Jets.

If Fuel Level goes MARGINALLY High,,,,
it sort of "Semi-overflows",,or "semi -floods "

That admits an Excess of Fuel to engine,,,causing the Stuttering You describe.

Ok,,,when You TURN the Bike while Riding,,it LEANS.

That Tips the Carb over to the side.

Same thing happens to the Carb and it's fuel ,,as when tipping a Filled Drink Cup.
It "Spills"

As is easy to Imagine,,
If a Cup is only 1/3 Full,,,You can tilt it quite a ways before any spills.

But if the Cup is Full to the Rim,,,ANY Tilting will trigger a spill-over.
Not Only TILTING it will spill,,
But Bumping it,,jiggling it will also likely spill a bit.


Carb Acts exact same way.
They are designed to have Fuel Level set to accomodate a large degree of "Tilting" as you Ride the bike around corners,,
And that Less-Than-FILLED level also allows the Fuel to slosh around inside the Carb over Bumps and dips and jumps,etc which motorcycles do.

So the FAULT is technically :Fuel Level TOO HIGH.
Causes are usually a bad/leaking Fuel Valve which fails to shut Off the Fuel Flow when proper level is reached.

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Turning /Cornering does not appear Related to Straight Line accelleration as anything which would cause Same Running Problems.

But in the matter of Excess Fuel Level,,it certainly can be.

If the Carb's Fuel level was simply Overflowing at a large Rate,,,
Bike would be running like the Choke was On.
Flooding,Running Very Rich,,,,Stuttering while simplyy cruising along.
And Probably Leaking Fuel as well.

But an Abnormally High Fuel Level---up to a Point--,,,,CAN run practically normal.
When You Accellerate,,,,there's a Drop in Vacuum but a RISE in Bulk Air Flow.
It causes a condition called "Pull Over".
Actually,,,it's "Transitional Pullover"

Instead of SIPPING GAS Under Vacuum thru the Carb's metering circuitry.....
The Sudden Vaccum Drop,,,"Drops" the metered Fuel Column.

Same as when a person quits sucking on a straw in a Soda,,,
The Fluid Column DROPS.
If ya ever look Closely,,You'll see it sorta BOUNCES a tiny bit.

But in a Carb,,,The Airflow Dynamics CHANGE at the same time.
When the Vaccum Drops the Fuel back into tiny metering orifices
Bulk Airflow and Velocity RISE in the Carbs Throttle Bore.

That has an effect Itself of Pulling Fuel from the Next Higher Order of Fuel Metering Circuitry.
Carb is Changing from one Metering Stage to the next.

So when Fuel Level is Marginallly HIGH---but NOT yet quite High enough to cause Full-Time poor running,,,,

What happens is pretty much a SPLASH of Excess Fuel gets "Bounced" into Carb's bore.

It's a Temporary Over-Rich/Excess fuel condition.

Thats why soon as it passes that intial Stuttering Phase,,,and You settle in at your new,higher rpm/speed,,,,all seems fine.

It once again Cruises normally and all is well.

What's happened is You've burned through that little Sudden Splash of Fuel,,,,and carb is once again metering thru it's jets & such Normally.

Exact same thing as when You Corner.

DIFFERENT ACTIONS---but Same Results/Symptoms/Outcomes.

A Splash of Excess fuel,,,which makes it sputter and stutter,,,
Then,POOF! It's burned and gone,,,and all seems well.

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If You had an actual JETTING Problem,,,such as Wrong Jet,,or LOOSE&leaking Jet,,,,
The Running characteristics would be Constant.

A MainJet Needle which is Too Rich,,,,gets WORSE after accelerating,,then going into "Cruise" steady throttle.

When They're mis-calibrated the bikes Stutters "All the Time" under cruise,,,and gets BETTER when you accelerate because of the introduction of sudden,extra air when Throttle is Opened.

NO Jetting/Mixture condition can cause Probs when Turning/Leaning/Jiggling Bike/Jumping/Bouncing,,etc,etc.

Fuel Slosh/Overflow is the ONLY cause of that.
Some Carbs(especially old,vintage ones) were particularly prone to that.
They'd run RICH/Flood turning in One direction,,,
And then LEAN/Starve for fuel when turning in Other Direction.

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REMEDY:
Basically it's a matter of Adjusting Fuel Level in Carb Bowls to correct Height.

In Reality,,it very probably should have the Fuel Valve assembly changed in both carbs.

It's RARE to have a Carb's float level just "change for no reason",,,especially street/road bikes.

Rising Fuel Level is typically a sign of a flaw or fault in the lil' valve assy itself.
Sometimes debris from fuel will cause a "slow leak".

But if Bike is more than a couple years old,,,it's wise to replace the fuel valves while you have carbs opened to make adjustments.

It's Not a terribly expensive repair to have a Shop perform the work.

Or,,anyone who is eve the least bit "nutsy-boltsy" should be able to do this type job theirself.

It's somewhat a Meticulous job,,,and carbs are a bit fragile.
So,,get a Manual or get some experienced assistance,,(Or Both:) if You decide to try it yourself.

Good Luck with it,,I'm sure that's all your trouble is,,and nothing more serious is very likely at all.

Answer 2

TXm42 sure talks a lot but doesn't say much. First I'll adress the "tilting" business. If the bike isn't moving, what he said is true but when you're moving and going around a curve, the bike is leaned into the curve and centrifugal force is going to keep the fuel more or less perpendicular with the bike. That's how fuel injection first came about in the 50's, with high speed race cars on flat tracks the fuel was forced against the side of the carb causing fuel starvation. Bikes leaned into the curve so they never needed anything like that. So much for that grand idea.

Nearly all if not all bikes these days use CV (constant velocity) carbs as opposed to the old throttle slide carbs. With the CV, when you twist the throttle, all you do is open a butterfly valve and the resulting vacuum then causes a rubber bellows to lift which lifts the throttle slide, thus giving more air. When the engine is stopped, you can crank on the throttle all day and you'll never open the throttle slide. On the old style throttle slide carbs the throttle cable is connected directly to the slides. What he said about whacking open the throttle (well what I read, I didn't have the patience or time to read it all) and dropping the vacuum, overly rich mixture and so on is true on the old style but not on the CV's. It sounds like to me that there's a "flat" spot in the carburation where the mixture is going lean. Many new bikes have problems with the idle mixture being set too lean. It's controlled by the little screw on the outward side of the carb in line with with the inlet and outlet of the carb. Yours may be covered with a plug or cap to prevent tampering. They can be removed, but if your bike is still under warranty, you had best not fiddle with them yourself. As far as the people at the bike place thinking you're nuts, no they don't, they just don't want to be bothered. If they had any smarts at all, they'd know about the all too common lean idle mixtures and that it takes nothing more than the turn of a screw to fix. I'd pester them because they should have replacement plugs, caps or the dab of paint so that you don't void your warranty. They sound like the kind of people that would jump at the chance to screw you over if something under waranty needed to be fixed.

Answer 3

Throttle Motorcycle

Answer 4

Hi, as a rider myself ! firstly id check spark plug , HT lead , but it sounds almost like a fuel problem , not enough going through to the carb which would make it stutter then stop completely , maybe there is a blockage ?
Just cos the bike is new the dealers shouldnt laugh at you for telling them this new bike has a problem !
Id take the bike straight back and demand they look at it , this is not only frustrating but also very dangerous for you as the rider . Good luck !
Sue

Answer 5

knowing the kind of bike would help a lot.

Most carb 250cc are 4 stroke - that would be a carb issue.

If you ride a 2 stroke then I would say that the needle valve is dirty - or you have the wrong jets in the carb.

Answer 6

It could be a leak in the carby gasket or the fuel level is not high enough in the carby bowl. the leak could be at the o ring where the carby attaches to the motor~~

Answer 7

I am going on a whim here.

But if you are talking about what happens when you suddenly open up the throttle -- it is quite common I would think in all makes to have a kind of momentary lag when you open it up suddenly. The carburetor takes a while/ a moment to adjust to the increased airflow causing a lag.

That is my idea -- if I am wrong let me know.

Answer 8

I say you got to adjust your carb for the coming winter. Does your carb have a heating unit? Adjust the idle a little higher or try bigger jets (not too much bigger). I know that's what I do on my bike.

Answer 9

Clean your spark plugs and check the gap, make sure the gap is within spec.

Check the oil level in your engine and if there's too much, let some out.

Check your air filters, they may be dirty.

Answer 10

Most Likely:
Float height in the carburetor and the carbs aren't synched