Ninja Ex250 Carb Issues

Hi there.
I recently bought 1996 Ninja Ex250, and it seemed to great until one day
fuel started leaking out of the carbs. The problem was a stuck float
that probably occured when the bike had sat for a while. Anyway, after
fixing the float problem, the bike now idles oddly. It runs semi-rich,
and has max horsepower when cold, but as the engine warms up the bike
begins to really loose horsepower. Also, as the temp rises, the bike
starts to stall, and I have to adjust the idle speed to about 3000 to
get it to run at all. Anything below around 3000 and the engine
nosedives into a stall. Also, after the engine has been reved, it gets
momentairly stuck at 5000 rpms before returning to idle. The carbs
appear to be perfectly clean (as they were just professionally cleaned
about two months ago), and I just cleaned the idle jets and reset the
mixture screws, so I'm really at a loss as to what the problem could be.
Any thoughts?
Thanks,
Jesse Vernon

 

Your idle jets and idle passages may still be a little plugged up, and
you can do the Berryman's B-12 liquid in the tank bit (3 to 4 ounces in
a full tank), or you need to disassemble the carbs and spray aerosol
B-12 through all the jets and passages until it squirts freely out of
the idle port and the transition ports, the idle jet, and the pilot air
jet.

Now, to reset the pilot mixture screws:

Even though the screws are actually controlling fuel/air mixture, we
think of them as only controlling the fuel when the mixture ports
regulated by the pilot screws are downstream of the throttle
butterflies. So, it's necessary to turn the screws clockwise to lean
the mixture, counterclockwise richens the mixture

When you reset the idle mixture, you turned the screws too far
counterclockwise, then you compensated for the rich idle mixture by
turning the idle speed up, now the throttle butterflies have uncovered
the three transition ports at idle, so there is the source of your rich
mixture.

The three transition ports are not controlled by the pilot screws at
all, the pilot screws are like fine tuning on the idle mixture
outlets...

The strategy of setting the idle mixture is to turn the pilot screw on
one carb about half a turn clockwise, and, as the idle speed picks up,
turn the master idle knob down. then go to the other carb, turn that
pilot screw clockwise, turning the master idle knob down again.

You will probably start with the pilot screws open about one full turn.

Soon the idle mixture will be correct, the engine will be idling at the
specified RPM. If you turn the pilot screws in more, the idle will get
rough, the engine will get hot at cruising speed and lack power.

You can also tell a lean idle mixture by the fact that you have to use
full choke to start the engine and it seems to take forever to warm up
and it's very "cold-blooded" until it's really warmed up.

That sounds like the original EPA-mandated idle fuel/air mixture
doesn't it? If your pilot mixture screws are open 1/4 to 1/2 a turn
extra, it might not pass some states' air pollution standards, but it
will start easier, warm up quicker and have better acceleration as you
just open the throttle.

Your pilot mixture screws will probably wind up at about 1/2 to 1/4
turn open...

 

Thanks for the awesome responses!
So, Rick:
Are idle jets and slow jets the same thing? Because I just checked and
cleaned what I was told were idle jets, and it didn't seem to fix the
problem. When you say the fuel level is too low, are you referring to
the level in the float bowls? Could a bad float height cause my problem?

krusty kritter:
I followed your steps for setting the mixture screws, and now they are
turned about a 1/2 turn from tight and the bike seems to be running much
better, but it still sticks at low RPMs though. Anyway, when I was
reseting them, I saw something alarming- there is a slow fuel drip
coming from the top of the carbs. It only seems to drip when the bike is
idling, otherwise, if I take it out and ride it, it doesn't seem to
drip. I don't think the float valves are stuck, but I guess it could be
possible.
Any adivce?
Thanks! Jess

 

Yes.. it sounds like your float is adjusted a tad too high for it to
leak out the top.... make sure there isn't anything in the way of its
operation before trying to make any ajustment... it may just be dirt on the
needle that can be wiped off easily, or it could in fact need adjusting....

Good Luck..
Fwed

 

There are a number of things that can cause high fuel level in the
float bowls.

The float valves can stick because of gum and varnish on the edges. The
float valves are usually square, sometimes triangular, using the narrow
egdes to guide them in the round brass seat.

There can be gum or varnish stuck to the float valve seat. Even a tiny
bit of crud that you can't see without a magnifying glass can stop the
float valve from seating.

The bendable tang on the float may be improperly adjusted. The floats
should be adjusted to the manufacturer's specs, plus or minus no more
than 1 millimeter.

Hollow brass floats can leak, so the float never rises high enough to
move the float valve shut.

Does the float bowl overflow when the engine isn't running?

The automatic petcock can leak if the vacuum diaphragm on the back of
the petcock leaks.

The automatic petcock can pass gasoline through grooves worn in the
rubber seal behind the selector lever.