Ninja Ex250 Carb Issues

Discussion:

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Jesse Vernon

2005-09-16 06:55:34 UTC

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

Rick Cortese

2005-09-16 16:03:40 UTC

Permalink

Post by Jesse Vernon
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

I would say slow jets. What happens is the slow jet is plugged so
nothing but air with throttle closed => engine dies. You screw in the
the idle screw to the point where it can get gas through the main jet
and it idles ~3000. When you run it at WOT the intake loads up with gas
then shutting the throttle off, where you did have mostly just air, now
has air/fuel mix and it idles at 5,000 until it burns off the excess fuel.

I am not familiar with your particular carbs. Slow jets are usually
mounted higher then the main so it could be fuel level is too low for
them to pick up gas. Some are mounted with O-rings to make a tight seal,
you could have a cracked one, I mean the bike is 9 years old. It could
just be an obstruction somewhere internally in the slow jet circuit.

krusty kritter

2005-09-16 16:11:11 UTC

Permalink

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...

Jesse Vernon

2005-09-18 19:24:39 UTC

Permalink

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

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fweddybear

2005-09-18 19:46:59 UTC

Permalink

Post by Jesse Vernon
Thanks for the awesome responses!
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?
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

krusty kritter

2005-09-18 20:33:49 UTC

Permalink

Post by Jesse Vernon
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.

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.

Rick Cortese

2005-09-19 03:06:25 UTC

Permalink

Same thing.

Were the idle jets plugged to begin with? Any sediment in the float
bowls? Sometimes when the source of the problem, like water in the tank,
isn't taken care of first the problems will keep cropping up.

If you take the carbs apart again you will see what I mean about the two
different heights. Float height/gas level is the thing I would check now
that you took care of making sure they weren't plugged.

Most idle mixture screws are set to be 'right' at 1 1/2 turns out. I can
think of exceptions like someone put in the wrong screw. If it ends up
running OK with them set between 1/2 to 3 turns I wouldn't worry about it.

There is a fuel/air path that goes around the carb slide and is on the
engine side of the carb. It would be a good idea to make sure those are
clean too. You can look for a pin hole sized opening on the engine side
to make sure. Losen the mixture screw and shoot some carb cleaner down it.

Since its a dual carb dual cylinder I am a bit puzzled. I mean it sounds
like something is happening to both sides. Bad gas/rusty tank kind of
thing could do it but Krusty's post made me think it could be something
else like the automatic petcock.

I have been known to pull out the drain plug on a carburetor after an
engine died to see if there was fuel in the float bowl. If you get
desperate you may want to try that.

Jesse Vernon

2005-09-20 22:41:37 UTC

Permalink

Alright, this is really weird.
So I disassembled the carbs and adjusted the float heights to about a
1/10 in out of spec to try and force the bike to run a little leaner. I
also adjusted the mixture screws to 1 and 1/2 turns out. The carbs are
clean, as well as the idle jets and whatnot.
So anyway, after reassembly, I drove the bike around for about 1/2 an
hour and it ran beautifully. So I decided to adjust the mixture screws
to get it to run optimally. But the mixture screws seemed to not really
have any effect on the idle speed, and after playing with the screws,
the bike began to stall again. So I had to bump the idle speed way up to
get it to idle normally and now it also sticks in the 3000 rpm range
again.
I have no clue what's going on. Any guesses?

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fweddybear

2005-09-20 22:52:27 UTC

Permalink

Post by Jesse Vernon
Alright, this is really weird.
So I disassembled the carbs and adjusted the float heights to about a
1/10 in out of spec to try and force the bike to run a little leaner. I
also adjusted the mixture screws to 1 and 1/2 turns out. The carbs are
clean, as well as the idle jets and whatnot.
So anyway, after reassembly, I drove the bike around for about 1/2 an
hour and it ran beautifully. So I decided to adjust the mixture screws
to get it to run optimally. But the mixture screws seemed to not really
have any effect on the idle speed, and after playing with the screws,
the bike began to stall again. So I had to bump the idle speed way up to
get it to idle normally and now it also sticks in the 3000 rpm range
again.
I have no clue what's going on. Any guesses?

If it ran beautifully, why bother tinkering with things? I mean most
people have a hard enough time getting their bike to run like you just
described. I would put things back the way you had them, and enjoy riding
the bike... if the plugs are burning a tan or brownish color, you have done
a good job....and should leave well enough alone....

Fwed

krusty kritter

2005-09-21 00:34:51 UTC

Permalink

Diagnosis: Idle mixture screws are open too far. Butterflies are opened
too far, compensating for excessively rich idle mixture. Transition
ports are uncovered by butterflies. Engine draws fuel through idle
mixture ports and transition port. Engine RPM hangs up when throttle is
opened and then closed because it gets far too much fuel for the load.

Prescription: Start out with idle screws open about half a turn. Turn
idle screws equally in clockwise direction. As idle speed increases due
to leaning out of idle mixture, turn master idle knob down. Turn idle
mixture screws equally in clockwise direction some more. Turn master
idle knob down to specified RPM. If engine begins to idle rough at
specified RPM, turn idle screws clockwise about 1/8th of a turn.

You will probably wind up with the idle screws about 1/4 of a turn out
from lightly seated. In the morning, when the engine is cold, it should
start on full choke without giving it any throttle. The idle speed
should not be excessively high, requiring instant choke adjustment and
a lot of fiddling. You should be able to put on your helmet and gloves
and zip up your jacket, etc., without messing with the choke lever. The
engine should warm up in a reasonable amount of time and not seem "cold
blooded"

When the engine is hot, the idle speed shouldn't be too high, nor
should it surge. Surging indicates idle mixture leanness. RPM hangup
indicates
idle mixture richness.

So far as the float height is concerned, the tolerance on adjustment is
plus or minus one millimeter, or 0.0394 inches. A float that holds the
fuel level one tenth of an inch low may cause fuel starvation at high
RPM.

R. Pierce Butler

2005-09-21 02:03:33 UTC

Permalink

If your floats are of the solid plastic variety, I'll bet they are porous
and need to be replaced. I had a Honda that had that problem. If you let
the float dry out it would run great for a while and then it would start
running too rich and eventually it would flood. Replacing the floats fixed
it.

pierce

Jesse Vernon

2005-09-22 20:49:23 UTC

Permalink

Pierce,
The floats are pretty new, but thanks for the suggestion.
krispy kritter,
So last night the bike ran out of gas, and after I refilled it, the hang
at 4000 rpms seemed to disappear. But there were A LOT of holes in the
carbeuration which definitely shouldn't be there. So I took your
suggestion this morning, turned the screws to 1/2 and fired it, but then
all of a sudden, gas started pouring out of the carbs and then the
engine flooded and died.
What's up with that?

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krusty kritter

2005-09-22 22:09:38 UTC

Permalink

Post by Jesse Vernon
krispy kritter,
So last night the bike ran out of gas, and after I refilled it, the hang
at 4000 rpms seemed to disappear. But there were A LOT of holes in the
carbeuration which definitely shouldn't be there. So I took your
suggestion this morning, turned the screws to 1/2 and fired it, but then
all of a sudden, gas started pouring out of the carbs and then the
engine flooded and died.
What's up with that?

There are a number of things that can cause flooding"

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. Or, maybe some trash passed through the
petcock, after having been stirred up from the bottom by refilling the
gas tank.

If your floats are still set at 1/10th of an inch low, you may
misunderstand the float setting procedure. When the carbs are upside
down on the bench, and the manual specifies that the float level should
be 15 mm from the gasket surface, but you decide to go 16 mm from the
gasket surface, that LOWERS the fuel level in the float bowl. If you go
to 14mm instead of 15mm, that RAISES the fuel level a millimeter. 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. The petcock may effectively be
in the PRI position all the time if the rubber seal becomes grooved.