Engine maintains revs after throttle is closed

[KnawJB]

Hi All,

I took out my KXF450 on the track for the first time this year, and noticed something odd regarding how my engine runs after the throttle is closed.

At idle - if I open the throttle to rev the bike, and close the throttle quickly - the engine revs for a long second or two before the revs fall.

If I accelerate hard into a corner, close the throttle and hit the brakes quickly -- the bike feels like it is still trying to accelerate (making for a very difficult late brake/turn). I usually do not clutch when I am in the proper gear, and because of engine braking.

The bike did not run like this before. Is the cause related to a/f mixture screw, jetting, or is this normal?

Any help is appreciated.

[FIVEWIDE]

You have an air leak somewhere .

[KnawJB]

Where would be the best places to start looking for an air leak (soapy water i assume)? Is there any other reason why this would happen?

[Dave2]

Generally, you would use carb cleaner because you want to find the leak and maintain a running motor. Carb cleaner is combustible, soapy water isn't. Check from the mixture screw area to the motor.

Sounds as though you ought to look at the throttle cable to. Maybe a kink or something...

[KnawJB]

By soapy water - I mean spray it on the outside of the engine, tubes, connectors... if it bubbles, then there be a leak.

Throttle cable in kinkless, and operates smooth with to full open/close positions.

[podfish]

right, but spraying something flammable tips you off when the engine revs. It's easier.

[the white stallion]

Hanging idle: as Fivewide states a Hanging Idle is a lean condition sypmtom and could all so be jetting. Since I assume this was not the case before you parked your bike you might have a varnished jet or be able to get rid of it with a fuel screw adjustment if it's not a air leak. Maybe bad gas.

1: check for air leak

2: Richen up fuel screw

3: clean jets.

[KnawJB]

Thanks everyone for the help. I will try to find an air leak, and also turn the fuel screw out a tad to see if I can get rid of this "hanging idle".

[honda53]

Your pilot jet is dirty, since the bike sat for a little while. Clean it out with compressed air and carb cleaner or buy a new jet the same size, could be hard to get clean enough.

[KnawJB]

The bike sat for about 5 weeks before I had time to fix the broken kick start bolt. Ran like a champ before. I'll check the pilot jet for sure.

Your pilot jet is dirty, since the bike sat for a little while. Clean it out with compressed air and carb cleaner or buy a new jet the same size, could be hard to get clean enough.

[FIVEWIDE]

Your pilot jet is dirty, since the bike sat for a little while. Clean it out with compressed air and carb cleaner or buy a new jet the same size, could be hard to get clean enough.

Thats also a pretty viable option, I'm not too familiar with the new 4t's but my buddies ktm250sxf has a slight hanging idle that is considered a normal thing, he fixed it by tweaking the air screw and messing with the jets, his problem didn't seem as extream as yours. ALso check your accelerator pump on your carb I'v heard of people putting o-rings on them to alleviate this.

[kawikx100]

you most likely have an air leak, get propane from a toarch and spray it around the engine it will inject and rmp will pick up! and that will help you detect you spot

[CamP]

Whenever a bike that once ran well, doesn't run well after sitting up, it's almost always a plugged pilot jet.

[schrode]

lean condition, which could be from an air leak or a plugged jet.

[Schneller]

By soapy water - I mean spray it on the outside of the engine, tubes, connectors... if it bubbles, then there be a leak.

QUOTE]

Your bike may be SUCKING air, not blowing air, so there wouldn't be any bubbles on the outside.

[zukboy789]

Throttle Cable. Lubricate it.

[brentn]

Holy moly, everyone seems to be screaming air leak except for two...

Fuel screw could be the culprit or a lean pilot or a clogged pilot, it MAY not be an air leak. So before you start spraying soapy water all over every crack, pulling your hair out cause you can't find the leak, check your carb out first and adjust the fuel screw properly.

My YZ used to have a hanging idle as well until I started understanding how to use the air screw and richened up the idle mixture. My old thumper did this as well until I started playing with the fuel screw.

[the white stallion]

By soapy water - I mean spray it on the outside of the engine, tubes, connectors... if it bubbles, then there be a leak.

QUOTE]

Your bike may be SUCKING air, not blowing air, so there wouldn't be any bubbles on the outside.

Word. Very good common sense.