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Kx85 carb issue(?)

No way

Viking Mothfukle
Joined
May 19, 2020
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33
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773
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Under the bus
I got a kid riding a 2014 kx85. It runs great and starts real nice and easy when cold but gets real fucky when hot. He's lightweight and can't kick it over hard enough so I or some other adult will have to start it for him whenever he's stopped during practice.

I aint 2 stroke savvy enough... what to I need to unfuck, overhaul or replace to remedy this crap?
 
I would do a compression test first. Doesn't take much longer than changing a spark plug. Will save a bunch of time chasing jetting when it could be time for a ring.


Manual says 119-185. I'd be wanting it above 150. If it was below that I'd pull it down.
 
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Nothing to add except that an 85 is the most dangerous bike an adult can ride.
 
coil failing when it gets hot

It does start though. Just needs choke and a firmer kick.
Tried two coils, no difference.

I would do a compression test first. Doesn't take much longer than changing a spark plug. Will save a bunch of time chasing jetting when it could be time for a ring.


Manual says 119-185. I'd be wanting it above 150. If it was below that I'd pull it down.

Ive had the engine rebuilt. No change.
I have 2 carbs and the current one is worse than the other which is why I suspect carb issues. Both are stock afaik. Could it be the Reed valve?

Nothing to add except that an 85 is the most dangerous bike an adult can ride.

This I can vouch for :laughing:
 
It does start though. Just needs choke and a firmer kick.
Tried two coils, no difference.

Bikes are not like cars or at least Japanese bikes are not like them. In close to 20 years I can count on 1 hand how many coild we sold.

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Ive had the engine rebuilt. No change.
I have 2 carbs and the current one is worse than the other which is why I suspect carb issues. Both are stock afaik. Could it be the Reed valve?


It could be carb related. I would look at reeds to be sure. Its hard to say unless you bought the bike new. If it was an x supermini bike or race bike there is no telling what has been done to it in the past.

How long since it had the rebuild?


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This I can vouch for :laughing:


a
 
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Usually, if the bike runs fine when cold, and gets worse as it heats up, it may be running rich (hotter air has less oxygen, making the problem worse).
Is there a particular part of the engine RPM's that it runs poorly at, or is it just hard to start after it gets up to temp?

I'd readjust the idle screw and fuel screw and see if that helps.
 
Yup, idle mixture to rich. this can be gotten around with the right timing of twisting the throttle open as you kick. Technique goes a long way. May need to drop a size on the idle jet. What altitude are you at?
 
Nothing to add except that an 85 is the most dangerous bike an adult can ride.

That's because a 100# kid will never have traction. Put an adult on there and that lightswitch powerband becomes a real motherfucker.
 
Usually, if the bike runs fine when cold, and gets worse as it heats up, it may be running rich (hotter air has less oxygen, making the problem worse).
Is there a particular part of the engine RPM's that it runs poorly at, or is it just hard to start after it gets up to temp?

I'd readjust the idle screw and fuel screw and see if that helps.

Runs great all the way. Just harder to start hot. Usually needs choke too.

Yup, idle mixture to rich. this can be gotten around with the right timing of twisting the throttle open as you kick. Technique goes a long way. May need to drop a size on the idle jet. What altitude are you at?

Track is maybe a couple hundred feet above sea level

When you say it was rebuilt were the crank seals done?

Yup. All bearings and seals.


We have two engines, the other one is an 07, also rebuilt and it behaves the same way with this carb.
 
Howsa crankcase oil level? Some kid believed that the gearbox should be filled until oil came out instead of using the dipstick marks. Got to the point the dirt bike was very sluggish & no power-
 
Is the plug new? Try loosening the gas cap before starting hot.
 
Howsa crankcase oil level? Some kid believed that the gearbox should be filled until oil came out instead of using the dipstick marks. Got to the point the dirt bike was very sluggish & no power-

Mid level glass as per spec.

Is the plug new? Try loosening the gas cap before starting hot.

Plug is new. Gas cap is vented and ventilation is functioning properly
 
Some morning coffee google foo provides multiple suggestions to kick it over with open throttle when hot as it could be trying to flood itself.
We'll try that next time he's out.
 
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