KDX250 Too lean?

[NOTTIES]

I have just completed my project: 1993 KDX250 rebuild.

I see on your site many of the riders suggests a pilot jet of 38-42 and a main jet of 155-158.

I currently have a 38 pilot installed and a 150 main jet.
Jet needle 1368.
Needle clip 2nd from top.
Have a FMF header pipe.
Standard reed valves.
B8ES spark plug.

I did not run the bike yet(waiting for front caliper).
Will then do the plug tests etc.

I would like to know if the extra leaner main jet will have the risk of letting the bike run too hot or cause any other damage?
Did anybody try the 150 option?
Will a 155 main jet be a safer option?

 

[julien_d]

Very difficult to say. You can start with what you have, and do some plug readings and playing with the air screw to get a good idea if it's running too lean. It does seem 150 would be a bit small. That pipe will lean it out some too, especially if you're loosing the airbox lid.

 

[sr5bidder]

Also keep in mind that it is winter at least here and the cold air is dencer so a 155 in winter might react like a 150

150 main is scarey
155 safer yes
158 safest
you should start big and work to smaller

Only two stoke cart racers run lean to the piont of being on the edge of total destruction.

or you can just see what the 150 does, it should only take a little while to sieze the piston / crank bearings.

What did you rebuild on your 250?

 

[dirt bike dave]

FWIW, when I ran my KDX250, I had a 155 main, and most of my riding was at around 4,000 - 6,000' elevation, using pump premium fuel with around 45:1 fuel/oil mix, with Maxima Super M.

On a dual sport run, I took it down to sea level and had some pinging/detonation under top gear accelleration.

IMO, a 150 would be too lean, especially if you are running at low elevation.

Your fuel makes a difference, too. I've got no idea how your fuel compares to ours.

 

[NOTTIES]

Thanks for all the advice - Will look for a 155-158.

Took it for a spin yesterday - not far - couln't wait for caliper anymore, will hopefully receive caliper tomorrow!!
Idles quite well starts easily, but another problem stuck its head out - float levels!!
Bike runs good until the carb gets empty.

Bike kept overflowing from overflow so I adjusted the floats to close the inflow needle early - but this seems to be too early!
It seems that my inflow needle doesn't function/seal too well.

I had a problem with my rear wheel hub that broke out so I replaced it with a Suzuki RM 125/250. That is when I decided to redo the bike.

Powdercoated frame
Replaced all bearings
Removed all unwanted paint on parts
New tyres & tubes
New sprockets & chain

At this stage I didn't do anything to the motor as it didn't give any hassles.

 

[NOTTIES]

Picture of the early stages.

 

[Matt90GT]

here was my setup on the US spec model:

@50:1 oil ratio (amsoil synthetic)

Jetting: 38/158 , R1367H needle 3rd clip from top, #6 slide

Be sure to install the KX (.26" headgasket (88-91 KX, raises compression))headgasket that is thinner than the KDX.

YOu may have to modify that jetting a bit for the non-us bikes since they are a bit different.


I had a ported cylinder and Vforce reeds. The bike FLEW!