How much difference can 1 main jet size really make

[jsnake]

Hi all,

I have a 01 kx250 that seems to be running extremely rich. It is 100% stock except for a FMF type Q spark arrestor. Typical rich signs of “bad low end response, oily plug, lots of smoke and spooge". The stock main jet is a 160, will changing to a 158 really make any difference or should I jump 2 sizes to a 155. What exactly is the measurement of jets? What is a 160 as apposed to a 158? Is it in millimeters?

 

[bclapham]

try the pilot jet first. Yes, one size can mean the differnce is it running OK and then running excellent. Buy a couple of pilots and a couple of mains and go and play!

 

[Rooster]

The golden rule seems to be go in one step increments when changing jets, don't jump two sizes, as you risk engine damage.

Good luck, :thumb:

 

[MXFastGuy]

1. Start by leaning the mainjet one size. It's the easiest to change and may fix your problem. If it seems to help, go down another size. If it doesn't help, put the 160 back in and proceed to step 2.

2. Move the e-clip UP one notch on the needle to lean the mixture.

3. Last, try changing the pilot to a smaller size (you sound way too rich to be helped by the airscrew, which is a fine-tune for the pilot).

I'm no jetting expert, but my practical experience speaks above.

 

[jsnake]

Great info, thanks alot guys!

 

[Eggnchips]

Hmmmmm haven't you got your order the wrong way round? Every article I have read says that you should tackle the pilot first.

Main Jet only affects 3/4 to full throttle and if you get that too lean then you're gonna melt a piston.

Try too work out which circuit is too rich and adjust that to give good running conditions.

Pilot jet up to half throttle
Needle position 1/2 to 3/4 throttle
Main Jet 3/4 to full throttle

Some recommend attaching a bit masking tape to your handle bars and marking the throttle position ( I think that was MX Tuner or Eric Gorr).

Eric gorr has a great website and MX tuner gives great advice on
www.off-road.com

 

[bigred455]

Jsnake I use boyesen ss stock reeds,I was getting a little sputtering up top pump fuel,I went from a 160 to a 158,and it cleaned it right up.I would start from the pilot and work my way up.If you are fine up to 3/4 GO FOR THE 158.1 STEP AT A TIME. Watch out for oem reeds on the kx.I was eating them up on my 00 in 2 hrs.I saw that kawa change the reed stuffer for 03 just as I suspected,The 00-02 went to a horizontal divider from the 99 which was just open.This caused too much fuel velocity going to that bottom right reed.The 03 has a vertical and horizontal line.The difference from a 160 to a 158 is very minute,but I did notice the clean up,I do not know the the sizes in mm.

 

[motometal]

Rooster, even though I know in theory or "by the book" you are probably right, experience tells me that the pilot circuit affects mostly closed throttle and just off idle. The needle and needle jet affect the most important, or most used area of throttle opening. Too lean with the needle will cause one heck of an annoying bog (as I experienced yesterday).