I put a fmf slip on, on my 06 Suzuki 125 drz-l and it is running a bit lean I was wondering what size I should go with on the jetting?

the level should be visible in the glass, get someone to help lean the bike one way and then the other , a small torch can help.

Changing to a K&N air filter changes the air/fuel mixture for your bike, and you're probably hitting an overly lean spot before things start to even out. For bikes still running carburetors, which I think you have, you can compensate for this by changing to a slightly larger idle jet. Your main jet is probably fine (it's kicking in after the lag); it's the crossover from idle to main jet that's causing the flat spot.
Talk to your dealership parts counter to get recommendations about jet settings. They may suggest, for example, to keep your existing idle jet and go with a different main jet. Expect to try a couple different setups to find one that works best for your bike and your riding style. I would highly recommend keeping your original jets, though, in case you ever go back to the stock air box.

Hard to give accurate info with out seeing the bike with jetting but I would suggest take a reading from the fuel air screw if the bike running better with the fuel air screw in carbi at 2.5 turns out or more then definitely increase pilot jet up 1 or 2 sizes if you get the bike to run well between one - two turns out on the fuel air screw that is where your jetting should be
I wouldn't be concerned with main jet at all - it should be fine for wide open throttle unless you feeling power loss when you open it right up
also if you are getting backfire or popping when de accelleration that also hints toward lean pilot so a size or two change in pilot jet would be worth trying

The carb on a DRZ 125 or KLX 125 comes pre-set from the factory with a very lean tuning. At first it may run well but once you get some time on it with a little build up and some dirt the lean setting gets even worse. To correct this you need to thoroughly clean the carb , specifically the fuel screw. Remove the carb from the intake and airbox rubbers and take off the slide cap. Drain the carb from all fuel. The fuel screw is in a recessed tube located on the bottom of the carb on the front side towards the cylinder. It may ahve a brass plug covering the screw. This is done at the factory to prevent tampering for emissions purposes. The plug can be removed by taking a very small drill bit 1/8" or so and very carefully drilling a hole in it. Put a small sheet metal screw in the hole and pull with pliers and it should come out. Make a note of the position of the screw and slowly turn it clockwise in to see what the setting is. Do not apply heavy pressure when you feel it getting tight stop. It is a soft brass screw with a fine point an is easy to damage. Typically the factory settings range from 1 1/2 to 2 turns out. This is way too lean for the bike to idle and run well or even start without the choke. back the screw out and remove the spring and O ring. Using carb cleaner with the hose attached insert it into the hole you just removed the crew from and spray it out. You should have a good stream of spray into the center bore of the carb. Re-insert the O ring and then the spring and fuel screw. turn it in until you feel it making contact and then note the position and back it out 3 1/2 turns. Put the carb back on and it should be able to start and run without the choke. Some bikes will still need a bit more tuning. The older carbs had a slide needle that had 3 to 5 positions on the needle that could be adjusted with the movable clip. Since this does not have that option you could go to a Yamaha dealer and buy the needle from a TTR 125 and install it as it is the same carb. This will cost you around $15.00. Or you could go to a hardware store and buy some 3/0 stainless washers and remove the needle and put 2 or 3 under it to shim the needle up just like the clip on an adjustable needle. You may spend $1.00 with this option. The bike should run better than new.

Hi and welcome to FixYa,

On a 2003 Kawasaki KLX125, these are standards:

• main jet - 145
• pilot jet - 17.5

Good luck and thank you for asking FixYa. Happy Holidays.

jetting is not an exact procedure. too many variable altitude, temperature, humidity, gaz.. does your bike presently run rich or lean ..ect..
i would go 1 size to start. it's safe to run an engine too rich but is risky to run too lean

They should be around 33hp at the rear wheel. They should wheelie ok, though you will need to try a bit for decent ones.

I have an 04 125 with a FMF fatty Factroy expansion chamber and a Titanium 2 FMF silencer (shorty) the carb settings are Pilot a size leaner than stock main IS stock jet needle is a setting toward lean and air screw is two turns out. My gas:oil mix is around 32:1 - 40:1.. My spark plug buget is killing me, and I havemessed with it til I was blue in the face can anyone help me?,don't worry I didn't take it wrong it is a good question, and let me put it this way.. I run it like a two stroke lol I have had more than my share of two strokes so I don't lolly gag it and usually for the trails I ride you really can't get out of 1st most of the timeI spend most my time around 3/4 throttle. When I 1st got the bike I had a friend messing with the idle screw on it cause it wouldn't idle at all and he messed with the mix screw by accident so I think it was screwed out more than two turns but I set it according to FMF recommendations with the pipe and chamber. The guy who had it before me raced it in a AMA class I don't know which so don't ask lol, so who knows what has been done to it other than what I can see,,,

I have an 04 125 with a FMF fatty Factroy expansion chamber and a Titanium 2 FMF silencer (shorty) the carb settings are Pilot a size leaner than stock main IS stock jet needle is a setting toward lean and air screw is two turns out. My gas:oil mix is around 32:1 - 40:1.. My spark plug buget is killing me, and I havemessed with it til I was blue in the face can anyone help me?,don't worry I didn't take it wrong it is a good question, and let me put it this way.. I run it like a two stroke lol I have had more than my share of two strokes so I don't lolly gag it and usually for the trails I ride you really can't get out of 1st most of the timeI spend most my time around 3/4 throttle. When I 1st got the bike I had a friend messing with the idle screw on it cause it wouldn't idle at all and he messed with the mix screw by accident so I think it was screwed out more than two turns but I set it according to FMF recommendations with the pipe and chamber. The guy who had it before me raced it in a AMA class I don't know which so don't ask lol, so who knows what has been done to it other than what I can see,,,