Jetting -- Where to start?

JHNguyen89 · Jan 18, 2010

New to the dirt bike scene and have learned that I need to rejet my KDX. Been reading quite a bit and am ready to go through this trial and error process.

That being said, what's a good starting point? MJ? PJ? And when going to the shop, is their a specific type of jet I need (other than the size)?

Riding in California on a KDX220. FMF Gnarly pipe, Vforce3 reeds, cut airbox lid, stock silencer.

Jetting is stock and the needle too.

sr5bidder · Jan 18, 2010

Keihin PWK jets are to be used

sr5bidder · Jan 18, 2010

start here

http://justkdx.dirtrider.net/carbtuning.html

jpgibson · Jan 18, 2010

mind you, if its running well maybe you dont.Best to have a look inside and see where youre starting from- better too rich than too lean.Stacks of info here and on thumpertalk ( where theres a whole post on kdx jetting-just google it).I bought all my MJ and PJ's from ebay.These bikes are so easy to get the carby off too! Unlike any other bike Ive had.One of the top tips from this site was to get a haemostat to hold the throttle cable spring up so you can get to the needle-pure genius!
good luck

JHNguyen89 · Jan 19, 2010

jpgibson said:
mind you, if its running well maybe you dont.Best to have a look inside and see where youre starting from- better too rich than too lean.Stacks of info here and on thumpertalk ( where theres a whole post on kdx jetting-just google it).

Honestly, I'm new, so I don't have much to compare it to. Though, I DO know that the previous owner never changed the jets from stock. He said he changed needle position, but doesn't remember where. I will be checking this stuff out tomorrow. I'm not positive, but I notice people usually re-jet when they add mods?

Like I mentioned earlier, it has an FMF Gnarly Pipe, V Force 3 Reeds, air lid is cut. I'm running 40:1 and in California.

On the other hand, I purchased a 145 and 55 jet today. Would like to see how it changes the bike.

Thanks for the info!

reepicheep · Jan 19, 2010

A 45 and a 142 with the middle clip position will likely run well. I don't know how that combo will effect long term durability.

JHNguyen89 · Jan 19, 2010

reepicheep said:
A 45 and a 142 with the middle clip position will likely run well. I don't know how that combo will effect long term durability.

Would I be better off running stock jets?

reepicheep · Jan 20, 2010

Stock is really rich on my 200, I don't know what stock is on the 220. Those are the jets I am currently running on a friends 220 I am helping him tune.

Overly rich is the safest, sort of. It'll spooge everything up, make less power, and foul plugs... but keep the bike from melting cylinders.

I've wrenched a lot of things, but this is my first dirt bike, and without a 4 gas analyzer dyno (which I sure as heck don't have) I am guessing at jetting. At about 800 feet above sea level, and about 30 degrees F, a 45 and 142 seems to be running well. That will "richen up" as the temperature goes up to normal summer riding temps, near as I can figure the mains can change by as much as 4 numbers between summer and winter riding. So I tried to get it spot on at 30 degrees, and I'll let it run a little richer as it gets hotter outside.

JHNguyen89 · Jan 20, 2010

Checked and I currently have 42 and 145 jets on the bike and needle 2nd from top. I bought 45 and 155 but am not sure if I should change both and/or needle?

Any recommendations? Seems to be running fine with no spooging, but no power in 1st, 2nd, and 3rd. People have told me the front wheel should lift if I crank the throttle, but it's not.

Bike: KDX220 2001
Mods: FMF gnarly, V force 3 reeds, cut air lid, 13/49 gearing
Conditions: 0-1000ft, 50-70F
Fuel: 40:1[/QUOTE]

JHNguyen89 · Jan 20, 2010

double..

reepicheep · Jan 21, 2010

Whoops! Sorry I missed that the first time. You are running an FMF Gnarly for the expansion chamber? Aren't those all torque pipes? Or do they make the rev pipe in a Gnarly as well?

On the 200, there are two FMF pipes... a torque pipe and a rev pipe. They have different part numbers. I think the torque pipe is a smaller expansion chamber (11" in circumference) and the rev pipe is 14" in diameter. All well and good...

But because of the different porting and geometry of the 220, the rev pipe acts like a torque pipe, and the torque pipe acts like... well... junk.

Could you have a torque (woods) expansion chamber on there instead of a rev (desert) expansion chamber?

JHNguyen89 · Jan 21, 2010

I think I may have the torque pipe. 13" circumference..

sr5bidder · Jan 22, 2010

the tab on the pipe under the tank will have numbers have you checked these #'s against the fmf sites?

reepicheep · Jan 22, 2010

Yeah, check the tab. I can measure the FMF Gold series Gnarly woods (torque) pipe on my KDX-200, and the Pro Circuit desert (rev) pipe on my friends KDX-220.

13" sounds big though... that would explain some of your issues if that is the case. Maybe somebody with a 200 and a desert pipe would want to trade...

julien_d · Jan 24, 2010

If you're in 2nd or 3rd, going slow and barely on the throttle, and then you crack it wide open, does it make a buuuhhhhhhwaahhhh sound and bog down a bit before it opens up? Or adversely, does it start to sputter and pop a bit before it starts to open up?

I second trying to get a REV pipe for that bike. Your jetting looks like it would be pretty close. I had fits jetting mine after a new top end. Where I ended up didn't make sense to me from where I started, but it's screaming now.

JHNguyen89 · Jan 24, 2010

julien_d said:
If you're in 2nd or 3rd, going slow and barely on the throttle, and then you crack it wide open, does it make a buuuhhhhhhwaahhhh sound and bog down a bit before it opens up? Or adversely, does it start to sputter and pop a bit before it starts to open up?

I will have to check this out a bit more closely when I get the chance to ride again on Monday. I've read through a lot of threads about the "buhwah" sound and bogging issues. And to be honest, I'm not sure if I have those issues.

I'm using this video as an example of bogging as I'm not sure what it is/was.

http://www.youtube.com/watch?v=qm6JRyPyBCM

And my bike never feels like that. If bogging is, the rpms dropping when I'm twisting the throttle, then my bike is not doing that.

Thanks for the help!

Jetting -- Where to start?

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