YZ250 Jetting Suggestions? Help/Insight?

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8/25/2018 12:43 AM

I live in NC. Elevation where I ride mainly is between: 1350-2600ft above sea level. Kinda of a jump but that covers every trail where we ride.

So my bike is really rich. Has oil thrown all over the back end, bogs on top when cold and takes a while to basically “unload” itself to somewhat rev out on top. I have had issues ever since I got it rebuilt. Before it surged (only a little bit, I would crack the throttle a few times and it would stop) when I would stop in the trail at those elevations stated above (issue wouldnt happen at my house or at the builder shop). So I went to a 58 on the pilot (stupid I know) but it stopped the “surging” i was experiencing. But now I am honestly tired of it running like crap so I have taken upon myself this week/weekend to get it running crisp like it should.

I did tried to do a pressure test myself and it wouldnt hold psi (as in it lost 3psi in 30 minutes). I drenched the bike in soapy water with no air bubbles anywhere. I have not lost any oil and didnt not hear hissing or anything from left side crank seals. Also I think I was losing air possibly at the reed boot connection of my tester because I had to use tape wrapped around the pvc fitting to make a snug fit and the tape kept wanting to “suck” into the pvc fitting while clamped into the reed boot.

So now I am just going to put it back together and mess with the jetting and see how it goes from there.

I have the keihin PWK38 (i beileve what came stock on the 10’ model yz?)
58 pilot
Unknown main (will get legit keihin jet tomorrow)
Needle 3rd clip
Stock reeds as well (extremely small gap showing light).
FMF Gnarly/Powercore 2 (plan on repacking tomorrow as well and getting the connection coupling)

I hate bugging people with repetive questions but I just want a good running bike for once.... thanks you guys!


8/25/2018 3:48 PM

Got the jets out of my carb.
Main - 172
Pilot - 58


8/25/2018 7:33 PM

You'll probably write off this advice but I had a 2007 YZ and I went to a NECJ needle and it was worlds better, much cleaner running and revving. When it comes down to it, the taper of the needle is the biggest deciding factor of how a bike runs and jetting is super important on a two stroke. You can order the needle for $10 off Rocky Mountain. It is much more forgiving for jetting because it's triple taper instead of double taper.


21.5 350 XC-F
96' XR600r

8/26/2018 5:30 PM
Edited Date/Time: 8/26/2018 5:33 PM

IF there are no mechanical issues with the bike other than your current jetting, which is very confused, and that you are using the stock OEM needle(N3EW) that came in the bike, This is an easy bike to setup jetting wise
You can use the stock oem needle, but I would recommend using a Suzuki NECJ in the #3 clip position to startout along with a 42 pilot jet and 172 main. Set the air screw at 1.5 turns out. If you want to fine tune it a little more, YZ 250's respond very well to using a #8 slide and leaning the power jet out to a 38 or 40. It really cleans the throttle response up.

NECJ--- https://www.rockymountainatvmc.com/oem-schematic/28

power jet/leak jet(same jet)--https://www.rockymountainatvmc.com/p/930/47202/Keihin-N424-52-Series-Leak-Jet


8/26/2018 5:55 PM

JD Jet kits make it super easy. They have great settings.


8/27/2018 7:37 PM

42 pilot? 58 pilot? isnt stock like 48 pilot 178 main? Not sure why you'd be way richer on the pilot and way leaner on the main.


8/27/2018 8:26 PM

dmm698 wrote:

42 pilot? 58 pilot? isnt stock like 48 pilot 178 main? Not sure why you'd be way richer on the pilot and way leaner on the ...more

Talked to a local suspension/motor guy and asked him what he recommended I should run as he sets local harescramble bikes up. Its the same setup our 09’ yz250 has currently. Going tomorrow to get a 175 main and a 48 pilot. Going to drop the needle as well down from 3rd position to the 2nd to fix the surging issue i was having before (would ride bike for a while and then it would surge while waiting in the trail, had to crack the throtte a few times for it to stop). Also might re pack my silencer...