JD jetting INSTALL on 2019 Yamaha YZ 250X

[@EarthSurferUSA]

I think your needle is still rich. You may have to go one richer on the pilot, but you have more grooves on the needle to go more lean than I did. One groove can be a pretty big jump, (depends on the taper of the needle I guess), and so far, I have gone the equivalent of 1.5 grooves with the 1/2 groove more lean needle, (still rich. just ordered the next leaner 2 needles). But if you want to creep lean with a lower needle, but not one groove, you can use little washers under the clip. I made my own on a lather that are the same diameter as the clip so nothing hangs up, and a groove is about a 1mm jump. If you make a washer to .020" thick, that will be about a half groove. But I would not be surprised if you end up one more groove. The smoke that comes out of the silencer is a pretty good indicator too. I found it great to have a little for start up, but pretty much clean after that, and the plug is a great light brown and dry color. If it burns all the oil, you get no exhaust mess. I remember racing amateur at the Pontiac SX after the pros were done tearing up the track for Saturday and Sunday, (a 2 race weekend in that dome). The pros raced all weekend, and the stadium did not get full of smoke until after the pros were done, until 2 short amateur races were run. What a difference in jetting. :) Doug in Michigan

[@jmarcinko1139]

178 main and 50 pilot is stock jetting which is slightly rich but but keeps the bike running cool and preserves the cylinder. The leaner jetting will run more crisp and be more responsive but also will generate more heat and wear the cylinder just a little sooner. Also you’ll likely be too lean this winter and will need to go back to a richer jetting but it’ll run great this summer. Enjoy! Also get a JIS screwdriver for the carb screws and avoid a normal Phillips to keep from chewing them up.

[@RIDEreviewRELOAD]

@@jmarcinko1139 yes the kit comes with a lot of different jets and a chart for your elevation and temperature!yes sir probably get this kit for my 125 too! what is a JIS screwdriver?

[@EarthSurferUSA]

@@RIDEreviewRELOAD I suspect that gentleman, (rich is not good either), is a candidate for a damaged piston, (as I posted above). I may be wrong, but I suspect he does not warm up that rich engine for 10 min to avoid piston damage. Now I know 2-stroke MX bikes with a carb are shipped rich. Always have been, and they all can use more exact jetting for your area. But this YZ is the fattest, richest pig of a stock bike that I ever tuned. I do blame the power jet quite a bit. It complicates jetting, adds electronics, (but seem to be reliable), and I don't think there is any performance advantage. In fact, it is a bit more of a pain to get them running perfectly.

[@EarthSurferUSA]

Actually, it would save some time with plug color if you used the black rich plug (as long as it still fires). As you lean out he jetting, it will burn off quickly and give you something to read. It just takes a while to get a new plug to color in a rich condition, and you can't get a good reading for a while. It has to darken up first. :)

[@RIDEreviewRELOAD]

Good info brother! Appreciate the input 🤙🏻

[@EarthSurferUSA]

Another problem with the rich stock jetting. PISTON DAMAGE!!!!! If your plug is black, you are not warming up the coolant quickly. It can take 10 min to get hot radiators, and the engine may never really get into a good operating temps, on the edge of fouling a plug. It is very easy for a rider to get impatient, and take off riding, and I bet a lot of YZ250 riders have done it,---and damaged their piston. I bought my bike from a mechanic/son at a dealership, who is not very sharp, but the bike only had about 3-4 hours on it. I did ride it for another 10 before I modified the engine, but I made sure the damn water was hot, (where 10 min comes from). I took out a stock piston that looks like it was ready to come apart. The crown and piston gets hot and expands, but the colder cylinder does not expand enough, so they make contact. You will get the "4-corner" scuff, but the worst is the piston crown side and rings being pushed into the exhaust port. There was so much contact there, (plating still great), it bent the material above the ring up, (making the ring groove wide. Ring could flutter and break), and pushed the material below the top ring into the bottom ring, (pinching the bottom ring, both losing compression). With my jetting much better, the water warms up in about half the time, and you are ready to go. I like to feel pretty warm radiator tops with my gloves on. That's pretty hot with them off. I know-------------------------there are a lot of damaged YZ250 pistons running out there.

[@EarthSurferUSA]

I have the 2019 YZ250, and the stock jetting is horribly rich, (MXA said it was perfect in their magazine test. They don't know how to jet a carb anymore.). I think the only difference between the YZ250 and YZ250X is the X model has a lower rpm tuned pipe. Compression may be slightly different too, but the 250 was lowered about a decade back, (I have both heads). I did a ton of mods to my bike, and the last is finalizing my jetting. I am in Michigan, and will not ride under 50 degrees temps at my age, so I will set my jetting on the edge at those temps. It will still be very good at 80 degrees. If you want, there is a mix trick you can do for different temps. If I set the jetting crisply at 50 degrees outside and run my gas mix at 40:1 (use a good oil), I can increase the oil to 32:1 for hot weather., (example, testing is needed IMO), so the fuel/air ratio across the board is a bit more lean. More oil mixed with the gas means less gas going though the jet, and that is what counts for the Fuel/Air ratio that you are jetting for. I want to know if that is actually a Yamaha needle, or if they make their own. So far with my jetting, (and I am still rich, and so are you. Smoking a lot still, rich needle), I have first modified the engine with the rich stock jetting. Woods riding, FMF Gnarly pipe, V-force reeds (no aluminum spacer block), and I lowered the cylinder by not using the base gasket (need a really good sealer, like 3M Nitrile #847 brown), that lowers the exhaust port to better match the lower rpm tuned FMF pipe, and raise compression (.048" squish left, was huge). I have to run 100 octane with no base gasket. That raised the compression and the burn temps, and the plug went from black to a dark brown. Still a long way to go. So far, I went with the 48 Pilot (50 was stock), a 40 power jet (50 was stock), and since one clip on the stock needle is still rich, put ion the next Yamaha lean needle, and still on the top groove, still rich. I just orders the last 2 lean needles from Yamaha. I may go back to the 50 pilot jet as it leans out the middle. I just dropped from the 178 stock main to the 175, but will probably end up with the 172 also, (using wide open test runs and throttle chops for main jet plug color. Gotta use the stock plug IMO.). One thing I know, carbs with out power jets were a lot easier to tune. I don't see any advantage of the power jet. This is not the best running bike I ever had. I would have it and worried about the electronics failing and going lean. But I have never heard of that happening. When you are burning all the oil, (no mess, and hardly any smoke), don't go any more lean. :) Oh, I modded almost everything on the bike, and I just modded the kick starter. Try rotating the kick starter on the shaft on spline to the right and look how it sits in there closed. Perfect. That is actually how it was drawn up, but the bottom casting of the kick start shaft area gets super close to one bolt on the clutch cover. So Yamaha just rotated the lever back one spline,--for like 2 decades. lol I just dremel cut the casting a bit for 1mm clearance, (just a shave), and man, it is amazing how such a little change, makes the engine area look so much better,-----------like it was designed that way. :)

[@EarthSurferUSA]

Doug in Michigan

[@RIDEreviewRELOAD]

You sound like a jetting Jedi!!! The force is strong with you 🤙🏻🤙🏻

[@jeffpaxton9172]

You’re going to have to rejet in the winter time now. With the stock jet you don’t have to do that. For me, not being a professional racer, it’s not worth the hassle of rejetting every season.

[@RIDEreviewRELOAD]

Oh I don’t race either. No problem at all, I only have to swap needle which takes a few minutes. Loving it!

[@EarthSurferUSA]

Winter time? (Michigan here). My bike is in perfect condition every spring (even when I wore them out faster when younger. Rode all summer), because I use winter time to make it that way. :) Back at the end of my "more riding" days, (around 2000), I found in my favorite sandy whooped out 40 mile loop trail ("The Little O". About a 3rd gear average trail on a 250 2-stroke.), I was actually warm in my summer riding gear. I just can't work that hard on that trail now, so 50 degrees is my new normal. :)