Converting a used Yamaha YZ250 2-stroke into an XC / woods weapon: Part 1

Take a well-used 2015 Yamaha YZ250 motocross bike and tweak it so it's perfect for some XC, woods or enduro riding and even a blast around a motocross track.
That's the goal of our latest project bike build. In this first episode, MotoHead's Dave Willet rebuilds the motor then fits some tasty mods such as a different cylinder and a cool new pipe, and bars and a whole host of Polisport plastic protection.

[@EarthSurferUSA]

MY ENGINE MODS ARE DONE, AND WORK GREAT: I just finished my 2019 motor mods and got a few chances to ride it so far. I do build 2-stroke engines for a living, so I know how to go the direction I wanted for a power curve, much more low end power. Trying to mimic the 1997 KX250 I had, and got close enough to love it. I don't think I lost much top end rev. I should have put a tach to it, before and after, but I did not think of it in time. Maybe I lost 500 top rpm. I won't miss it. The mods. We have the same pipe and V-force reeds. I am not using the aluminum spacer plate between the reed cage and the cylinder. This decreases crank case volume a bit, and you get a better hit. The stock reed cage, with the "reed stop plates", the upper plate blocks off the rear reed boost transfer port quite a bit, so you will get better flow there with the V-force reed. Now, since the FMF pipe is tuned to peak at a lower RPM than the stock pipe, I lowered the cylinder by not using the cylinder base gasket and I still have .049" squish clearance, which is good. We have the same head, 2015 and 2019, (sealing it with a good sealer is very important here, and doing a good job.). What this does is 2 things. It raises compression (have to use 100 octane fuel, mixing 110 race fuel with pump 90 octane "Rec fuel", no alky" sold a pumps in Michigan USA.), for better power pretty much through out the curve. And it lowers the exhaust port timing to better match the lower rpm tuned FMF pipe, (a major key to great running engines, matching the pipe to the porting). I think the pipe is on the mellow side (I do have a Pro Circuit to use, and I bet it is not as smooth in the power delivery, but more of a hit.), as it does not pull/hit as hard as that MXA bike of the year KX250, but it is smooth and powerful. At 190lbs, I can short shift it with stock gearing and it does not fall on its face. And it is easy to loft the front end at the lower rpm's when I need, (the Dunlop MX14 scoop tire really stick out with this power curve. Withe the high rpm curve, you did not hit good power until you were moving pretty good, and any decent tire hooks up well there. But this tire is a stand out for a lower rpm power engine. I estimate it is making 48 plus hp, and the higher compression cleaned up the stock jetting pretty well. I have never lofted the front end before when shifting the gears in the sand with out trying, (even with great engines), and that tire is hard to beat for what ever it can sink its rubber teeth in. Grass? Wow!!! Dry clay,---maybe not gas it hard. :) Drop that cylinder, remove the reed plate, jet it again, (It will run quite a bit more lean, (but the stock jetting is pretty good with it as stock, it was rich enough to leave droplets on the back of my jersey, lol.) and you will love it. I thought it may hit too hard, but I think it is now the best engine I ever had/built. No flywheel weight and I don't think I have the need, and the mods are easily reversible,--but you won't do that. I promise. :) Doug in Michigan.

[@gareththornton3917]

Looking forward part 2 buddy

[@EarthSurferUSA]

I think in 2011, Yamaha went with a bit less compression head also, (not sure on the year for the change). I bought both of them and measured them. For a bit more mid range punch, if you remove .0085" (.216mm) from the gasket surface, it will be the same CR as the older heads. I lowered my bike, (1" in the seat and .75" off the suspenders), getting the Gnarly pipe, V-force reeds, and the higher compression head,---for starters. :) I am also making my own mounts for "bark buster" style Yamaha hand guards, because I don't like a clamp on the taper of my pro taper bars. I think it is going to make a wonderful woods bike. Our ground in Michigan USA is soft enough to try that nasty looking Dunlop MX14 rear scoop tire too. :)

[@hurleyhastzler719]

shes a beauty.. cool classic yamaha of troy inspired grafix kit.. 👍

[@nkreigline01]

Apex, Smart carb and GYTR flywheel transformed my 250x completely. I can lug anywhere and rip wide open. So smooth now

[@EarthSurferUSA]

You should have learned how to jet the stock carb IMO, (they are always rich from the factory), and save money on a smart carb.

[@nkreigline01]

@@EarthSurferUSA I had the stock carb jetted fine. It worked as it was designed. The transitions on the different circuits are fine if you like that kind of hit. The smart carb is noticeably smoother than stock. There are six other 2 stroke bikes in the garage, 3 of which are twin cylinder if I want to play with jetting.

[@EarthSurferUSA]

@@nkreigline01 OK, I never tried a smart carb, (but got good at jetting), but I can see they can be smoother in transitions through the rpm range. I can only go by what I know about carbs, and what I have experienced of course,---and stuff I have read. I take it that you hit a nail on its head when you just said "If you like that kind of hit". That leaves it to personal preference. To the performance of my 2019 model though, I wanted a much bigger hit down low, (it is made to try to keep up with 4-strokes on a fast 4-stroke track IMO, screaming like a 125). My stock jetting is very rich with the stock motor, (almost fouling), but I left it stock to kick off the motor mods. I just started it up so far, (riding it later today,--I got delayed with cancer), and I can already tell the needle jetting is much better, (not a lot of smoke, and clean revving from the start.) I think that is because of the higher compression, burns hotter. Here is what I did. it is all reversible. Bolt on's: 1) FMF Gnarley pipe. It is a lower rpm target pipe for more low end power than the stock set up. 2) V-vorce 3 reeds, (no stock aluminum spacer, for a harder hit). lighter reed tension opens earlier to help fill the bottom end at lower rpm's. The stock reed upper "stop plate" does block off the 5th transfer tunnel some, and the back of the cylinder could be filled better with V-Force reds, (or cut the stop plate). Mods: 1) Lower the cylinder by not using the cylinder base gasket. Must use a good sealer, because you don't want a leak there. This lowers the exhaust port to less duration, and should better match the lower pipt rpm target, (A big key to good porting, is a pipe that matches the porting peak power rpm target). Compression is higher because of the lower cylinder, and I have to run 100 octane gas or higher octane. Power valve operation is not a problem with the slightly lowered cylinder. 2) a tad bit of porting, (not a window change, other than lowering the cylinder), an opened the 5th port tunnel just a bit, for insured good flow. That is about it. All is reversible (93 octane if you put the cylinder base gasket back on). So lower that cylinder, and se how you like it. :)

[@EarthSurferUSA]

With my stock head, (to about 2011 I believe), will still have about .049" squish clearance after lowering the cylinder. About .040" is as tight as we should go on a 250 IMO. The earlier heads have a bit more compression, and should squish at about .041". Still good for 100 octane gas. I used the newer head with the .049" squish. I would use the older, higher compression head if I raised the cylinder back up with the stock gasket and switched back to 93 octane fuel.

[@colinlloyd1543]

I've had a lot of these bikes and yes put a COMPLETE wr400 transmission not just the 3,4,5 like what's in the 250x and then change the front sprocket to a 15 tooth and the diff is amazing, awesome, transforms the bike, real good

[@douglasbuckler8934]

A 98-00 WR400 transmission.? I’m looking that up right now. Thanks!!

[@hankevers]

I have a 2018 yz250 set up for xc. It’s got apex xc dome head And then most of the mods you’ve shown here. I’m super interested in what you’ve got to say about this bike after putting in hours

[@EarthSurferUSA]

Fork alignment: Everybody on YT does it poorly, and the fork legs are not parallel. For your bike, (same as my 2019 YZ 250 2-stroke I am sure), have a metal lath cut you a 1" diameter piece of aluminum round stock, and face off the ends exactly to a length of 5.661" (143.8mm). With that, you tighten up the axil clamp on the brake side and leave the other side loose. Then put the bar stock between the plated fork legs as low as you can (wheel off the ground), and it fits though the front brake disk. Use a small rubber mallet to tap over the lose fork leg until the round stock is tight, and tighten the last clamp. Be careful removing the round stock, (can use a bit of grease on the ends if you wish), but the soft aluminum should not be able to hurt that hard fork leg plating if you keep the ends clean of grit. Now your legs are straight, sliding smoothly, more plush action, less wear and tear on the bushings,-------and don't be surprised to find you will need stiffer springs because the forks are not "wedging" themselves as they go through the travel. I bought a set of upper triple clams that offer rubber mounted handlebars, (BRP racing USA,---fantastic quality, aerospace worthy quality.), so I measured the distance and made faced off the stock. I had to go back to stiffer springs. :)

[@EarthSurferUSA]

I hate the abstract art Batman plastic on the new YZ's so much, I put on a 2014 front fender, went Ronnie Mac or the arrow tips, and use a Frisbee for a front number plate. 175 gram. I was looking for a more vintage plate, set the Frisbee there, and started laughing,----so I mounted it. :)

[@apuuvah]

SmartCarb SC2 and FMF Gnarly torque. I don't think you can build a really good hard enduro 2-stroke without them.

[@WTF.416]

You need some billit rad guards 👍

[@Coopdegras]

Are we getting an update on this soon Dave? I've sold my tpi and planning on doing something similar with a yz250.

[@jacobhanson9009]

Just find a yz250x

[@Mkruzer]

Would have love to see some other items besides plastics and a pipe. That was a long video for basic plastics. Suspension changes, head changes, carb adjustments, fuel mix, tires, controls, gearing, flywheel.

[@EarthSurferUSA]

I can't my 2019 out until spring. I like whooped out 3rd gear sandy trails the best. Here is the set up I just finished with. I do build 2-stroke engines for a living, so I think it will work out well for a lot more bottom and mid range. If it is to strong down there, I can always reverse a thing or two. I am not worried about losing up to 1000 top rpm. I won't need it much. 1) Pipe is that FMF Gnarley. 2) V-force 3 reed with out the stock aluminum spacer. (More crankcase pressure, and no stock reed stop plate to block off the rear reed transfer port tunnel.) The reeds are designed to give better lower rpm flow. 3) I did very little porting. Just cleaned all the goobers, touched up the champers, and opened that rear reed tunnel 1mm on the cylinder side. The fuel feed into that port tunnel is terrible with the stock reeds because of the reed stop plate, )which V-Force does not have.), blocks off that port tunnel about 50%. 4) Lowered the cylinder .024". Did not use the stock gasket. Sealed the cylinder to the cases with a good gasoline gasket sealer. This lowers the exhaust port to better match the lower rpm pipe rpm range. It also raises compression with about .050" squish clearance, (good, but can be even tighter). Have to run 100 octane fuel. Should hit harder down low for the sandy whoops. 5) Wiesco piston kit. The stock piston is pretty weak. It was damaged from the previous owner not warming up the bike well before he rode. 6) Jet the carb well, (first thing in the spring). If it hits too hard, I will add the reed spacer first. If still hard, (I doubt it for my tastes), I can raise the cylinder up with a thinner gasket. But I want it to be strong down there. Not that I will always use it, but I like to get my front end over unexpected things easily from low rpm's when needed.

[@brentroy03]

We get it your sponsored by polisport

[@robertwaynereoo3636]

If you don’t mind me asking mate, what fuel ratio do you run in this kind of bike?

[@EarthSurferUSA]

32:1 with a good oil. That is pretty standard for the best ring seal for a 250cc 2-stroke MX bike, but the EPA (environmentalists) want you to run less oil. Ignore them. :)

[@Sers0812]

Where will you get the power for the headlight from

[@gavinbailey748]

it's own battery, like a little atv battery

[@terryfinch9319]

Where is part two??

[@pedrofernando176]

✌👍

[@trav1369]

They struggle a lot on extreme enduro, I've ridden mine for a year and half and raced XC, hard, and Extremes. And extreme enduros is too gnarly for the yz, too much power no grip

[@EarthSurferUSA]

Again, just because everybody else is doing it for no real reason in reality,----we ride with a rotated forward freestyle bar set up. It is a irrational era we are entering.

[@chadrides914]

Bro there is nothing different about turning a yz250 into a woods bike. EVERYONE around here has been doing it since 2000’s. They arent different for the sake of being different. Sorry to burst your bubble.

[@EarthSurferUSA]

Since the late 1980's here. My 2019 YZ needed more work top make it a good woods bike than I have ever had to do. Motor, lowering suspension, get rid of the Batman plastic, etc.