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The Ultimate YZ-250 Horsepower Resource
We know there are more ponies in there!
The shifter kart guys are getting 62 horsepower out of the YZ 250 engine! These go-karts just flat-out haul butt and rival Indy cars in performance.
Our 2006 YZ has about 15 hours on the piston and rings. We calibrated our Hamlin Dyno so a stock 2006 Yamaha YZ-250 reads 46.5 HP @ 8,500 RPM. This is very close to what is published in the motorcycle magazines. All of the below inexpensive modifications work together, so we won't dyno test again until we are finished.
There were very few changes from 1999 to 2017 with the YZ-250 engine. I raced an 1999 YZ in the past, and don't think the new ones are really that much better. I really liked the feel of the old steel frame a lot more than aluminum. There was not that many changes at all from 2005 to 2017 with the whole bike. Even the 2006 suspension is better than most new bikes today.
Amateur racing in the 250 class, the stock 250 YZ eats 250 thumpers for lunch and will hang with the Honda CR, Kawasaki KX, and Suzuki RM two strokes just fine.
YZ 290 295 300 324 345 big bore kit 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014
Racing in the open class at the pro level, the Yamaha YZ250 is 4 to 5 horsepower down on the hot 450s. We are going to make things even.
Getting Bolt-On Dyno Horsepower
Our goal was to increase horsepower that can be used on a motocross track by intermediate and expert level riders. The objective was to boost power in the upper mid-range of the powerband. If fact we did just that. Our finished Yamaha YZ 250 with the hot-rod mods feels like a 300cc bike. You can go through corners a gear higher, as It pulls so dang hard! So far, reliability has been excellent.
DEP MX Pipe with Pro Circuit 304 Long Muffler
DEP is the magic expansion chamber for the YZ-250. The Pro Circuit Platinum (Works) pipe makes the same power on top end, but the DEP exhaust makes more power everwhere! All other YZ-250 pipes on the market are not much better than stock! The DEP exhaust feels like a big bore kit! The DEP pipe with the Pro Circuit 304 long silencer has to be good for at least 2.5 horsepower throughout the powerband. The quiet Pro Circuit 304 Factory Sound long muffler helps with top-end overrev. My guess is any long muffler will work the same. The DEP exhaust is more heavy duty than stock. Another note: The European racers say the HGS YZ pipe from the Netherlands is the best you can buy. The HGS exhaust is difficult to find here in the USA. I got the HGS pipe on radar.
YZ DEP Pipe For Sale
Pro Circuit 304 Factory Sound Muffler
Pro Circuit Platinum Pipe
HGS Exhaust Systems Site

Boyesen RAD Valve Kit
The Boyesen RAD Valve adds more power punch to the mid range and top end. In fact the low end power feels crisper too. This mod has to be good for at least 1 horsepower as you can really feel it on a stock bike. We have never tried it, but many say the Moto Tassinari VForce reed cage setup is just as good or better.
Buy Boyesen RAD Valve
Moto Tassinari VForce Reed Cage
RB Designs Head Modification
What a performance bargain! Send your cylinder head to RB Designs with a little money and the solder squish test piece (read more on their website). They will modify the head squish shape for more performance, but you can still run 91 octane pump gas. You may only get an one HP increase. However you get this little boost everywhere in the powerband! Your bike will just run so darn good and will love you back!
RB-Designs Website
Also See PhatHead Racing
K&N YA-2506XD Air Filter
I have always been a fan of foam air filters on dirt bikes. However the K&N air filter has to be good for at least one extra horsepower. You can tell right away something is different as your bike is begging for a jetting change. It flows like you have no filter on your bike at all. After getting the jetting sorted out, your YZ250 really comes alive on top end. I keep an extra K&N YA-2506XD filter in the truck to swap out between motos if the conditions are really dusty. They are easy to clean! For closed course racing, the K&N is a must have. Save your foam filter for trail rides.
K&N YA-2506XD Filter For Sale
AMR Racing Monster Coil
There are some that claim the Monster Coil gains 2 HP over stock. I would be happy with just one little pony. The AMR Racing Monster Coil increases spark 34%. You know 32:1 premix does not burn as well as straight gasoline. Getting more power with 34% hotter spark does make sense. You can feel the difference as throttle response has that "instant power" feeling. It also feels like the engine pulls harder through all the RPM range.
AMR Monster Coil Yamaha Page
Black Ops Moto Coil Site (expensive!)
JD Jetting Kit
The JD Jetting Kit does not really add horsepower, but releases the hidden ponies in your engine. Getting the jetting spot-on really makes the above modifications come alive! The carb jetting ties it all together. So many times I have I seen guys bolt on power parts, and then complain there is no difference.
James Dean Jetting Website
Run ATF In The Transmission
ATF Type F is for older Ford vehicles. The ATF "Type F" specification works the best. I know some of you are already freaking out. I have run automatic transmission fluid in two strokes transmissions since 1975. The factory teams used ATF too. The gearsets and clutch are a big rotating mass, and the thinner weight ATF will add some power. In 4th and 5th gear, you can feel your bike pulling stronger. Shifting in cold weather will improve too. You can abuse your clutch all day long without it singing. The YZ clutch friction plates are a lot like the clutch bands used in car automatic transmissions. Car transmissions take 10 times more abuse than a bike! The ATF will run cooler and provide excellent shifting and clutch performance. Change the ATF every other weekend of racing. I always use quality brands of ATF like Quaker State or Pennzoil. Do not use synthetic ATF! If you do experience some clutch slippage, switch to heavy duty clutch springs.

Powerful as 22LR! 1250fps Pellet Rifles. Starting at $120

Talk about cheap ammo. Pellets are about $12 per 1000. Wow!

Free and Cheap YZ Horsepower Tricks
1. Retard Ignition Timing. The factory CDI unit advances the ignition curve to keep pace with the powervalve. The CDI may advance the ignition too much! There are many YZ guys that say they can get a mid-range horsepower boost just by retarding the timing 1 or 2 degrees. Loosen the screws and turn the stator plate counterclockwise. Just move the stator about 1/16 of an inch using the guess method. Tighten the screws, then try it out. If your bike is harder to start and pings in the mid range, you turned it the wrong way. It may take a few tries to get it right.
2. Use a Thinner Base Gasket. The OEM cylinder base gasket is .025 to .030" thick. There are companies (Moose) that make thinner base gaskets .012 to .015" thick. Using a thinner base gasket raises your compression a little. This will boost power in the mid range and low end without really changing top end power. You may need to adjust powervalve slack when you are done. If you use the retard ignition timing trick above, you should have no problem running pump gas.
3. Hillbilly Pipe Tuning. I picked up this trick from my flat-track racing days. It worked wonders on my 99 YZ. It will add 2 horsepower on top end. Cut 3/4 inch (19mm) out of the header of your stock exhaust pipe and braze it back together. I used a power chop saw for a straight and smooth cut. Make the cut on the straight part between the flange and spring hooks. Make sure there are no burrs on the inside. Weld on the piece you cut out as extra reinforcement. Your bike will lose some low end power, but just scream on top end. You can use some aftermarket pipe spacers to tame the beast if you think you cut too much. When you are done, you may have to fight to get the mount bracket hole lined up. It is best to buy a cheap used pipe on ebay for this trick. To really fine tune, add 3/4 inch length of tube between your expansion chamber and muffler. Better yet, just use a longer muffler.
4. Remove The Reed Block Spacer. Some years of YZ did not have a reed block spacer, but most did. The ones that did't were slightly faster (2001). Removing the reed block spacer will add top end power and you will sacrifice some off the bottom. When you are done, just use one gasket and make sure it don't leak. Do not do this trick on 1998 and older YZ 250 bikes as the reeds will stick in too far.
5. Use Racing Castor Two Cycle Oils. About 90% of the shifter Kart guys run racing castor because it adds power and keeps 16,000 RPM engines together. Racing castor oil is the very best lubricant there is for high RPM two strokes. You will gain horsepower using it! Even with all the modern technology, synthetic ester oils can't come close to castor lubricating strength! About six years ago we put a modified Honda CR500 hillclimb bike on the dyno and got an 3.5 HP gain switching from a popular synthetic blend to Blendzall 460 Racing Castor (with the green lable). We were shocked!
Two years ago we switched to Klotz BeNOL in my son's YZ-144, and there was a huge difference in power. BeNOL has to be good for another 500 RPMs! My boy started pulling holeshots on 250 thumpers! He is now running Blendzall in a stock TM 125 and it pulls harder and revs into neverland.
I just started running Blendzall 460 in my 250, and I can tell a big difference on sand hills at the dunes. It's like a turbocharger kicks in at 7,000 RPM and keeps pulling way past redline! It makes your engine feel like it can never blow-up. Blendzall 460 has to be good for at least a 2 to 3 horesepower gain! When the race season starts, I am going to run it in my bike.
Another positive is you will get 40% more life out of your engine. Racing castor two cycle oil is not for everyone. You will need to clean powervalves about every four races. EasyOff oven cleaner does a quick job! Seafoam also cleans good. Mix your oil and gas the same day you race. Shake your gas can about 10 times before you fuel your bike. Castor don't mix with gasoline very well below 40 degrees F. Use racing castor for racing, not for trail riding. For 250cc run 32:1 ratio. For 85 and 125cc, run 24:1 to 28:1 premix. You may need to dial-in your jetting. Top brands: Blendzall 460 Racing Castor (green lable), Klotz BeNOL, Burris Hi-Rev, Castrol R40, Castrol A747, Motorex Racing GP, Shell Advance Racing M, and Maxima Castor 927. Smells like cooking french fries!
Buy Racing Castor Oil

Adding Some Track Horsepower
Track horsepower does not always show up on the dyno, but helps you go much faster. The track horsepower tips below should be worth at least five horses or more!
Rear Sprocket Swap
Most MX bikes are geared too high for motocross. This also includes the YZ 250. Goin up 2 teeth on the rear sprocket (from 50 to 52T) is like getting a four horsepower boost! No kidding! You can now power through 2nd gear corners in 3rd, catching 4th as you exit the turn! When going up 2 teeth on the rear sprocket, your top speed will still be over 65 MPH! Going up 3 or 4 teeth on the rear does not work so well, as you will be shifting too often to cover the same ground. Sprockets are cheap horsepower! Some of the new steel sprockets are light weight, plus they last twice as long as aluminum. YZ250 Sprockets
TMV 9 Ounce Flywheel Weight
This should have come on the YZ250 from the factory. Back in 2004, all of the team Yamaha pros used a flywheel weight on the YZ 250. With the 9 ounce weight added to the flywheel, you will gain traction without loosing any power. You will notice more drive out of corners, and less chance of locking up the rear wheel during hard breaking. I chose the TMV brand because I know it will fit under the ignition cover. I have used light 8oz and heavy 11oz flywheel weights, and the 9oz feels just perfect. It gives my bike that "European" feel.
Buy Flywheel Weights
Heavy Duty Clutch Springs
I guess any brand of stiffer heavy duty cluch springs will work. I bought some Barnett clutch springs for my 2006. They worked too good. Fanning the clutch coming out of corners, the hookup was too wicked. My solution was to use ony 3 of the Barnett heavy duty springs, along with 3 stock springs. Now the performance was perfect! I can feel a great advantage in loamy and sandy soil. For only $22, this is a bargain performance mod. Note: You can also use the YZ 450F springs. The 450F springs are just like the heavy duty aftermarket clutch springs. Don't mix springs on older models with the five spring clutch. Barnett Clutch Springs For Sale
Huge Rear Tire
Size does matter! Get the biggest tire that will fit. With the performance mods, your YZ250 will make open class power. You need the same size rear tire as the 450 4t. Right now I am running a Pirelli MxExtra-X 110/90-19. You need to get this two stroke muscle to the ground, and a huge tire is the best way to do it! Traction with a two stroke bike is critical. Cheap Motocross Tires

Things to leave alone
1. Don't mess with the power-valve! There are those that claim they can get a few extra horsepower messing with PV shims and springs. The YZ power-valve works perfect the way it is. I bet the same Japanese guy that invented the VCR, designed the YZ-250 power-valve. This engineer really tried to impress us!
2. Don't use an O-ring chain! Avoid the X-ring chain too as they will rob your bike of massive amounts of power. Use the same money to get a premium heavy duty solid roller motocross chain from Regina, Tsubaki or RK. Regina Gold is my long time personal favorite. (520x114 is stock)
3. Don't stack base gaskets and mill the head (hillbilly porting job) In theroy, this trick raises the exhaust port to increase top end power. It don't work unless you can stack 5 or 6 gaskets without leaking. With 2 or 3 OEM base gaskets, you will not feel any real power increase. Save yourself some pain. Years ago someone offered an 1.5mm thick aluminum spacer with two base gaskets that would do the trick. I can not find the source.
4. Don't use an 12T or 13T countershaft sprocket This is too small and could cause extra stress on the transmission shafts and bearings. The smaller sprocket will also cause fast wear on the swingarm chain guard. Better to change gearing with the rear sprocket.

Big Money YZ-250 Horsepower
Those of you that want to build a wicked powerhouse, below is the equipment and mods to get it done.

Eric Gorr Cylinder Porting with Head Work. Choose the More Mid-Range package. For less than $250, many consider this a real bargain. Eric Gorr is who the factory pros turn to for more power. With the above bolt-on mods, this porting/head service should put your Yamaha YZ 250 over 53 horsepower. Another tuner named George Babor at BPM Racing Engines is the wizard of the south. He has a reputation of getting massive power out of MX bikes. It may be worth it to give him a call.
Eric Gorr - Forward Motion Site
Finish Line Engines - Porting
BPM Racing Engines - George Babor On Facebook
Lowell Horning Two Stroke Porting

295 & 300cc Big Bore Kits. Cubic inches is the fastest and most reliable way to more power. I rode an Athena YZ-300 some laps around the track, and it has more power than most mortals need. It was good hard hitting power too! An old guy that races a YZ300 in our local vet class gets holeshots every time, smoking the 450 and 525 thumpers! Eric Gorr also offers an 295cc big bore service using your stock YZ250 cylinder. Eric claims their big bore YZ is more reliable than the stock bike! Depending on the porting and head work, we estimate 52 to 59 HP with a big bore kit. Eddie Sanders is offering 325cc and 345cc kits. I bet they flat out rip! This would be a good excuse to get that Keihin 41.5mm PWK Carburetor! Once you go Big Bore, you will never be satisfied with an 250 again!
Athena Big Bore Kits - Cylinder and Head Included!
Eddie Sanders 325 & 345 Kits - This is BIG!
Lukes Racing YZ-300 Big Bore Kit
Pinsonnault Racing 295 and 305 Big Bore Kits
More YZ250 Big Bore Kits

APT SmarCarb 38mm Cast You need to place your order now and wait. The demand right now far exceeds production. The APT SmarCarb is a big revolution in carburetor technology. People are saying the APT SmarCarb is better than fuel injection! You can adjust jetting on the fly with external knobs. People were gladly paying $750 for the billet version. Now the cast version offers the same performance at half the cost. I also hear the new Lectron Powerjet carburetors are just as good. These carburetors will get every ounce of power out of your engine!
APT SmarCarb Page
Lectron Carburetors Website

Vortex X10 Programmable CDI The X10 CDI allows you to select up to 10 different power map modes. You can really tune your power curve! From dyno test done by Vortex, there is almost an 2 HP gain over stock. This is a simple 10 minute performance mod. All you do is swap out the Yamaha YZ factory CDI ECU unit. Vortex Website

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