Yamaha YZ250F Big Bore

Schlava
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Schlava 11/27/2019 7:33am
11/27/2019 7:33am
Emailed cylinder works and they said kit should be available mid December
22Ryann
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22Ryann 11/28/2019 3:23am
11/28/2019 3:23am Edited Date/Time 11/28/2019 3:29am
Dirt Bike TV did some light motor work to this build & had great things to say, think it's only a stock ECU and high comp piston and ported head


sparker245
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sparker245 2/28/2020 12:41pm
2/28/2020 12:41pm
Well, the Cylinder Works 270cc Big Bore Kit is finally available for the '19/'20 Yamaha YZ250f. Got mine last week and rode it for the first time at Prairie City (Hangtown National Track) yesterday.

Clearly more low end and mid range, which, of course isn't surprising. Pulled a one tooth smaller sprocket on the back easily even with some track elevation change. No loss of revs....and the YZF revs a lot. Other than a FMF slip on and the Travis Preston Map installed, my bike is stock. Have to admit that I was looking for a little more mid range hit, but the power is really good but linear.

Have a '19 KTM 350sfx also, and although the YZF big bore is an improvement from stock and fun to ride, don't expect it to be equal to a 350. Gonna try some mapping changes this weekend since it clearly need more fuel (some popping on decel). Don't want to go all out on engine mods, so I need to accept that for a relatively inexpensive mod the results were positive.
2
TSCHAM101
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TSCHAM101 2/28/2020 1:08pm
2/28/2020 1:08pm
Jamie at Twisted development built my YZ270F.. with some port work and a Vortex and the thing is a blast to ride.. i enjoy it alot.. the thing is a monster, and with good technique i wont have a problem off the gate with the 450's unless its a really long start like Glen Helen then i might get pulled.. but most people in vet classes dont really know how to start so i can still get holeshots..

But honestly, if money were no option, i would have gone with the KTM 350 and then put KYB suspension components on it. the only reason being is i cant take a 250f to glamis without running into heating issues and i dont want to grenade my bike.. the 270 is phenomenal at the track.. i believe it pulls about 24ft.lbs of torque, which is a significant difference over stock at around 20 ft,lbs of torque.

The Shop

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HondaMan66
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HondaMan66 2/28/2020 1:39pm
2/28/2020 1:39pm
racerx217 wrote:
I had a 290 and just like the other guy said good bottom end but nothing up top. Then i did a 269 cylinder and a...
I had a 290 and just like the other guy said good bottom end but nothing up top. Then i did a 269 cylinder and a +3mm long rod that made it to be a 285cc. That was a really good setup. In all reality though if i could take it back I would had not done either bike and just got a 450 earlier in life. Tons of money wasted on mods that made the bike unreliable. Both bikes blew up with about 15-20 hours on them. If you need reliable faster bike get a 250 smoker or a 450.
A long rod does not increase displacement. I think you meant to say a stroked crank. The rod could be a foot longer and the displacement does not change. Just realized this in an old post. Never mind.
PTshox
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PTshox 2/28/2020 3:47pm
2/28/2020 3:47pm
I plan on doing one of these (2019) BigBore kits this year. Who do you folks like for mild porting and ecu remaps on the BB'?
cwtoyota
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cwtoyota 2/28/2020 6:00pm
2/28/2020 6:00pm
Vet57 wrote:
Currently have an 18' and wondering what impressions the 270cc kit did for this bike? Anybody do this mod and how did you like it versus...
Currently have an 18' and wondering what impressions the 270cc kit did for this bike? Anybody do this mod and how did you like it versus stock displacement. Thx
It can go either way. I rode a 270cc Yamaha in some sand dunes where you really notice the difference in power.
He had camshafts and some other mods that made the bike run pretty well.

On the other hand, if you just install a big bore kit without doing other work to correct the power curve, you can often end up with a bike that makes a ton of torque through the mid range and runs out of RPM very quickly.

If you plan to do a whole proven engine package, it might be worth it. If you plan to just switch out a cylinder, maybe a 450F or a 350SX-F would be a better choice.
NeedMoto
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NeedMoto 2/28/2020 6:53pm
2/28/2020 6:53pm
PTshox wrote:
I plan on doing one of these (2019) BigBore kits this year. Who do you folks like for mild porting and ecu remaps on the BB'?
Jamie at Twisted, hands down!!!
2
sparker245
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sparker245 3/27/2020 9:58pm
3/27/2020 9:58pm
sparker245 wrote:
Well, the Cylinder Works 270cc Big Bore Kit is finally available for the '19/'20 Yamaha YZ250f. Got mine last week and rode it for the first...
Well, the Cylinder Works 270cc Big Bore Kit is finally available for the '19/'20 Yamaha YZ250f. Got mine last week and rode it for the first time at Prairie City (Hangtown National Track) yesterday.

Clearly more low end and mid range, which, of course isn't surprising. Pulled a one tooth smaller sprocket on the back easily even with some track elevation change. No loss of revs....and the YZF revs a lot. Other than a FMF slip on and the Travis Preston Map installed, my bike is stock. Have to admit that I was looking for a little more mid range hit, but the power is really good but linear.

Have a '19 KTM 350sfx also, and although the YZF big bore is an improvement from stock and fun to ride, don't expect it to be equal to a 350. Gonna try some mapping changes this weekend since it clearly need more fuel (some popping on decel). Don't want to go all out on engine mods, so I need to accept that for a relatively inexpensive mod the results were positive.
Rode my '19 YZ250f (270) today for the first time with the ECU remapped by Twisted Development. Amazing! Much better in every way.
kage173
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kage173 3/28/2020 7:41am
3/28/2020 7:41am
350
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sandtrack315
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sandtrack315 3/29/2020 10:51am
3/29/2020 10:51am
I’d get some head work, exhaust, ECU, run ras gas, and call it a day. I wouldn’t want to try to sell a 250F with a big bore kit.
trailriderxc
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trailriderxc 4/11/2020 4:11pm
4/11/2020 4:11pm
hubbardmx50 wrote:
My buddy actually just did this to his 2015 and claims it he loves it. He was back and forth on just getting a new leftover...
My buddy actually just did this to his 2015 and claims it he loves it. He was back and forth on just getting a new leftover bike or doing the big bore kit but decided to do the big bore. I believe he used an Athena cylinder and also fitted a new Yosh system to go along.
The big bore kits are awesome if they're done right you have to send the head off to the machine shop and make sure is milled perfectly flat ported change the cams add a pipe Tokyo mods ignition and you will have a monster and it will be reliable.the biggest issue with big boring a carbureted bike is the jetting if you get the jetting right with the other mods it will rev out. With the fuel injection bikes use the tokyomods ignition and injection controller and you have the same results.
walent215
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walent215 4/12/2020 4:45am
4/12/2020 4:45am
Talisker wrote:
Never really understood this. Your going to spend a lot of money, spend time, and loose some reliability. Not to mentioned if not properly set up...
Never really understood this. Your going to spend a lot of money, spend time, and loose some reliability. Not to mentioned if not properly set up will be just as slow, if not slower, that a stock 250F. Just buy a 350 and be done.
I would much prefer to be riding my bike, getting better on it, that constantly working on it. In my opinion, if you are looking for more bottom on a 250F you need to work on your corner speed. Spend money on lessons and practice, practice, practice corners until they become natural.
thephoenix wrote:
What you are forgetting is that the rotating mass and gyroscopic behavior of that 270 is going to be a lot less. That translates into a...
What you are forgetting is that the rotating mass and gyroscopic behavior of that 270 is going to be a lot less. That translates into a lighter feel. Flicking the bike into and out of corners. Like a 125 that can smoke a 250 or 500 back in the day (think O'mara at des nations).
rob162 wrote:
Not only that but you really do not lose any reliability doing a big bore and i have never had a big bore, a reasonable big...
Not only that but you really do not lose any reliability doing a big bore and i have never had a big bore, a reasonable big bore i should say, come out worse then a stock motor.


For reference my oldest BB build is a built 08 kx250f that's a 269 cylinder works with high compression piston, cylinder head done by Jesse Williams flat face valves dimpled backsides, pc cams and springs built to to get every ounce of useable hp out of the head. Crank i did when i worked at crank works knife edged, lightend and balanced to new piston using a hot rods rod with nj roller crank bearings Barrnett clutch basket hinson hub and plate, carb was done by dicks racing his full mod list. A custom built and tuned MRD exhaust system and a work ignition I sold my soul to get to help pull it all together spinning at 14,000 rpm. Running on 3 gallons leaded race gas to 2 gallons premium pump. This bike hauls ass.
An we have over 175 hours on this engine spec. Only thing we have had to replace is clutch packs, 2 re-rings, 2 piston and 1 set of valves with recut seats. Everything else has been perfect.
I've also did for a friend a crf269 he's got 50+hours on it all stock and everything's perfect.
Do you recall your jetting specs on the 08 BB? I have a friend that just did an 08 kx250f Bb and he is having a heck of a time jetting it, even after sending his carb to tokyo mods!
trailriderxc
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trailriderxc 4/12/2020 6:03am
4/12/2020 6:03am
thephoenix wrote:
What you are forgetting is that the rotating mass and gyroscopic behavior of that 270 is going to be a lot less. That translates into a...
What you are forgetting is that the rotating mass and gyroscopic behavior of that 270 is going to be a lot less. That translates into a lighter feel. Flicking the bike into and out of corners. Like a 125 that can smoke a 250 or 500 back in the day (think O'mara at des nations).
rob162 wrote:
Not only that but you really do not lose any reliability doing a big bore and i have never had a big bore, a reasonable big...
Not only that but you really do not lose any reliability doing a big bore and i have never had a big bore, a reasonable big bore i should say, come out worse then a stock motor.


For reference my oldest BB build is a built 08 kx250f that's a 269 cylinder works with high compression piston, cylinder head done by Jesse Williams flat face valves dimpled backsides, pc cams and springs built to to get every ounce of useable hp out of the head. Crank i did when i worked at crank works knife edged, lightend and balanced to new piston using a hot rods rod with nj roller crank bearings Barrnett clutch basket hinson hub and plate, carb was done by dicks racing his full mod list. A custom built and tuned MRD exhaust system and a work ignition I sold my soul to get to help pull it all together spinning at 14,000 rpm. Running on 3 gallons leaded race gas to 2 gallons premium pump. This bike hauls ass.
An we have over 175 hours on this engine spec. Only thing we have had to replace is clutch packs, 2 re-rings, 2 piston and 1 set of valves with recut seats. Everything else has been perfect.
I've also did for a friend a crf269 he's got 50+hours on it all stock and everything's perfect.
walent215 wrote:
Do you recall your jetting specs on the 08 BB? I have a friend that just did an 08 kx250f Bb and he is having a...
Do you recall your jetting specs on the 08 BB? I have a friend that just did an 08 kx250f Bb and he is having a heck of a time jetting it, even after sending his carb to tokyo mods!
40 pilot jet 165 main a boyesen adjustable leak jet or they call it mechanical injection... I know it sounds too lean it's not trust me it makes a world of difference
1
walent215
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walent215 4/12/2020 6:25am
4/12/2020 6:25am
rob162 wrote:
Not only that but you really do not lose any reliability doing a big bore and i have never had a big bore, a reasonable big...
Not only that but you really do not lose any reliability doing a big bore and i have never had a big bore, a reasonable big bore i should say, come out worse then a stock motor.


For reference my oldest BB build is a built 08 kx250f that's a 269 cylinder works with high compression piston, cylinder head done by Jesse Williams flat face valves dimpled backsides, pc cams and springs built to to get every ounce of useable hp out of the head. Crank i did when i worked at crank works knife edged, lightend and balanced to new piston using a hot rods rod with nj roller crank bearings Barrnett clutch basket hinson hub and plate, carb was done by dicks racing his full mod list. A custom built and tuned MRD exhaust system and a work ignition I sold my soul to get to help pull it all together spinning at 14,000 rpm. Running on 3 gallons leaded race gas to 2 gallons premium pump. This bike hauls ass.
An we have over 175 hours on this engine spec. Only thing we have had to replace is clutch packs, 2 re-rings, 2 piston and 1 set of valves with recut seats. Everything else has been perfect.
I've also did for a friend a crf269 he's got 50+hours on it all stock and everything's perfect.
walent215 wrote:
Do you recall your jetting specs on the 08 BB? I have a friend that just did an 08 kx250f Bb and he is having a...
Do you recall your jetting specs on the 08 BB? I have a friend that just did an 08 kx250f Bb and he is having a heck of a time jetting it, even after sending his carb to tokyo mods!
trailriderxc wrote:
40 pilot jet 165 main a boyesen adjustable leak jet or they call it mechanical injection... I know it sounds too lean it's not trust me...
40 pilot jet 165 main a boyesen adjustable leak jet or they call it mechanical injection... I know it sounds too lean it's not trust me it makes a world of difference
Thanks alot bud I just sent this over to him!
trailriderxc
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trailriderxc 4/12/2020 6:25am
4/12/2020 6:25am
40 pilot jet 165 main a boyesen adjustable leak jet or they call it mechanical injection... I know it sounds too lean it's not trust me it makes a world of difference

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