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Big bore for a 250 xcf-w or not

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Im a ok rider but I want a new bike but im getting older so going balls out is not the only way to ride for me now. I'm on a 1998 250 ktm mxc 2 stroke but love the bottom end on that bike. im 180 lbs 5' 11" and i just dont want to get some thing that will tear me up but i dont want driving Mrs. Dasiy ether.

I want a fast but light front end but lot of bottom end to do tech. stuff on a single track so 2007 ktm 250xcf-w 4 stroke with the 320 thumper kit or what?

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Ive got an 07 with a 300 kit, helps the bottom quite a bit, but it still is not a 2st 250 power wise. Got it to have a 4st with a plate to ride in the forest. good bike, just not a power house.

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I don't believe in big bore kits. Your carb, cooling system and crankcase are designed for the dispacement your bikes comes with stock. Sure, you can rejet the bike and put an aftermarket pipe on it, but everything on your machine is designed to get the most out of your displacement. You you go screwing with the displacement then your bike becomes way more finiky and picky and a pain in the ass. I don't know anyone who has raced a big bore kit throughout a whole year and had good things to say about it.

look into a newer bike. A ktm 300 would be what your looking for, 08 or newer

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I am loving my TR 2007 320 XCF-W! I just had to rejet and it has way more grunt than the 250 and still runs out to a great top end. I am not a racer, but keep a pretty good pace on single track. Cooling system is keeping up just fine.

Cheers,

MrHix

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I don't believe in big bore kits. Your carb, cooling system and crankcase are designed for the displacement your bikes comes with stock. Sure, you can rejet the bike and put an aftermarket pipe on it, but everything on your machine is designed to get the most out of your displacement. You go screwing with the displacement then your bike becomes way more finiky and picky and a pain in the ass. I don't know anyone who has raced a big bore kit throughout a whole year and had good things to say about it.

look into a newer bike.

Not true at all.

My super strong '08 HT 350 XCF-W has always been very reliable, and has been as 'un-finiky' as any of my big bore RFS bikes. I have ridden this bike for over 3 years and have 6,625 trouble-free miles (315 hrs) on it. The engine has never been touched and still runs as strong as ever, plus the valves have never required re-shimming. Many of these big bore 250's have been successful and trouble-free in racing.

Edited by Chas_M

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meh, still not a believer. I've still heard more big bore horror stories than ones that worked out. Any time your removing metal from a cylinder and bringing the walls closer to the water jackets is comprimising it's durability.

and you still can't argue that these race machines are completely designed around the displacement they have from the factory. There's always someone who has good results and always someone who has horror stories over the same thing, but if I want more displacement, I'll buy the whole package and not just a larger cylinder. KTM offeres every power characteristic imaginable and they do so with all the testing and refinments nessesary before they sell it. If I had deep pockets and needed to push the limitations of the class I race in to be competitive, I would bore my bike out. But I'm not an xc2 racer or pro motocrosser, so I'll leave my bikes stock and reliable and learn how to ride instead of trying to make up my time in the straights

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meh, still not a believer. I've still heard more big bore horror stories than ones that worked out. Any time your removing metal from a cylinder and bringing the walls closer to the water jackets is comprimising it's durability.

and you still can't argue that these race machines are completely designed around the displacement they have from the factory. There's always someone who has good results and always someone who has horror stories over the same thing, but if I want more displacement, I'll buy the whole package and not just a larger cylinder. KTM offeres every power characteristic imaginable and they do so with all the testing and refinments nessesary before they sell it. If I had deep pockets and needed to push the limitations of the class I race in to be competitive, I would bore my bike out. But I'm not an xc2 racer or pro motocrosser, so I'll leave my bikes stock and reliable and learn how to ride instead of trying to make up my time in the straights

I've put a Thumper 300 kit on my 250excf and it has transformed the bike. I didn't do anything to the head/cams and still get 39HP at the rear wheel and more importantly (for me) is it's now got low end power, much much more than before. It's made it a lot more fun to ride.

It's not all about the extra power that a big bore gives you, infact i wasn't interested in more top end power, just more bottom and mid power that the original engine lacked.

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well maybe I can't argue with this 300 kit. I still don't like the idea but if it's working for everyone here that has one, then I guess it's legit. Maybe it's cuz ktm makes all their own big bore kits that seem to work better than the kits you get out of a motosport mag. I still wouldn't do it, but I guess there's proof here it's ok

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There is plenty of evidence that these big bore 250 engines last way longer than their 250 counterparts. The main reason for this is that the big bore engines provide plenty of power and torque at much lower RPMs without the necessity of constantly bouncing off the rev limiter.

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Like all things.... We get what we pay for. I've had great success with a 300 big bore kit from thumperracing.net. It's installed on an '08 250 XCF-W.

The cylinder's machined water ports really improved the cooling. We ride when it's 100 degrees out here and it didn't overheat like it did last summer.

I agree with another statement that the bike doesn't have to work as hard as it did when it was stock.

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I agree with another statement that the bike doesn't have to work as hard as it did when it was stock.

you can short shift it and still make progress, something that you couldn't do on the stocker!

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meh, still not a believer. I've still heard more big bore horror stories than ones that worked out. Any time your removing metal from a cylinder and bringing the walls closer to the water jackets is comprimising it's durability.

and you still can't argue that these race machines are completely designed around the displacement they have from the factory. There's always someone who has good results and always someone who has horror stories over the same thing, but if I want more displacement, I'll buy the whole package and not just a larger cylinder. KTM offeres every power characteristic imaginable and they do so with all the testing and refinments nessesary before they sell it. If I had deep pockets and needed to push the limitations of the class I race in to be competitive, I would bore my bike out. But I'm not an xc2 racer or pro motocrosser, so I'll leave my bikes stock and reliable and learn how to ride instead of trying to make up my time in the straights

Its time to start BELIEVING.

My son has a 250 sxf that he races in hare scrambles. Full season of racing with a Athena 280 kit. running great. The lower end is amazing.

I agree that the bike was built around the 250 displacement- And any time you add hp to any motor you are putting more stress on it. But I see my son using the added hp at a lower rpm. so in the end I think it equals out.

I also think guys install a big bore kit after they blow up the bike. But all they did was replace the cylinder and piston. The crank will have 1or 2 season of racing on it. The the big bore finishes it off.

And all the horror stories are all over the Internet- but the successful ones are not posted 100 times.

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