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2008 KX250f performance question

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Hey

I just built a 2008 kX250F supermoto race bike. The crank and bearings were just replaced with hotrod stuff and a stock piston and rings was added. The head was not touched. I rejetted using the Team green needle and specs. It starts and runs fine with a great linear power delivery. Almost too smooth if you ask me and it seems to sign off quickly at the top. This is my first kawi dirtbike. It has a Hinson slipper but i don't think that is causing a problem because other 250's were not pulling away down the straights. Just curious if anyone has a suggestion for better top end. Oh and it has a ti powerbomb header and a Q4 can, yes I know but we have sound resrtictions at our tracks.

I was thinking about cams but I am not sure. I rode my buddies 07 YZ250F with a web cams and hi comp piston...same top speed perhaps a titch faster but the power band comes on with a hit, my kawi is simply flat. Curious if that is normal.

Cheers

Ross

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Since it is a Supermoto I am running 13-45 right now...I will bump it up to a 48 rear for the next session. I am thinking I will try the stiffer slipper clutch setting to see if that helps along with dropping to a 170 main jet. I am hoping to get it on the dyno this week time permitting.

thanks

Ross

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Good head porting will help substantially...

and there are cams available that produce great power...

So all depends on how much you want to spend and how fast you want to go.

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Sorry I am just curious as I had not ridden a KX250F previously, is the power delivery usually that linear, ie: not much of a hit...a smooth acceleration, almost too smooth. I know the possibilities are endless but I am just curious.

Thanks

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My guess would be the gearing. and maybe check the jetting again. I assume you have already checked valves..... I know you super moto guys need a lot of top end and I hvae never tried that combo. I can't imagine an 07 YZF being faster with the same mods. The KXF has a killer engine stock. Did you try asking on super moto junkie?

my .02

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The motor was just rebuilt except the valves as they were fine and in spec. Yes we need a ton of top end thus I am thinking of going with stage 2 hot cams.

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I checked the valves tonight...all in spec...right between the min and max. As I said before the crank, piston rod are all new...head was not damaged. Air filter is clean and sealed along with the carb.I am now going to check the Hinson slipper clutch....I will pull the clutch plates to see if there are wear or burn marks on the steel plates.

Anyone else have any suggestions...it is running well I was just expecting (hoping) for more of a hit.

Cheers

Ross

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An engine designed for motocross will not be the ultimate SM weapon.

In MX we want long, smooth power delivery that doesn´t need to be shifted a lot. It´s a typical midrange engine, yes it´s considered to have good top-end, but that is compared to other MX bikes.

On the road you want peak horses and are always free to shift into next gear.

You have a lot of work ahead of you to successfully shift the powerband higher and not loose all flexibility of the engine.

Remember it´s a small engine that really is going head to head with 125´s.

That´s why most SuMo´s are 450 and bigger.

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Hey d-k. Yes I know the popular sumos are the 450. I ran those for the last two years and wanted to try something different and lighter. I think the stage 2 hot cams will help me out with the top end. Anyone chime in with experience. As for jetting I am running the team green settings with their suggested needle, 175 main. 42 pilot. It runs great.

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If it was me the first thing i would do is get the head ported for SM, all your bolt on stuff doesnt add up to much when compared to a good port job.

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Agreed. This bike for sm needs some major work to get it comprible to what youre used to with the 450s.

You should get it ported and add a higher comp piston and probably a more agressive cams other then Hot cams. I would even consider an ignition.

If you have money might even consider shaving the head to raise comp again and running race fuel.

Realy in the world of supermoto power is everything not allways weight.

You should also consider replacing the bearings with ceramic bearings.

You have alot of money to spend to get this thing to compete with the power of the 450s.

Good luck

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pu it on the dyno yesterday....cold and raining so it may be off...32.8 the first two and then 32.6 and 32.5 HP...couldn't do the Air/fuel test due to the screen in the Q4...wondering if the stock exhuast would breathe better than the Q4...any suggestions...

I am running the NCYS needle (TG), 4th clip, 42 pilot and 168 main. A slight hicup at 11000 RPM...clip??? sea level elevation.

cheers

Ross

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