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450 Carb on a 250 Big Bore

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So I punched some quick numbers as I'm getting ready to big bore (290cc) my WR250F this winter and found that the increase in displacement is 16% and the area opening increase between the 39mm FCR and the 37mm is 11%. Is it worth bolting on a 39mm or 40mm carb (or boring the stock carb) with a big bore kit for the top end's sake? Or will the intake and exhaust ports ultimately restrict the flow? FYI, I will also be running Stage 1 intake and Stage 2 exhaust cams with a full FMF exhaust, (debating a port job as well).

Any good knowledge is appreciated!

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I agree. They had to add an accelerator pump to this carb to make it work with a 250. There are jetting recommendations for a stock carb on a 290, not the 450's carb on a 290. You would really be out in left field, but it would be interresting to learn and watch you go thru the pain!! LOL

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The jetting would be a trial and error experiment. However, I think you could guess reasonably close to start. For example, a 40mm carb is 16% more area than the stock 37mm, which matches the increase in displacement (16%). This means that the velocity through the carb would basically be equal to the original 250 at any throttle opening. If a proportional amount of air and fuel is required for the larger displacement, I think you would essentially use the same jets as the velocity creating the venturi effect across the jets would be equivalent. Am I way off base with this?!? It would probably end up more trouble than it's worth, but thought it would be an interesting discussion. As a side note, I see the new 2012 YZ250F is running the 39mm carb.

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Send the carb to Jesse Williams at williamsmotorwerx.net and tell him what you did to your bike and how you ride and he can hook you up.

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The '12 yzf250 has a 39mm carb. The Starr Racing yam's run the 39mm carb.

I think this is a good idea. If you couple the larger carb with more agressive cams' and a mild port job, the 290 should be able to get some of the rev back.

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I bore them to 38.5mm.

Porting is a very good idea with big bores, on top of bored carb.

Jason, bike all good since last transmission update?

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Yup, shifts as smooth as butter :busted: I'm really spoiled by my bike now, it's so powerful that when I hop on a stock yz250f it feels like a slow pig with a flat tire.

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Absolutely. Never again will I buy aftermarket motor mods and just unknowingly toss them at a motor. You can't just bake a cake by throwing whatever is in your cup board into a mixer, it take's a recipe, or a chef.

Every bike I get from here on out will have your motor package's.

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Thanks Jason!

So you've done that before i take it?

Most think its too expensive, and try to piece together a mod bike, if done right you can get the best bang for buck working with a good performance shop!

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I agree. They had to add an accelerator pump to this carb to make it work with a 250. There are jetting recommendations for a stock carb on a 290, not the 450's carb on a 290. You would really be out in left field, but it would be interresting to learn and watch you go thru the pain!! LOL
Don't forget about your carb manifold and airbox boot. Might be the hardest part of the project.

Note that all FCR carbs have an accelerator pump. It should not be a pain at all jetting the larger carb.

All the FCR carbs have the same basic body and the same external dimensions so installing a larger bore carb will not be a problem.

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Jesse, it's good to see you back on this sight as you disappeared for a long time. I guess those damn customers were keeping you away from the 'puter!

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Yea, I'm trying to be more interactive. This has always been my favorite forum... but summer seems a little slow around forum. More fun when people like to talk about bikes because they aren't riding them!

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if you are going to port your head,i strongly advise you to have someone who has alot of experience with 5valve yammie heads do the work.you can really mess one up if done incorrectly.the 5 valve heads flow characteristics are like no other,especially the 2010 and up 250f yammies.beware!!!

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