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Kx250 big bore - I can't leave well enough alone

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Hey guys

A while back I picked up a 99 kx250 for 400. Guy said it had "motor problems". I tore Into it and checked clearances and all I could see wrong was the top end was shot. So I replaced it with a WISeco top end and away I went.

Well I also noticed when I was in the engine that it had a wiseco crank assembly and we all know the reputation of those... I just figured it would either hold up or it wouldn't and either way I would be rebuilding it this winter.

Well it's winter. I have a disease known as Million Dollar Man Syndrome that makes it difficult for me to put things back together without making them bigger, faster, stronger....

I have been looking for info on big bore kits. The problem is that there is so much Info on them it's tough to weed through. I'm looking specifically at the 301 from Kustom Kraft. It seems to be an all inclusive option and that they have a good reputation. Any experiences?

Is there anything different/extra I would need to do wen rebuilding using a big bore that I wouldn't have to do normally? I was planning on buying an OEM crankshaft/rod assembly. Is that ok with a big bore?

And the big question for me is how hard is it to make it run right once it's together? I can jet a carb, but I'm certainly no guru....

What about reliability and longevity? Is it much different than a standard 2t?

I have never done a big bore before. I am a little leary....

Thanks for reading!

Edited by Kawi916
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We built a 04 KX295 early this year and like slowoldfart's friends bike it is smooth and fast. I used a oem crank assy that had a season on it with no issues so far. We tried several different pipes. The stock pipe made the powerband very explosive. The rare Enduro Engineering pipe made it very smooth, similar to a KTM300's powerband. Whats funny is that the EE pipe when used on the same bike when it was a 250 made the powerband pretty aggressive! We settled on a FMF Gnarly which was the best of both previous pipes. Its jetted for 110 cam 2 race fuel and we had the compression bumped up. This is my wifes bike and she loves it! We will be building another one this winter based on a 03 KX250.

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So I'll talk to the manufacturer about the crank and the jetting for that matter. They will probably have a suggestion.

You guys don't have any significant problems then I take it?

This bike could use a replate and a top end anyway. That alone is gonna run $275+. So $525 for the 301 is only ~$250 more. Seems like a cost-worthy investment.

Anyone with experience with Kustom Kraft specifically? I've rrad good things, but that doesn't always tell the whole story.

Zz3gmc: is yours the Eric Gore as well? Good experience with that brand?

Thanks for your input.

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I have the same disease.. I have perfectly good running bikes but always seem to want to add something..


I saw this and thought it was pretty cool.





Edited by hawaiidirtrider
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Thanks. Yes that's an important consideration. Here's a faq from forwardmotion:

Q. Is the price of the 295 pistons really expensive?

A. Click to reveal:

The price of the 295 piston is the same as a 250 piston, plus our custom Wiseco has anti-wear coatings, 2 rings, more surface area to transfer the heat, and a smooth intake to boost the engine’s over-rev. Replacement cost is $160 and average service life is 50-100 engine hours.

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He got back via email. Took almost a week. But as long as I know someone's alive out there I'm happy. If I get in a rush I'll call. I guess I've tried to contact so many companies that just never respond that I'm a little jaded.

Thanks for the info you guys.

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