I picked up a used KLX 250s powertrain recently to use for a project build. I would like to install a big bore kit, kickstarter, HD clutch springs and rewind the stator to support some electrical add ons. The bike will be likely be 70/30 street/trail. My bike is a stock 2012 KLX250s with about 4,000 miles on it.

I have seen a lot of reviews of people who were very happy with the results of their big bore conversion, but i have also seen two or three who have had issues. Since I have an entire donor engine, I am leaning towards the BB351 and if it does not work out, I can always put the stock motor right back in the bike and not lose much riding time.

I plan to do the work myself. I am not a mechanic; this will be the most involved engine work I have ever done. From the research I have done so far, (reading topic threads and watching videos) it looks like a very doable DIY job. Just take your time and follow the instructions.

Questions for discussion:

1)I did not get a carburetor with the motor, so I need to solve that issue. Should I go with the Bill Blue pumper carb, look for a stock replacement to modify, or just use the one currently on the bike and re jet it as suggested?
2)Where is the best place to send the stator to have it wound? (I plan to add High Output headlight and auxiliary off road lights as well as power outlets for phone and GPS).
3)Which big bore kit should I use? (Stock 300 Kawi, BB331, BB351, Other)
4)Which pipe is the best choice for a BBKit?
5) what is the best source for the kickstarter set up? Is this mod worth the money?
6) The donor engine spins over freely with the starter, should I set up a test stand and confirm that it runs or jump right in to the BB kit and roll with it?

I know this topic has been flogged to death, but I thought I would kick it out there and let the kind and happy warriors of the internet guide me.

 

No idea where you are, but unless it was a really hot arid area I'd do the 351. The big bore will increase the mid range power tremendously. They don't always add to the top end speed, because that is mostly affected by the cams, but the power build is in the mid range where it is most used. Mechanical issues usually fall to the mechanic with a proven kit like the B&B kits. Bill likely has jetting for a stock carb down pat. His 34 probably is darn close. A number of riders have done the Mikuni TM36-68 pumper. I am doing one with a 300 kit. The pipe is probably the least beneficial of all hop ups so I'd say consider sound over performance. Have some respect for people and get one that isn't going to rattle windows when you ride by.

One thing I'd have tried if I had the money would have been a plain TM36-2 flat slide. It is the old non-pumper used on most two strokes and on some four strokes. My SR500 had a Mikuni VM36 on it, the round slide non-pumper. They're way less expensive and the fact that they worked on a lot of older four strokes indicate the carb should work on a KLX and be simpler to deal with when it comes to jetting. Only issue might be fitting, because they are shorter.

 

Only assistance I can offer is on the issue of carburetion.
You have two choices - go experimental, or go non-experimental.
Which you choose depends on whether or not you want to personally "tinker up" proper fueling thru the use of a dyno or perhaps a Wide Band O2 kit. This option will be necessary on all carbs other than the two listed below.
If you do not:
1. The Bill Blue pumper kit will have pretty good fueling with only the need to richen the main jet slightly.
2. A stock carb - using a 351 setup on it - has already been determined, and is easy to replicate.

 

1) The pumper carbs are great. I would go with the BB pumper if you just want an easy bolt on. I'd go with the tm36-68 if you want a bit more top end power. I went with BBs mikuni and am happy with it.

2) Baja designs used to do stator rewinds for the KLX. I don't see it on their site anymore but you could call them.

3) I went BB351 but there are multiple good options. I'd agree with above that most issues probably come from install error.

4) Whatever you can find at this point. I don't think you will see that much performance difference between the aftermarket options. Just note not all slip ons will fit all headers if you start mixing and matching brands.

5) webike.net was the best source I found for the Kick start kit. Note it will say its for earlier bikes, that is because you will need a different RH footpeg mount in order to allow clearance for the kick starter. Find an 06-07 KLX250 or an older KLX300 peg mount if you do the kit. If it's worth it is completely up to the individual. Mine came in handy very soon after I installed it.

6) depends how much time and energy you are willing to expend. I would probably just start tearing down the motor and inspecting it.
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