I own a 2013 KLX 250s, I have 9900 KM on it since I bought it new, I love it, for town use. My problem is I do a lot of highway KM (1hr - 2hr rides with plenty of stops) and the KLX, well, is not fast enough out of the box for highway. At 90 KM/hr I rev up at 6000 RPM which is pretty high. So I geared up to 15t front, it goes 95 km/hr at 6000 RPM, which is still not enough, I would like 100 KM/hr at like 5000 RPM max, so I thought of a big bore. Anybody can suggest me a big bore and if they go good on the highway? Thanks! I was looking at the 351 Big bore, but is that enough for highway riding?

Thanks.

Current bikes :

2013 KLX250s, FMF Powercore4, FMF Megabomb, Uni Air filter, stock motor 15tf.

 

Little singles need to rev to make any power-my 351 buzzes along at 7k+ for hours without any complaints. I think that your goal of 5k at highway speed is only going to happen on flat ground with a draft behind a semi. If low rpm cruising is your goal than you have the wrong machine-a big twin would be a better fit.

 

My 650 thumper is turning 4000rpm @ 100km/hr. I think with a big bore kit you may get it to run @ 5000rpm, but if a hill comes, you'll probably need to shift to 5th.

 

Its the gearing, bore change wont make it rev any lower just accelerate faster.

 

I was like you and wanted a bike that was slightly more highway capable.

Stock bike wont handle a taller gearing very well, but you can big bore the bike and then up the gearing to give you a few more km/h tops. (google gearing commander and play around with it). A big bore in itself will do the same rpm/speed as your old bike, no difference there.

This bike will never be a good highway bike and when I got the big bore, I fell so inlove with the increased power that I would just feel bad about gearing it up again. I kept the stock gearing and now I just accept that I am slow on highway because I am so damn fast off the highway that its worth it.

I think the bike sounds fine at 7000rpm/110km/h.

 

The KLX is also a bit on the light side to be spending much time on a highway at higher speeds. More mass would feel better/safer.

 

I had the same goal as you, got an amazing deal on a 351 kit (literally $200 new after core and carb sell) and it does accelerate quicker and it is a bit unusuable since I have a 13 tooth front sprocket (runs out of gear WAY too fast). If you want to run highway go get a 650. I'm going to end up needing a 650 for what I do, 80% road(40miles daily commute), 20% dirt (fast open desert on the weekends). And I like to carry a passenger sometimes, although better with the 351, the klx is really a single man's bike so to speak.

 

sounds like you need a bigger bike unfortunately.

 

That is what I was going to say.

 

I'll just chime in to say that with a 351 and 15 tooth I don't have any problems cruising at 100 kph (62 mph) with plenty of power to spare for passing or climbing. I wouldn't worry a bit about spinning the motor at the resulting ~6200 rpm.

That being said, I've never owned a 'big' bike and I'm sure they are much more pleasant for long, high speed runs... but if your main concern is that you're over-revving the engine, I wouldn't worry about it.

As others mentioned, going to big bore won't lower your revs, but it does make the bike seem a little less frantic at higher speeds, just because you don't have the throttle held WFO.