Hi all,

I see there are many threads arround Big Bore and most of the success stories are based on carburetor version of the KLX250. I'd like to start a thread with EFI owners who actually tried to install Big Bore kits available on the market and either succeeded or failed. Please, tell your story and share it with world!

My story is the following:
Installed Bill Blue 351 big bore kit with EJK tuner that came with it and couldn't get it working right. Power delivery is not quite smooth, startup issues occur (hard to start up, might need more powerful battery which there aren't a lot in the market to fit KLX 250S), but the biggest problem - bike stalls when revving up to 6-10k rpms and dropping the throttle. It's not in condition to ride and I am considering either going back to original OEM 250cc setup or ordering one of the 292cc kits and try again.

What's your story? Is anyone able to get any kind of Big Bore kit to work flawlessly and what are the mods you needed for it?

 

I rode a Honda Grom 125 for years and was (and still am) active on the forum. It was full of stories of big bore engines that blew up. It taught me that if I want a bigger engine, buy a bike that comes with a bigger engine.

 

I used to think that way, but learning more and actual experience changed my opinion.

Only reason for most big bore breakage is human error. I have a big bore KLX650 that has 50,000 miles on it. I know there are several riders here running 300, 331, and 351 big bores for quite a long time. The only time there seems to be problems are when big bores have other performance adding features, like high compression. The other is when someone just plain screws up. High compression does seem to affect reliability a bit.. Putting a Vulcan 1500 piston in the KLX650 did not significantly change compression, so all it was is the same thing as doing a rebuild with an oversize piston. Same with putting a 300 kit on a KLX250. Just ask @TNC about his experience. Not everyone has the skill to do it right and it seems those are the ones that break.

I've seen a number of multi-cylinder big bores that work as well as OEM when done professionally. Fact is some OEM bigger engines are done by enlarging bores - you know, like your 300.

 

You said the right word... "professionally".

I think the most common mistake is when the strength of the bottom end of the engine doesn't match the increased force of a larger piston.

The 300 has a stronger connecting rod, bearings, and crank to handle the increased force exerted by the larger piston. There's no problem even full out to redline when everything in the engine matches the strength of everything else.

( My bike on the dyno )

 

You know that for a fact? Or is it that the 250 had a gear on the crankshaft end for the electric start and maybe that there is a bit of weight difference for the 300?

 

Just what I read. I also looked up the OEM part numbers. They're different cranks. For example, KLX300 crankshafts are #13031-0936 while KLX250 crankshafts are #13031-0757.

All I know about weight is what the specs say. Mine's a "California Smog" bike so it's 2 pounds heavier at 304. I removed 16 pounds and added a 3 pound rack for a net weight loss of 13 pounds.

 

I would expect the number to be different since there should be some minor differences in weight to deal with a bit more weight of the piston/rings/pin having more material.

I will say no one here that I've read about ever had a crank break running any of the big bores. So not too concerned with that.

 

[QUOTE=klx678;553564]I would expect the number to be different since there should be some minor differences in weight to deal with a bit more weight of the piston/rings/pin having more material.

Both the 250 and 300 crankshafts are listed as including a matching connecting rod. Many of the Grom big bore failures were rod related.