I have recently completed the installation of a Bill Blue 331 kit in my 2009 KLX 250S. IIRC, the factory ignition-cut rev limiter comes in at 10,500 RPM on this bike. I am wondering if reving the 331 too high could cause any damage. I don't imagine it really would; seeing as how everything in the cylinder head is the same I'd think it would be ok for short bursts at least...? I've had it up to nine grand a few times and it seems okay. Haven't taken it past that yet for fear of damaging something in my nice fresh motor lol... Hoping someone with more knowledge than myself can tell me if I can sit there in second or third gear and just hold the throttle wide open and let it rev to the moon, or is something going to let go in there? With all the money and time I've put into this thing, I sure would hate to grenade the motor... Thanks in advance.


EDIT: For the Google researchers out there, I've since made the determination that 9,000 RPM is the "red line" of the Bill Blue KLX 331S engine (come to think of it, I think that was what he had "guessed" it would rev out to when I asked him about this, before I purchased the 331 kit...), but the engine really isn't comfortable above about 8,000 RPM, so I don't go above that unless I really need to.

 

You can beat it just as hard with the big bore kit as without it. I would not worry about it at all. Just make sure you change oil on a regular basis. Remember though that sometimes revving it to the limiter is pointless, considering the bike makes most of it power around ~7500 RPM.

 

True. Especially with more displacement, there's just no need to rev to the moon. The head and valve train is just as good after the bigger bore as before, so no real concern there. On the increase in rotating mass with a bigger bore, it would be good to see actual piston weights for a real comparison, and the higher compression "might" have a little impact on engine stress at high rpm. Still, all of this is probably well within the design durability of the KLX engine. The lower end of the KLX is quite tough. Though not concerned, I would be curious if there might be any effect on the engine due to the balancer assembly perhaps not being "tuned" to the bigger bore if any notable weight in the rotating mass occurs. I'm no engineer and claim no knowledge on that issue.

 

i believe there is less reciprocating mass, at least on my cycle pro kit. My 331 piston was noticeably lighter than the stock slug. i wish now i would have weighed them side by side.

 

All I noticed was increased vibration at idle, and when I'm in the desert spinning the rear tire (no wind noise so I can hear it) around 9k my bike makes a pinging noise(extremely fast), like someone hitting a metal pipe with a rock. No damage as of yet, doesn't sound like piston knock, nor is it detonation either (spark plug is in good shape).

 

Interesting. I guess if there's any issue with the balancer assembly not being optimum for the new rotating mass question, I guess it would apply to an assembly being too heavy or too light. Still, like I said, I am no authority on it whatsoever. I tried doing some reading on motorcycle balancer assemblies and how an engine might be affected by a notable change in rotating mass but didn't find anything definitive. The whole thing might be a moot point. I was just curious and not making any statements of fact. You know what they say about "a little knowledge".

 

Yea I know the bike makes power down low and agree there wouldn't usually ever be any point in revving it past 7,500, or even past about 5 or 6 thousand as far as power output is concerned. However, there will be times off road when revving to the moon will definitely be beneficial. There will be situations when doing gnarly hill climbs where second gear (even with the big bore) is too tall to get going at the base of the hill (example big ruts, rocks in the way...), but where you have to be able to go reasonably fast to make it up the latter parts of the hill, in which case first gear would need to be used. I know there is a clutch lol but sometimes having to slip the clutch can take away from your ability to keep your momentum and stability/tracking in order as well as you could with a solid grip on the bar and not being on the clutch. In these cases, first gear will be low enough to do the super tough, really low speed stuff at the bottom, and if you rev it to the moon, can still get up enough speed to do the latter part of such a hill. As in, being able to go 30 mph in first gear, vs like 24 mph if you don't totally rev it out... By the way I don't like doing down shifting from second to first half way through a hill climb, either. Bad things happen, if you know what I mean lol. Of course the same basic concept applies to second/third gear. Second gear power with ~45 mph if you rev, vs ~35 mph if you don't (where third gear is too tall at the base, but you need to HAUL the freakin mail to make the whole hill). I know these situations will be a.) very rare, and b.) for the most part possible to work around. I know second gear has plenty of torque and should be workable for 99% of hill climbs. I just think of every possibility.

 

Anyway thanks for the responses. Although the cylinder head (valves, cams) is the same, the piston is slightly heavier. I believe I remember reading it is the same weight as the Kawasaki 300cc piston. Not much more than stock but it IS heavier. I suppose the only thing the slightly heavier weight might do in theory would be putting a little more stress on the bottom end, through the con rod. That being said I do know the bottom end is quite robust and should be able to handle that fine, from what I understand. What if my jetting were a little lean...not getting enough fuel especially at those RPM causing excessive heat buildup which could cause damage? Has anyone here actually gone to 10 grand plus on a 331 or 351? With any regularity? No damage?

 

I see 15k on your bike; how many miles do you have on the big bore? I seem to hear the same noise sometimes, after the big bore, and it's been driving me a bit insane. For me it is very faint though, barely audible, so much so that I've questioned if it's really there lol. I also am certain it is not ignition pinging. It doesn't happen under load at lower RPM, only when under heavy throttle at really high RPM. I was theorizing it could have been one of the smaller cam chain-side head bolts rattling loose lol. Assuming the worst I suppose. I'm pretty sure what it actually is is some part somewhere on the bike (probably not even in the engine) rattling a little due to the high tempo of vibrations caused by big bore high RPM. I also wondered, perhaps foolishly, if you might actually be able to HEAR the sound of the new piston rings seating...as I seem to remember the same noise on my TW200 after the rebuild. Pretty sure though that it's just some little part somewhere on the bike, tapping against another from vibration. Oh yes and, speaking of vibrations, the big bore motor definitely does vibrate more than the stocker. It doesn't really bother me personally, and isn't THAT much more, but for sure you will have a bit more vibration after going big bore.

 

Whew glad it isnt just me!!! Same situation only high rpm lots of throttle. Could be anything on the front end of the bike, no necessarily the gnone (as you said). I like the big vibes at idle, makes me feel like a I have a big thumper lol

Bike has 16k now, big bore at 3000 or 4000 (so done at 12 or 13k i have to confirm though)