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Isaac LRRS/CCS #871 ECK Racing | Spears Enterprises | GMD Computrack Boston | Pine Motorparts/PBE Specialists | Woodcraft | Street & Competition | MTag-Pirelli | OnTrack Media
Bike: SV650, Bride of Frankenstein
125s have a very narrow, specific range of RPMs within which they make power. If you're outside that range all that happens when you twist the throttle is the engine flips you the bird, says it's "le tired and going to take le nap now..." and does nothing. Literally and figuratively, nothing. If it's feeling generous the exhaust note will change to that of a open mouthed yawn. Meanwhile, you'll be wishing for a set of pedals to try and keep the bike moving.
125 GP motors are highly tuned. They make power between about 10K and 13K revs. If you let it drop below ten and open the throttle, it feels like hitting the brakes. LOL. You will have to downhift once or maybe twice to get back to going forward. Now you have lost 15 mph, and half a second.
I liked Kurlon's answer because it made me chuckle, but I liked Paul's answer because it was informative.
Roland Arsenault
LRRS and USCRA #763
2012, 2013 and 2015 Big Fish Small Pond Champion
"The 4 board is an upshift marker, not a brake marker"
Does anyone race 250s at LRRS? Can't seem to remember any ever being there. Or maybe just a couple?
A man of many names...Jay, Gennaro, Gerry, etc.
How difficult or practical to throw a yz250 motor into a 125gp chasis or something like that? the 250gp's are 2 cylinders though, aren't they?
Your comment of power on/off just had me think about it. Its what I miss most about my prior yz250. The 125s I have ridden are much much more tame. I crave the 250 powerband.
A man of many names...Jay, Gennaro, Gerry, etc.
I remember coming out of 2 one time on a 125 and twisting the throttle and nothing, so I downshifted and almost launched the bike out from under me! Literally! When they are on, they are ON. The trouble I had was going from my 350 to a 125 and vice versa. My 350 is a great bike, but is SUCH a pig compared to the 125. Oh and the amount of shifting you have to do in one lap is crazy. I guess once you are fast, maybe it isn't as many shifts, but my foot and leg were tired after 2 laps!
I think I am more of a twin girl, but the 125 was definitely an experience and I would ride one again. Do you still love me Paul
You can take the girl outta Jersey.....
USCRA #75
Street- 03 Suzuki SV650
Track- 71 Honda CB350
Do the 125gp's pack a greater punch than the 125mx bikes? I think 125mx bikes are awesome machines and are by no means slow. They're just relatively tame compared to their bigger 250 brothers.
A man of many names...Jay, Gennaro, Gerry, etc.
There is still one 250GP machine, or at least there was last year. I had a ball racing that lil screamer in GTL.
250 and 500 MX smokers have been jammed in GP chassis by quite a few people. As Paul noted a 250MX motor doesn't make anywhere NEAR the 70+ a 250GP twin puts down. The 500's don't quite make the same peak either but have a FAR wider torque curve. Both like to vibrate their donor frames to bits, or so I've heard. There is lots of speculation that a Quadzilla motor may be the ideal candidate, 500 two stroke single that's built for quad abuse, lots of build potential, AND has a counter balancer... but I've yet to see a completed one turn laps.
The middle ground is to just motard a two stroke. You'll be on a chassis you're familiar with, with a maint book that is similar to the GP machines, slotted into multiple classes you'd be competitive in. It's still not a GP bike though, one of these days I'll try one...
KX500 has a counterbalance as well I believe.
A man of many names...Jay, Gennaro, Gerry, etc.
I never saw any reference to a counterbalanced KX500, but if there is such a variant, I know the line is highly regarded as an open class motor.
I just remember people talking about putting those in quads over a CR500 motor as they were less prone to rattle apart.
A man of many names...Jay, Gennaro, Gerry, etc.
I think you could get away with claiming a paint mixer powered by a cammed and badly carb'd HD Big Twin is a viable swap compared to a CR500 if less vibration is your primary criteria.
Last edited by TTD; 03-20-15 at 11:09 AM.