Hello all. I am a new member here, and this is my first post. I have been around drag bikes for the past 5 years (currently 17). Have an 11 second Harley Sporster and a 14.80 second Yamaha XS400. I just picked up a 1992 RM250 and am setting this bike up for the dragstrip like everything else i own. Plans are to put street tires on it, a misc. cheap front and a Meztler ME880 120/90-19 on the rear (originally meant for the front of the Harley Vrod, plan on turnin the tread around,anyone know if this works?). I am also plannin on taking the shock off and putting on a solid rear strut, for better 60ft times and to lower the bike to about 4 inches off the ground. Does anyone know the best way to lower the front forks? Was thinking about either using a front end strap and mount it to the brake caliper mount and the mount where the fork guard goes on the opposite side...if it is strong enough, or cut the springs. Im gonna rip the airbox out, throw a clamp on filter on it. Also gonna put on drag bars. Right now it is geared 14/47 (stock is 13/49). I plan on running 14/41. Think this will be enough for the 1/4 mile? Also, who makes the best pipe, FMF or Pro Circuit? Is it safe to run with no silencer? What about reeds, Boyesen or Vforce? Also, anyone know the best tach I can hook up to this? And one more thing, how far can you rev this thing, i know NOTHING about two strokes and have been short shifting the thing. Finally, what kind of ET/MPH can i expect?
Setting a 1992 RM250 up for the strip! Need HELP!
- Thread starter AaronRM
- Start date
Matt Fisher
Member
- Apr 17, 2002
- 136
- 0
Couple things that come to mind, in no particular order.
1) The Mikuni TMX carb on there sucks. If you need a needle, plan on it taking a month+ to arrive.
2) The clutch isn't the strongest. Keep in mind that an aftermarket basket and hub my be in your future.
3) Tying down the front end sounds like an nasty accident waiting to happen. Broken tie-down= big pain. Cut the springs.
4) You're going to be totally wound out before the 1/8 mile comes up. Once you gear it to keep pulling to the 1320 mark, it'll suck coming out of the hole. The tranny's a close ratio deal.
5) The Harley tire should work fine. After all, the bike's only making somewhere between 40-45 hp.
6) Tach could be interesting. The RM produces no voltage to run one, and you'd need a tach that works with a 2 cylinder 4 stroke motor for it to read properly. Either a very small constant loss battery, or a RMX stator (slight loss in hp) will take care of the 12v issue.
7) Don't be in a big hurry to yank the airbox. After you get it jetted right (did I mention the carb sucks) then take if off. You may find that the clamp on produces too much turbulance, lowering power.
8) The port timing on a 2 stroke generally keeps you from overrevving them. Running it lean, however, will put the hurt on the motor. That RM comes with a 360 main stock. You may very well end up with a bigger main (on my 92 RM250 I ended up with a 340 and a leaner needle, leaner pilot).
9) At the end of your run, do not chop the throttle and let the compresion slow you down. Doing so keeps the rpm's really high, while the carb's not letting in enough fuel. And since the motor's lubrication is in the fuel... Pull in the clutch or better yet push the kill switch with the clutch out.
10) Leave the silencer on there. It'll produce more top-end power than without one (unless your jettings wrong to begin with and by some miracle pulling the silencer fixes it). Also, whatever pipe is listed as a "top-end" pipe would probably suit you well.
11) Hell if I know what kind of et/mph you'll have, but with drag bars a 200# bike ought to 60' REAL well.
Good luck.
1) The Mikuni TMX carb on there sucks. If you need a needle, plan on it taking a month+ to arrive.
2) The clutch isn't the strongest. Keep in mind that an aftermarket basket and hub my be in your future.
3) Tying down the front end sounds like an nasty accident waiting to happen. Broken tie-down= big pain. Cut the springs.
4) You're going to be totally wound out before the 1/8 mile comes up. Once you gear it to keep pulling to the 1320 mark, it'll suck coming out of the hole. The tranny's a close ratio deal.
5) The Harley tire should work fine. After all, the bike's only making somewhere between 40-45 hp.
6) Tach could be interesting. The RM produces no voltage to run one, and you'd need a tach that works with a 2 cylinder 4 stroke motor for it to read properly. Either a very small constant loss battery, or a RMX stator (slight loss in hp) will take care of the 12v issue.
7) Don't be in a big hurry to yank the airbox. After you get it jetted right (did I mention the carb sucks) then take if off. You may find that the clamp on produces too much turbulance, lowering power.
8) The port timing on a 2 stroke generally keeps you from overrevving them. Running it lean, however, will put the hurt on the motor. That RM comes with a 360 main stock. You may very well end up with a bigger main (on my 92 RM250 I ended up with a 340 and a leaner needle, leaner pilot).
9) At the end of your run, do not chop the throttle and let the compresion slow you down. Doing so keeps the rpm's really high, while the carb's not letting in enough fuel. And since the motor's lubrication is in the fuel... Pull in the clutch or better yet push the kill switch with the clutch out.
10) Leave the silencer on there. It'll produce more top-end power than without one (unless your jettings wrong to begin with and by some miracle pulling the silencer fixes it). Also, whatever pipe is listed as a "top-end" pipe would probably suit you well.
11) Hell if I know what kind of et/mph you'll have, but with drag bars a 200# bike ought to 60' REAL well.
Good luck.
First off, I think taking a MX bike 1/4 mile drag racing is a bad idea. A 1/8 mile maybe, but not a quarter. Secondly, MX bikes, from airbox to silencer are tuned machines. You can not successfully remove the entire airbox on a current MX bike and expect it to run right if it all. Take a look at FMX. For the sake of having more areas to grab ahold of the bike while airborn some of these guys expeirmented with no airbox. It didn't work at all. In the exhaust department a little known fact is that 40% of a 2 stroke engines power is because of the exspansion chamber. You'd might actually hurt performance by removing the silencer. Two strokes need some back pressure. Try a shorty style silencer. I don't know about using straps or cutting the springs in the forks down. Why not have a spacer cut to prevent the forks from topping out hence lowering the bike safely. It probably wouldn't hurt to put a fresh piston and rings in this machine to restore it's performance. Another thing to remember is MX bikes were not designed for pavement use, or full-traction. Running it down the road is rough on the engine (as a whole). I wouldn't think you'd need a tach, learn to shift that bike by ear. It'll let you know when it needs another gear. Get a pipe designed to allow the bike to rev out further if your doing the quater mile with it. Either brand (Pro Circuit or FMF) will be similar.
- Thread starter
- #4
Thanks for the responses. Do you guys think i could throw a Quadracer 250 6 speed tranny in her? Seems like alot of the parts interchange. As for the forks, what do i do, just put in a spacer right on top of the fork? I believe I'll leave the air box. Any suggestions on carbs? Im so new to 2 strokes, i dunno where these things make power. Does the power band come on strong when the sound of the motor smooths out? As far as takin it to the drag strip, i have a large debate goin through my head...Dragstrip, or back-road corner carver? I dont know the difference in the RMX and RM, think this RM motor would be suitable for street use?
xsnrg
Member
- Jul 20, 2004
- 728
- 0
Have you read this? http://www.dirtrider.net/forums3/showthread.php?t=96360 May give you some insight. I'm thinking you should find a 500 to start with. You could run it with no engine mods and still beat that 250 modded out.
