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Small power improvements

Discussion in 'Technical and Troubleshooting Torque' started by Tack, Feb 6, 2005.

  1. I was just wondering if making small changes to things makes much difference to power on 250 cc bikes. Easy things like changing exhausts or air boxes. Nothing major just things that might make the motor run a bit more sweetly or crisply..if you know what I mean.

    My bike is a '91 Kawaski GPX 250R. Do they have restrictive mufflers on them? Maybe the standard exhaust has been fitted just to meet noise restrictions but could be a bit safer, as in cars can hear you easier, plus give a freer reving engine (plus sound better as well?).

    Is the air box restrictive? It is mounted under the seat with two venturi type ports in the lid. It seems a bad place for air as its right over a hot motor and the holes in the lid look quite small (Although it seems a good place to keep water from entering!) The venturi inlet size seems quite small and the air box volume is small. So was just wondering. Any ideas?

  2. One mod I have read about for the GPX is to change both sprockets with less (or it could be more) teeth which will give you more power but less top end speed.

    I read about it on the overclockers forum but the thread appears to have culled (too old) :( .
  3. go down teeth on the front and up teeth on the back and it will improve acceleration (no power gain) but you will lose top speed.
  4. Yep the muffler is restristing your bike, as is the airbox,the air filter, and the carby's. the sproket idea will give you more "push" in first and second but make fifth and sixth feel sluggish.
  5. for the short period of time your on a 250cc is it really cost effective to modify the bike with aftermarket pipes and such?
    After all what you gain is usually at best 5hp :)

    The GPX is a good 250, save your money and buy good rubber that sticks like shyte, then learn to harrass all the big bores through the tight twisties. Trust me through most of the twisties a 250 is hard to shake if it has good rubber and rider.
  6. You should be able to gain 10% without too much hassle. 10% is 10% on any bike.

    The good thing about doing it on a 250 is that you can use that extra 10% every day. On a 1000 you would be kidding yourself if you thought you were going to use it.

    You also have the advantage that you only have 2 cylinders. This means that is costs less to modify then a 4.

    Go for it. Pipes (muffler at least), filter, degree the cam, advance the timing, run good petrol, jets. That should get that 10% or maybe a bit more.

    Any more then that will become expensive and unreliable. Also keep in mind that you won't get your money back on any mods and if they are too radical you may even decrease the value of the bike.

    Have fun
  7. Brian, what tyres did you have on your Across?
  8. Hi Yas, Dont stuff with your air box, It is the best it can get, Swapping to pods can be an expensive exercise. It mmay create the need for carbs to be re jetted to account for the extra air getting in, IE , with pods your bike will run leaner causing valves to burn out and other dramas, If you can source a decent aftermarket pipe, thats the easiest way to make it a little better, and I do mean a "little".From memory the gpx has about 40 HP. A gain 5%over that is not really worth it in my opinion,a pipe will make it sound heaps better.A popular mod in the past was to grab a pipe off a cbr 250 and go to ya local muffler shop and he will make it a single system unit, creating more back pressure ,hence a little more torque. Be sure to run the single system on the RHS of cycle so as not to foul the sidestand .