I have been trying to get a little more top speed out of my sons Kymco 250mxu.
The carb is very small 18mm .I have increassed the main jet from 98 to 102.
put one tooth larger front sprocket on.
Ported the exhaust. and put the 10,000 rpm limiter on.
We have picked up about 1/2 second in the drags.
So now I want to try something different.
I beleive the 18mm carb is too small for a 250cc, so I have put a Keihin PE24 on the atv which
has a 95 main jet in it but I am told it is normally furnished with a 130 main.
So I am going to put a 115 main in and see what happens.
One other thing it has very little top end and would never reach the orignial rev limiter.
Does anyone have any ideas? Thanks

 

If it has a CVT drive (Automatic) you can reduce the roller weights this will move your power band peak closer to the horse power peak. It allows the unit to rev higher rpms befor the CVT cuts in on the line (rear break on, rev up on the line, then at the flag release the break and floor it).

see: CVT Tuning @: http://www.off-road.com/atv/kidskorner/clutching1.html

You may want to look into a Racing CDI ignition box. ( It raises the rpm rev limiter, and advances the timing. (some say it works great and others say no)


Larger Tires will increase your top speed but it steals horsepower to turn them over because of size and weight. (go 2 ply to save weight & dont go overboard in size)
http://www.denniskirk.com good selection of tires & cheap shipping


Add more air flow through the carb. This means you jet more gas, that is more power (gas is your friend) A good air filter UniFilter or K&N Filter and a good racing exhaust will move maximum air flow through the motor then re jet the carb so your gas/air mix is correct.
If you want filter mod pictures, look at picture gallery page of sb05 I used his mod and it worked great.

If you go up in jet size and don't add more air you well run rich an carbon the plug. if you add more air flow and don,t re jet you may run lean and burn a piston. after each change test run it on a clean plug then check for rich or lean condition.
Black = running rich
Brown = air/gas is correct
white or grey= running lean

 

I have the Kymco Mongoose that I/we have raced in 2- 12 hour races. That's all over so I jumped into the motor. Here is what I've done/learned::
I installed a big bore (265cc's- Thanks Poweroll). While in there, I ported the intake and exhaust. Flowed the inlet tube from the carb.
It has a "desert toyz" pipe on it. They also sell a big carb set-up (I see you don't need that). I went to a 110 main jet w/the stock carb.
It is still slow as heck (compared to my R) but does run better than stock.

IMO, there is nothing more to do. This machine runs great but will never be a high revver.
I feel that boring the inlet/exhaust of the head to allow more fuel would help, but alot of fuss. Poweroll has a cam too, but it's alot of $$ and again, I feel the gain would be very minimal.
I did change the sprockets to a smaller rear.

Desert toyz has modded one all out (cam,port,carb,pipe) and they have the same reults. They got like 5 more HP, but little rpm gain.

Clutch tuning will not effect top speed and my clutch works very well in stock form (I've been into the clutchs but didin't see anything to increase/modify at this point).
I wish I had better news for you. [img]i/expressions/face-icon-small-sad.gif[/img]

Give the MXU to the wife (mine loves it) and get a more suitable racer for the kid. [img]i/expressions/face-icon-small-smile.gif[/img]