Author Topic: How I modded my KXR 250 / Mongoose 250  (Read 2810 times)

SilWo

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How I modded my KXR 250 / Mongoose 250
« on: July 27, 2021, 07:54:58 PM »
Hello everybody, I registered to this forum to share the mods that actually made a difference on my quad, because there is very limited info on modding this quad on the web. I am only going to tell you about engine mods (but please get at least some wheel spacers).

 -I replaced the stock rollers with some 20g flying roller 2 rollers. They behave really well with the stock variator, but after around 1500-2000km I decided to hit a really rough trail near me. After some beating, I noticed that the quad was sluggish if I came to a stop and then hit the gas. It felt like I was trying to start a bike with 2nd gear in. I assumed that a roller broke and was stuck upside down, so the variator was not able to go to the lowest gear ratio. I opened it and I was right, but to my surprise, the other rollers also were very worn out. So I went to the store and I got some 19g simple cylindrical ones. The engine's performance improved a lot (compared to stock), as it revs higher and runs in the powerband. This is a very simple mod that you can do to any cvt engine and it will make a big difference.

-I also had an aftermarket pipe and header, but it didn't make a big difference because the already poor mixture from the factory got even poorer.

-Now the mod that made the biggest difference. One day I decided to take the airbox lid out and went for a ride. The engine was running VERY lean, it was sputtering a lot and I think 5-7 out of ~20 horses had gone to sleep. Then I knew: Kymco was restricting the engine with the airbox. The snorkel that the airbox sucks air actually has a very small diameter. I made 4 3cm diameter holes in the lid (this is almost like running with the lid off) and I put it back on to make it a bit more silent (I actually liked the induction sound of the engine with the lid off, but it was too much). At this point, I had removed the restriction from the intake and the exhaust, so I had to give the engine more fuel to compensate for the increase in airflow. The stock PWK carb had 98 main jet, 35 pilot, and the needle in the middle of 5 slots. After many tries, I went up to 115 main, 42 pilot and one step richer on the needle. I think I could go a bit higher than that, but it runs very well and I decided to leave it there. That made a BIG difference, and the bike pulled a lot harder than before all that while still running the original air filter. Also, the throttle response got WAY better. That was the time when I finally could wheelie the quad sitting. With a K&N and 120 main, 45 pilot, and maybe one more step richer with the needle it will go even faster. And by faster I mean accelerate better. To get more top speed with a CVT bike if it can already hit the rev limiter you have to change the sprockets (or make the rev limiter kick in at higher revs lol).

All these mods (except the pipe) cost less than 50eur if you decide to do them yourself.

Please feel free to share the mods that you have made. As I stated in the beginning, there is very limited info on modding this quad on the web, so that could help someone.

P.S. I know my English is not the best, sorry for that.

Stig / Major Tom

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Re: How I modded my KXR 250 / Mongoose 250
« Reply #1 on: July 27, 2021, 09:00:53 PM »
Well done!
Thanks for your input!
Stig
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Rural Ohio

And, I'm feeling a little peculiar.

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