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Yamaha kx250 engine in a blaster?

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Hey guys, Im getting a blaster frame from my neighbor, complete with suspension and wheels. I just so happen to have a 99 kx250 in my garage also.

I think you know what my question is... :):)

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I hope you are a good machinist/fabricator, or have deep pockets.

Alot of work is involved. Motor mounts, frame reinforcements, and 250 2t motors rattle like crazy.

Ive been there and done that, motor swaps are cool and unique to everyone else, but 90% of the time the builder (if they do it themselves) is tired of it by the time the project is completed

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what would u do with the chassis? assuming that not making it runnable is not an option

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I take it youre asking what to do with the roller youre getting. If you must make it a running machine try to just find a blaster motor and throw it in there. Quick, easy, reliable. No need to worry if the frame is going to hold up or how bad it will vibrate or any of that bs.

Find a cheap blaster motor and put alittle work into it, or part the roller out and put it towards a bigger quad.

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Trial fitting a KTM 250  and KTM 380:




The 380 sits at a different angle and would not fit. It is possible to put a 300 cylinder on the 250 but it makes more power than the tires can put to the ground anyway.


Detail of the front motor mount with just enough cut off to fit the motor:




Trial fitting the rad and electrical:




Trial fitting exhaust, note the black sooty oil. Engine has run at this point to test things.

These pipes have to be welded together to the right lengths or the engine will not be in tune, only make half the power.




Getting closer, note the big header flange to make removal possible. Rad in, electrical done, it is a roller.

Bottom engine mount was an issue. MIG welding is dicey on a chrome moly frame. 




I runs, but the carb is at a 45 degree angle. Takes fancy jetting to make it run. How do you get over that?

Power is awesome. The chain broke on a tryout and jammed here.




Carb issue resolved. Much better idle and low speed performance. Vent lines off carb.

Needed for in water use. Blaster actually floats but carb is often submerged. Uses the well sealed aircleaner.




Intrepid Blaster operator taking in the rays. Awesomely fast and light quad but a bit high sped with the close ratio box.




This was a lot of work, even for a guy who has done a lot of swaps over the years.

There results are a success but it is always an engineering project. Every trip out to the woods had me changing something.

I am working on a stock engine Blaster, taking advantage of its simplicity and lack of a rad and better gearing.

A few mods on the engine will make nearly as much power. Here are some examples of what can be done to get 40hp out of a Blaster:


Modified head, nearly doubles the stock power:




Stuffed intake. Forget about hogging out the ports, fill them in!

Port windows are larger but all the extra volume is filled in with epoxy to increase velocity, improve jetting and throttle response at all speeds.




I've done it both. Fix up the stock Blaster motor. It is a wonderful simple design capable of over 40hp.

40hp in a Blaster means power wheelies at over 60mph. 40hp in a 300 lbs Blaster trumps a 700 Raptor.


Steve Best

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