A built 150 can often beat a stock 250 from what I've read. Most guys with the 250's want to jump up to a larger engine as well a lot of times. Usually looking into a 400 or 500cc engine swap. So no matter where you look, guys are always looking for more power.
I suggest you look at some of the other things as well. How are the tires? Does the 150 only have a single disc brake or are they on the fronts as well? What type of suspension are both karts using? (specifically what is the rear drive set up?) Where and how will yu be driving the kart mostly? You may want the smaller framed 150 for trails. Also how will you be transporting the kart? My 150 fits right ino the bed of my truck. Not sure if a 250 would?
To be honest, if you're going to be sinking $$ into the engine's anyway, you might want to consider an engine swap right from the start. If you can get a good solid chassis for cheap, you might be able to find larger (motorcycle, ATV, snow mobile) engine for cheap too.
Just somethings to think about.
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My Yerban assault vehicle: 2.2mm stroker crank, 62mm Nikasil cyl, Taida large vavle head, American made valve springs, ported intake manifold, TM 28mm carb, stock CDI, Bando coil, TK exhaust, stock CVT except for the 14g sliders, 13/40 internal gears, 16T drive, 31T axle sprocket, 22" rear tires, and ONE BIG POTATO CANNON mounted on top..........
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