The swap is under way! Short explanation: 250 is more than enough for me off road. I'm no expert rider. But it doesn't cut it for road use, and the constant wot takes its toll on the little 250. I sometimes forget that the 250 is NOT the banshee motor I love so much, it just won't put up with the same endless abuse at wot. The banshee motor (my first choice) is really wide which makes everything about a swap into a dirt bike harder so the next best choice is a 500 single. kawasaki style
what we have here is a 2002 yz250 with a fresh '88 kx500 motor sitting comfortably inside the rails. Zero cutting required. If i tell ya'll how easy it was maybe more people will start to do it
The swingarm inside dimension is different so I measured from countershaft sprocket face to engine swingarm bolt surface on both motors. It is 0.375 further on the yz250. This means to line the sprocket back to where it should be, I have to remove .375 from either the inside of the chain side of the swingarm, or the chain side of the kx500 rear engine bolt surface. After disassembling the bearings from the swingarm, it looks like the easiest choice is going to be to cut it from the inside of the swingarm. I just chose swingarm mostly because i can get a whole new one for like 50 bucks on eBay if i mess up. This means that instead of the 2x needle roller bearings that are in there now (on each side), ill need to get either one that is .375 less wide that the 2 current ones together, or 2 bearings that are .1875 less wide each. throw the seals back in and it should all be fine. at that point when the sprocket is lined up properly I need to do the same thing to the other side of the swingarm narrowing the engine or the swingarm. Ive read a lot of people who have done swingarm swaps and they seem to think they have to run a dirt bike bearing so they give up. Come on, theres companies that make bearings to any spec. it may cost 20-30 OMG for that one bearing but who cares. I spend that on premix oil every weekend I ride so that argument is void. In any case, the chain alignment doesn't scare me because 1. I can shim the engine with washers if i have to and 2. the countershaft sprocket is held on with a circlip not bolted like the YZ250 motor is. This means that I have .1875" of forgiveness right there, making my job easier.
surprisingly the stock carb goes into the carb boot and the air boots goes right on the back. too easy! but it shifts the carb upwards like 1.5 inches due to the bigger motor and because of this it puts the throttle cable in a 90 right as it comes out the carb hitting the frame right forward of the rear upper shock mount. the air boot slightly rubs the spring but i could live with that. right now i am looking into early 90's kdx boots. I want to run a lectron carb as well and they are slightly taller apparently.
with the rear motor mount in, i can run a bolt through either the front motor mount, or the bottom motor mount, but not both. When one is aligned, the other is off by .5". This is much better fitment that i thought.
Also the lower rad crossover tube (between the bottom of the 2 rads) is squished between the frame and the engine. In fact, it is sitting .75" above the exhaust port squished by the cylinder just below the head. Maybe Ill go the smaller diameter hose possibly hard line if i have to, or run the hose another location.
Almost forgot to mention that the behemoth 3.2 gallon tank fits flawlessly! ok well not quite but considering how big the motor is i thought Id have to run a pocket flask for a gas tank. Nope! the tank slides right down and stops about an inch up from where it should. Dangit what is it hitting!!! well, its hitting the cylinder head. Fortunately, as I push down on the tank, the cylinder just splits the forks of the tank and the tank settles into its position. After a few heat cycles, maybe the plastic will deform to fit the engine that is stretching it
I think that covers it all, what do you think
450XSM · updated 
panagiotis · updated 
Troy H. · updated 
Timmay510 · updated 
