Hi, I found this old topic. i recently finished a KX500 sf project. I had with the 93 chassis and a 86 motor. the problem: the carburetor behind the cylinder is too high and would not fit under the tank and the airboot. I've then built a wedge from 2mm to 10mm. now the carburetor sits lower. and its perfect to airboot and under the gas tank.
side effect is that I have since made my the same time a spacer. works quite well. a smooth power from bottom to top which makes the 500 easier to ride with these bears performance.
I have simultaneously built a 10mm reed spacer for my 83 KX250, that i use for vintage racing. now waiting for the spring in germany to get it to install and test, how much more power then bottom and mid range. in stock its from bottom like a modern 125 and then between mid range and top explodes like a rocket. which makes it difficult on hard or slippery ground ride. i want more from bottom to mid range. hope of mid to top is a softer transition with the same peak power.
here I have a few pictures of it on the 500 engine. more to see on my website