Kawasaki Kx420 Hop-Up

Kawasaki KX420 Hop-Up

A 443cc Kit Makes It Run.....Now, If We Had A Kit For The Other Problems

We were highly disappointed with our '81 Kawasaki KX420. Several things didn't seem up to open class standards. But by far the most disturbing thing about the KX was the lack of horsepower. We complained to Kawasaki and they offered us a Kawasaki-approved kit that makes the 420 into a 443.

The kit consists of a new crank assembly and a handmade pipe, a cylinder spacer and gaskets. Because the Electrofusion cylinder can’t be bored, a stroker kit is the only way to increase the engine’s size. The kit is available from Kawasaki dealers for $230. But it’s not a bolt-in deal. The center cases require some machining for clearance. We let Kawasaki install the kit and when they returned the bike, we entered it in a local Pro motocross at De Anza Motocross Park.

The increased power is instantly noticed. The bike wheelies easier and getting the hole shot on uphill starting areas is easy. It’ll pull Yamaha YZ465s and open Maicos by two to three bike lengths. Engine vibration isn’t as bad as stock but the level is still higher than other open bikes.

The 443 easily got the hole shot in the first moto. And it led all 15 laps of the moto. Not without mishap, however. Ten laps into the moto the engine suddenly started vibrating worse than normal. Next lap, the clutch lever became impossible to pull. It didn’t slip, but control of it wasn’t possible. God, this is a new motorcycle, what else could go wrong in its first race? Next lap we found out. The rear suspension stuck at the bottom of its travel.

Action in the pits between motos was hectic. The next moto was three races away, not much time to try and fix all of the things that broke the first moto. The clutch proved easiest to fix; the clutch nut had come off. The nut and threads on the shaft were cleaned and the pieces Loctited, then the largest wrench in the pits tightened it down. The aluminum head stay wasn’t repairable. We decided to ride without it. The collapsed rear suspension was the result of the shock spring retainer clip folding up. The shock had to be removed, the spring removed, and the clip hammered into a reuseable shape. Because the clip folded up the first moto, it seemed reasonable to expect the same act from the bent up part. As security, we wired an 8mm alien wrench across the top of the clip .... and hoped.

The clip held for 15 laps of the second moto but the clutch nut didn’t. It came off again the second lap. Our tester still won the moto, but we don’t know how. We also had tremendous problems fouling plugs on the starting line. Unless the fuel petcock is turned off until the engine starts to run out of gas, then tuned on until the float bowl is full, then off again, the bike will foul the plug. Kill it dead. Not simply dirt or carbon foul it, it’s gone electrically. Never did figure out what the cause was.

Handling is much improved with the increased engine power. The front end is lighter and the extra power makes the bike easier to place where you want it. Although the power increase makes the bike competitive engine-wise—and we won on it—the total package isn’t a match for a 450 Maico or a Yamaha YZ465 or a Honda CF450R when equal riders are aboard.

But, by far the most disturbing thing about the bike is the way it disintegrates under use. The clutch nut problem is probably curable. The head stay breakage and shock clip bending are problems that existed last year. No excuse for them this year. The head stay breakage is simply an annoyance; who wants to replace it every moto or try and build a stronger one? The shock spring clip problem is unacceptable. A buyer of a $2100 racing motorcycle shouldn’t have to worry about the rear suspension collapsing over every jump. Ours failed on a part of the course where some degree of control could still be maintained. What if it had happened part way down Carlsbad’s torturous high speed downhill? The constant replacement of the kick start lever is ridiculous. A buyer shouldn’t have to carry a six-pack of kick start levers.

Motocross bikes have been advancing at an accelerated pace the past two years. The open class, a class not previously taken too seriously by many manufacturers, all at once has many great contenders. Unfortunately, Kawasaki’s KX420-443 isn’t one of them.