Let's face it, when a thumper gets really hot, it can be a bitch to start after a stall or crash. Add race-induced fatigue, and starting can get even more difficult. The air in a hot cylinder of an improperly killed motor is usually very rich with fuel. Kick the motor over and open the throttle and you get even more fuel. Kick it with no throttle, and you get no air, so the rich condition still exists, but now your leg is tired. A hot start valve can solve this problem by injecting cooler air into the cylinder, making a hot engine much easier to start. The process involves taking cooler air from the filtered side of the air box and delivering it through the valve, into the intake manifold between the carburetor and the cylinder. Slowly kick the bike to get to the intake stroke. Then open the valve and kick to start the engine. The cooler air will be sucked into the cylinder, and the engine will fire right up...

You might be able to find a suitable valve somewhere, but I couldn't, so I bought the hot start valve for a YZ/WR-400F, p/n 5GR-14340-00-00, for $35. Both the air box and intake manifold must be modified. The tubing and nipples can be found at any hardware store. Installation was relatively simple...


Working clean is the key here. Since you're drilling into the intake manifold, it is vital to ensure no metal drilling fragments make their way into the cylinder. Stuff a clean rag into the manifold beyond the area to be drilled. I dabbed grease on the part of the rag I could see inside the manifold and coated my drill bits with grease. I figured these steps would help contain any stray metal fragments.

Manifold Drilled for Inlet Tube

I used 3/8 inch outside diameter copper tubing for the manifold inlet tube, because it matches the tubing on the Yamaha valve.   First, I drilled completely through the manifold using a #20 bit, which matches the inside diameter of my copper tubing. Then I used a 15/64 inch bit and drilled into the #20 hole, stopping short of drilling completely through the manifold by 1/8 of an  inch.  This provided a stopping point for the manifold inlet tube, matched the inside diameter of my inlet tube with that of the #20 bit hole, and insured there was nothing protruding into the intake manifold to interrupt the fuel flow.  I cut the inlet tube to 5/8 inch length and secured it into the manifold with JB Weld. Click for a Mechanical Drawing

Manifold With Copper Inlet Tube Manifold With Copper Inlet Tube

Again, an emphasis should be placed on neatness.  Take steps to ensure no metal drilling fragments remain in the manifold when the stuffing rag is removed.  I used compressed air to blow the surfaces clean and carb cleaner to remove any remaining grease.  Be sure to thoroughly clean and smooth the cut surfaces of the inlet tube before inserting it into the drilled manifold.  Use a pipe cleaner and solvent to ensure the inside of the tube is equally clean.  Allow the JB Welded surfaces to cure while you work on the air box.

Air Box Outlet Nipple Between Spring and Reservoir

Outside of Air Box With Plastic Outlet Nipple

Inside Air Box With JB Welded Brass Threaded Cap

I decided to put the air box outlet on the left side of the box between the rear shock spring and the reservoir to give me the shortest possible run of tubing.

The air box must be removed to be modified -- it's not easy, but if your KLX looks anything like mine did, you really need to pull the box and clean it anyway. I drilled the air box and inserted a plastic nipple. I used large rubber washers on the inside and outside for a better seal. On the inside, I used a brass tube cap to secure the nipple from inside the air box.  For added security, I used JB Weld to hold the cap on the threaded end of the nipple.  The washers were sealed to the air box inside and outside with rubber cement which would allow the seal to remain flexible after it cured.

Allow the rubber cement and JB Weld to cure thoroughly before reinstalling the air box.  Since Kawasaki apparently designed the entire KLX motorcycle around the carburetor, reinstalling the air box and carb are a bit of a pain.  In my case, it's comlpicated even more because I used a Mikuni pumper carb which is larger than the KLX's stock CVK.  Be prepared for a fight to make it fit.  I installed the filter housing first, then bolted on the rubber boot.  The bottom left 8mm bolt on the boot is a bear to get on, but this way was still easier than installing to two pieces already bolted together.

Hot Start Valve Bolted to Voltage Regulator Mount

Finding a location to secure the valve is a trick.  The goal is to be able to reach for the valve quickly without fumbling around.  I fabricated a small mount from 1/16-inch thick galvanized metal, and drilled out each end. Fortunately, the threads on the Yamaha valve match the threads of the 8mm bolts on the KLX. Using two longer bolts, I secured the bottom of the mount behind the voltage regulator and mounted the valve to the top as illustrated here.  I cut my tubing to the proper lengths, attached some hose clamps, and the job was done. 

There are other places to mount your hot start button, of course. Terry Cable, for example, offers a universal kit for $90 (p/n 33-HSS0004) that mounts to the clutch or front brake perch, and for an additional $10, you can get their remote, handlebar-mounted actuator. The rule of thumb seems to be that a higher location will bring cooler air into the engine.   Here's how one KLXer mounted his hot start button...