(This article originally appeared in the January 2003 issue of TransWorld Motocross. Get all this and more delivered directly to your mailbox every month by subscribing!)
Solving Carburetor Problems with Team Chevy trucks Kawasaki’s Randy Lawrence
Mechanic: Randy Lawrence
Photos: Donn Maeda
T ext: DW
You finally get the nerve to jump that big double at the local track. Everything is timed perfectly, but during the landing you feel an unexpected surprise when your bike mysteriously cuts out and loses power completely. Landing from jumps and having your bike “bog” or “hiccup” feels terrible and simply isn’t safe. Most people that have ridden for any length of time have experienced this before, especially two-stroke riders. There isn’t just one answer to solve the problem, but there is some basic doctor work that can be performed to help make your bike run correctly again. We talked to master mechanic Randy Lawrence to figure out how to get rid of the “hiccups” and get back on the gas. He broke it down in a few basic steps that anyone can do at home. So sit down, lean back and see if the old dog can teach you a new trick to keep your bike running the way it was meant to.
“First things first, you have to start with a clean bike. The smallest grain of sand or dirt in your carb can ruin your day really fast. Be safe and keep things clean. Working on and around carburetors just requires some basic hand tools, contact cleaner (a mechanic’s best friend) and an air compressor.”
Looking For The Obvious
“Stock bikes are built really well right now, but carbs are still mounted where they can get beat up. One of the first reasons that a bog can occur is if the fuel in the float bowl splashes around too much, not letting the jets get the proper amount of fuel. Try these steps first, as they are good starting points that can often solve the problem without much effort.”
Step 1: The first thing to do is check your vent hoses and make sure they aren’t clogged or kinked off by an awkward bend. Clean them up and use electrical tape to hold them together. Don’t use zip ties, as one could get over-tightened and close off a tube.
Step 2: Check and see if the carb drain bolt has been hitting the cases. Even if it doesn’t look bad, you have to put some foam or rubber pad between the float bowl and the engine. If your carb hits the cases hard the bike will bog due to the jolt, which causes the fuel to splash around and the motor to bog.
Step 3: On Ezra’s KX250, the works carburetor sits really close to the motor. For a little extra breathing room, I grind off the webbing on the Keihin carb’s drain bolt.
“Once you know that the carburetor’s outside surroundings are good, the next step is to make sure everything inside is okay. Follow these instructions to dial-in your carb’s internals.
Step 1: Remove the carburetor from the bike and completely disassemble it. Look for any dirt or glue in the float bowl’s O-ring, which may be causing the problems.
Step 2: Using contact cleaner and compressed air, clean the carb thoroughly, blowing out all orifices with compressed air. (Be sure to have some eye protection, because it’s no fun getting contact cleaner in your eyes). Clean each jet, and after blowing them out hold them up to light to make sure there is nothing clogging them. Everything has to be spotless, and you should be able to see right through the hoses and jets.
Step 3: Reassemble the carb, but leave the float bowl off. After putting all of the jets back in, blow air across the face of the openings. This will create a vacuum and suck out anyy little pieces of brass that can get broken off from your tools. It’s easy to make small scuffs in power jets, so be careful.
Step 4: Next, check your float height. I recommend referencing your service manual for the OEM setting. Remove your fuel line, clean it out and reattach the fuel intake nozzle to the carburetor. With the carb upright, lightly blow air into the fuel hose. Use your hand to move the floats and needle to the point were the air won’t flow. While holding it there, you can visually see where the float height is. On most bikes it will be level or just slightly higher than the flat base of the carb. If it is incorrect you will have to bend the little tab on the float to set it. This can be tedious, so take your time; bending it the slightest bit can make a big change.
Step 5: Check your work! Fill the fuel line with contact cleaner, holding the floats and needle closed. Let the floats down slowly so you can stop at the first sign of fluid coming out. Once that measurement meets the OEM standards, double check that everything is tight. Put the float bowl back on and install the carburetor back onto the bike.
“I always set Ezra’s float height on his factory Kawi carburetor a little higher than level for Supercross to help eliminate splashing in the tight rhythm sections. Watch Yogi’s bike at the next Supercross race. There will always be gas coming out over the flow hoses, probably more than you would ever expect. For outdoors, I run the float height perfectly level to make sure no gas is wasted and that it doesn’t pour onto the cement start gates.”
The Jetting Game
“If the bog just won’t go away and everything is mechanically sound, you’re probably dealing with a jetting issue. The main cause of the landing ‘hiccups’ in regards to jetting is usually improper air screw adjustment and pilot jet selection. Many people are intimidated by adjusting the air screw on the carb. The only way to learn is to play with it. Once you learn how to adjust jetting properly, picking the right pilot jet is easy.
“During the Nationals, I’m constantly changing the air screw position throughout the day, as weather conditions often change. A basic rule of thumb is that you want your air screw 1.5 to 2.5 turns out. Therefore, if your bike runs better at 1 turn out, it is a sign that you probably need a richer pilot jet. The ‘bogs’ can happen easy if you’re lean off the bottom. Here’s a quick way of checking this in the pits.”
Step 1: Take your bike off the stand, start it and let it warm up, until you can feel heat in the radiators.
Step 2: Now that it’s warmed up, make sure that your idle screw is adjusted so the motor stalls out slowly. Start by turning the idle screw in until the motor will idle, then back it out slowly until the motor stalls out.
Step 3: Get the bike running again and pin the throttle for a quick second before letting it die down to just above where it wants to stall out. Then, twist the throttle as fast as you can. If it’s lean, it will “hiccup” or “bog” for a quick second before it revs out. If that happens, take a small flat blade and try to adjust the air screw so it revs out cleanly.
“This technique works well, but it isn’t bullet proof. The easiest way to really get things dialed in is to play with jetting and carburetor settings on practice days so you know how to adjust the carb for different conditions. Once you know the characteristics of your bike, race day will be a breeze.”