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Product Code: 012




By Mike Unger

In today’s struggling economy, racers are turning to an old friend to allow them to race while keeping costs down. That friend is the Yamaha KT100 2 stroke engine. These engines are simple, easy to work on, easy to get parts for - and while they are not as powerful as the TAGs and Shifters - they are much easier on the pocketbook. With some help from my friend and engine builder Bill Willis of Powersports, this article will explain how to rebuild the Walbro carburetor in order to keep your KT100 running strong while saving yourself some cash in the process.

What does a carburetor do? In simple terms, a carburetor mixes the fuel and air into the right combination so that combustion will take place inside the engine in the most efficient way. The Walbro carburetor does this by pumping fuel using rubber diaphragms, metering holes, and vales. Like anything, these parts occasionally need attention and rebuilt.

Before we get into the details of the rebuilds, I have a couple recommendations regarding carburetors. While you can use a completely stock carburetor off the shelf, it won’t allow your engine to perform at its peak like a blueprinted carburetor. A blueprinted carburetor is a fancy term for a carburetor that has been gone over in extreme detail. These carburetors have all of the manufacturing tolerances taken out, specs matched to the kind of engine you are running and overall brought up to the limit of the rules without being illegal. This is the kind of carburetor you should start with, and is the kind of carburetor that you should learn to maintain yourself. I recommend sending a stock carb to a reputable engine builder and get it blueprinted. In most cases you can get one done for less than $100. Of course you can run a completely stock carburetor but my experience says it will cost you a couple tenths in laptimes.


Start by removing the airbox from the carb. Since the airbox is technically part of the intake track, it needs attention as well. If you are using a filter inside the airbox, remove it and clean it thoroughly with a good cleaner and allow it to dry. The inside of the airbox will be coated with oil and need cleaning as well. This is best done by using a little bit of brake cleaner and a paper towel. Make sure the inside of the airbox is spotless and perfectly clean. Just remember that when you put it back on the carb whatever foreign particles are not cleaned out will end up in your engine.

Next it’s time to take the carb off the engine. Typically this means using a 4 or 5mm Allen wrench to loosen the 2 bolts holding the filter cup and carburetor to the engine. Gently remove the bolts and remove carb from the intake manifold. Take care not to tear the gaskets between the filter cup and between the carb and the intake manifold. If the carb sticks to the gaskets, gently tap the carb with the end of the screwdriver to pop it off. In most cases you can get the carb off the engine without damaging the gaskets. But in some cases you will tear them and they will simply need to be replaced – no big deal.

Before you start taking things apart, I suggest you clean the outside of the carb thoroughly with carb cleaner or brake cleaner. The last thing you need is for dirt on the outside of your carb getting into the inside during your rebuild. While you are cleaning, inspect the throttle cable attachment for wear and for play in the throttle shaft itself. Too much wear and the pieces will need to be replaced.

Once the carb is clean, it’s time to get into minor surgery. There are 2 kinds of rebuild kits available: the diaphragm only version and the complete kit. We will be going through a complete rebuild in this article so you will need a complete rebuild kit from your local kart shop.

Things You Will Need:

• A very clean place to work

• 2-3 Bins to organize and hold parts

• A small Phillips screwdriver

• A small standard screwdriver

• A small pick

• A pop-off gauge


I suggest completely disassembling and inspecting first, then step by step assembly with new parts. Let’s walk through the disassembly first.

Starting with the pumper side (that is the side that has the 90 degree pulse fitting on it), remove the four bolts to uncover the pumper diaphragm. You will find a paper gasket that will likely stick to the cover and a rubber or nylon mesh looking diaphragm. Gently peel the gasket off and scrape any leftover gasket from the cover. Place the gasket and diaphragm in the small bin of old parts.

Now closely looking at the carb side of the pumper, you will notice several orifices for the fuel passages. You will also notice a small screen covering the largest of the holes. This is the final filter to catch particles from the fuel line before it enters the engine. Often you will find this screen full of debris. Your complete carb rebuild kit will include a new screen so with your small pick, carefully pop the screen out and place it in the discard bin.

Now moving to the metering sides of the carb, (the side with the hole in the center) remove the four screws to uncover the metering valve. When you remove this cover be careful not to force the cover off. Older style diaphragms have captive centers that positively attach to the fulcrum arm. If you force it straight off you can bend the arm. To avoid this issue after the cover is loose slide the cover toward the high and low speed needles. Underneath the cover you will find the diaphragm against the cover and another paper gasket. Just like the pumper side peel the diaphragm off the cover and place in the discard pile.

Looking at the metering side, you will see a lever arm attached to a small needle-like valve. This valve opens and closes with the pulses of the engine to provide the proper amount of fuel to the needle circuit. This valve will eventually wear out. To remove the fulcrum arm spring and needle, remove the single Phillips head screw that holds the fulcrum arm axle down. As you remove the screw, put your thumb over the fulcrum arm to avoid the spring making all of the parts fly everywhere. After the screw is removed, gently remove the fulcrum arm, spring and needle for inspection.

The fulcrum arm spring is something you need to reuse and pay attention to. If you have a blueprinted carb, it is likely this spring is special because it is cut to produce a predetermined set pressure or ‘pop off’. The valve inspection is relatively simple. The tampered end of the valve is made of rubber and is graphite coated. The coating will wear away and show the orange rubber underneath. If the bare rubber is visible it is time to replace it. Of course if all is good, place it with the fulcrum arm and spring for re-use.

While working on the meeting side, you will see a half-moon cover with 2 flat blade screws. Remove the screws. With a small screwdriver, gently pry the cover to get it to pop off. Underneath you will find another diaphragm and gasket. Discard these with the others.

For the final disassembly remove the low speed needle (the big T) and the high speed needle and inspect. Look for scratches or grooves around the taper. It is rare to damage one, but if it is damaged, place it in the discard pile and purchase a new one. Also, under the high speed needle you will find an o-ring and a washer. Remove both of these. If the o-ring is worn, replace it. Like the needles, it is somewhat rare to wear one out.

The final step of the disassembly is to thoroughly clean every circuit, and surface of the carb. For this job I recommend an old tooth brush and some brake cleaner. Use the toothbrush to scrub the outside of the carb free of any grease and grime. Spray into each hole and circuit to make sure the entire circuit it perfectly clean. Safety glasses are recommended. I can tell you from experience brake cleaner in the eye hurts! After the carb is thoroughly cleaned I recommend you let it dry completely before re-assembling. Brake cleaner and rubber diaphragms don’t generally get along so well.

Reassembling the Carburetor

Before we start reassembling things, you should be aware that the rebuild kit you purchased is made for more than one version of the carb, so there will be parts and gaskets that you will not use for your carb. I will explain how to identify the ones you want to use and which ones to discard. Also in several steps the order at which the gasket or diaphragm is placed is important, so please take note.

We will reassemble the carb in reverse order starting with the high-speed needle. Put the spring on the needle, then the copper washer, then the o-ring. Screw the needle in until it stops. Don’t tighten too hard as you can damage the needle and the carb itself. Next, screw in the low speed needle until it stops, using the same discretion.

Starting with the metering side, find the 2 half moon paper gaskets and the matching rubber gasket in the rebuild kit. Use the paper gasket with the dumbbell looking slot with holes on each end, and discard the other paper gasket. Place the remaining paper gasket down on the carburetor first then put the rubber gasket on top of the gasket. Then put the steel cover on top and tighten the 2 taper flat blade screws.

Next is the fulcrum arm and metering valve assembly. For this you will use the new valve and the old fulcrum arm and spring. Place the spring down in its hole in the carb and put the new valve into the slot on the short end of the fulcrum arm. Carefully lower the fulcrum arm down into the carb allowing the metering valve to slide into its hole. Slide the fulcrum arm axle into its slot and tighten the small Phillips head screw.

You can adjust the fulcrum arm height on the carburetor. This height is the depth the fulcrum arm below the top 
surface of the body of the carb. Typical fulcrum height is 0.045 to 0.060 in. This number is dependent on your engine builder’s preferences so you will need to talk to him to get the correct number for your engine. To measure it you can use a simple depth gauge or many kart shops sell a fulcrum height gauge. Adjusting it is a simple matter. Using your thumb gently hold the metering valve in place while pushing down or prying up on the fulcrum arm with a small screwdriver. If the fulcrum arm height is too low, you will need to pry the arm up. If the arm is too high bend it down.

Now it is time to check the pop off. Using a pop off gauge with a small rubber nipple attachment, you can quickly measure and set the pop off. To properly set the pop off you will need to make sure the valve seat is wet. To do this, squirt a small amount of WD40 or similar product into the area around the metering valve. Gently press the fulcrum arm down a couple of times to make sure the oil gets down into the valve seat. Now using a pop off gauge, place the nipple of the gauge into the large hole on the pumper side of the carb that had the screen over it during the disassembly. Slowly pump up the gauge until the carb needle audibly pops. The highest pressure you see before that sound is the carb’s pop off pressure. Like fulcrum arm heights, pop off pressure varies greatly depending on the kind of engine you are running (Super Can, Pipe and so forth) and the engine builder’s preferences. Usually pressures in the 9-12 psi range are normal but to get it right for your particular engine contact your engine builder and use his recommendations. If the pop off is too low you will need to increase the strength of the spring. To do that you need to stretch the spring out. If the pop off is too low you will need to soften the spring. This is typically done by cutting coils of the spring off. Whatever direction you need to go keep in mind that slightly stretching or cutting even half a coil of spring can be a big effect on pop off pressure. As you can image, setting the pop off is an iterative process and can take some time. Don’t rush it and get it done right.

After the fulcrum arm and pop off pressures are set, it’s time to put things all back together. Looking at the metering side place the paper gasket on the carb body using the small pins cast into the carb body to properly hold the gasket in place. Next find the metering side diaphragm in the kit. This will be the one with an aluminum disk in the center with 5 holes in it. Look closely at the pin in the center of that disk. If you are looking at a simple pin with no notch in it you have the new style, non-captive diaphragm. If the pin has a notch in it you have the old style. I haven’t seen a difference in performance between the two styles but the way you install the two styles is. For the non captive one you simply put the gasket with the aluminum disk face down on top of the gasket. For the captive style you will need to slide the pin into the fulcrum arm slot holding it in place. Like the paper gasket the diaphragm will line up with the locating pins cast into the carb body. With that all lined up correctly, put the metering cover over the rubber diaphragm and tighten down the four small screws.

On the pumper side, you will need to locate the pumper diaphragm and pumper paper gasket. The gasket will be the one that matches the skeletal shape of the inside of the pumper cover. Carb kits usually come with two kinds of pumper diaphragms. One will be black and made of rubber the other will be a brown-ish color and have a mesh look to it. The brown one is intended for use when alcohol is ran though the carb but some builders claim this one will outlast the black rubber one. I haven’t noticed a difference so you can choose as you like or if in doubt ask your engine builder his preference. Either way with the pumper cover on the bench put the paper gasket on the cover using the locating pins as a guide to help orientate the gasket. Next, place the diaphragm on the gasket again using the pins as a guide. Now hold the gasket cover in your hand, and with the diaphragm up, place the carb down on to the cover. The locating pins will let you know that you have it all together correctly. Now put the carb down on the bench with the pumper cover up and screw the 4 medium size screws down snug.

Putting it all back on the engine is an easy matter. Just remember the gaskets between the carb and the manifold and the carb and the filer cup. Like the other gaskets, inspect and replace as needed. Some filter cups use a rubber o-ring in place of a paper gasket. It will need inspection and replacement as needed too. Put your newly cleaned airbox back in place and your carb is now in top form and ready to work at its peak.

So there you have it – A complete carb rebuild that anyone can do. It does take some time and patience but it will save you some money in the end. For the quicker rebuild when you are having issues at the track I suggest only replacing the two diaphragms on the metering side and on the pumper side. I don’t suggest messing with pop off or fulcrum arm height at the track. Most of the time you are experiencing troubles at the track it is because the diaphragms are worn out. Also, setting pop off or fulcrum arm height is not something you can easily do in the pits. Save it for the workbench at home.

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