Knowledgebase: Modifications & Upgrades
How to install an aftermarket carb on your pocket bike?
Posted by Scott S. on 22 October 2009 08:19 PM
A carburetor mixes gas with fuel for the engine. Most 47cc pocket bike engines come with an OEM floater-based carburetor. Replacing the stock carb with a higher performance carb can give you a number of advantages. More expensive, especially brand name Japanese diaphragm carbs such as Walbro or Mikuni feature larger gas throughput, finer gas atomization, and multiple air/fuel ratio adjustments, allowing you to take your mini bike's engine to a whole new level. Note that properly tuning a carburetor requires skill beyond the scope of the article. |
The first step is picking out the correct carburetor for your engine. If you are working with a 47cc or 49cc two-stroke engine, you have a variety of options, mostly depending on your budget and engine type. Keep in mind that frequently, on 2 cycle engines, such as those on our Cagllari, GP MTX, or Mini MX dirt bike models, an aftermarket carb mounts to a different engine manifold. Be sure to check that the carb, if sold by itself, is a direct OEM replacement or if it needs the additional hardware. Look for "HP carb kits" that include the necessary hardware.
Step 1: Ensure you have all needed hardware to mount your new carb.If you purchase a complete kit such as the Walbro WYK 47cc kit, you will have everything you need except an air filter. Never ride your minibike without the air filter attached, as you will quickly damage your engine! Note, on some bikes such as the GP RSR, there is not a lot of room for a large air filter, and you may need to use a UFO style filter.
Step 2: Remove the stock air filterUnless you already have a "K&N™ Style" filter already on your bike, you will be tossing your stock plastic air filter. Begin by removing the two phillips screws that hold the stock box air filter in place.
Step 3: Remove the old carburetorYou will need to drain the gas tank, or pinch the fuel line, before disconnecting it to prevent gas from leaking all over your work, creating not only a mess but also a flash fire hazard. Disconnect the fuel line and the throttle cable (by unscrewing the OEM carb's top cap). Then, unscrew the two bolts holding the stock carb in place to the engine manifold. If your carb kit requires a replacement manifold, you can also remove the stock manifold at this time. Be sure to clean the manifold surfaces and preferably replace the gaskets when installing the new carb.
Step 4: Install the new carburetorMaking sure everything lines up, install the new manifold / carb assembly, making sure you tighten all bolts well without over-tightening. You will need to feed the throttle to the new carb. Depending on the model, you may need to use an arm and a spring on the throttle cable to properly tension it. It is also a good idea to zip-tie the throttle cable to the frame of the pocket bike once it has been attached to the carb to prevent unnecessary movement and keep it from accidentally touching the exhaust. For Walbro kits, you will usually mount an assembly consisting of a velocity stack (the round adapter to which you attach the air filter), new metal choke, and the carb. Some carbs, such as the 19mm Walbro's, have a choke already built-in. Check that the diameter of the choke or v-stack that you using are not smaller than the diameter of the carb. Also, be sure you are mounting your carb the right way. Usually there are two small holes on one side of the carb that will line up with the holes on the engine manifold.
Step 5: Attach the air filter and enjoy and tune the carb.Attaching the air filter may get tricky if you are not working with a lot of space. It is best to plan out your build in advance or call/email us for advice as to which filter will best fit your pocket bike so that it doesn't interfere with the frame. Most carbs come with factory settings dialed in, and will require very little if any adjustment (particularily idle speed and air/fuel mixture, or high/low screws if your carb has them), to run well. Be sure to let your engine warm up completely before fine-tuning the carb.
Please exercise patience when tuning your new carburetor. Remember that it will need to be adjusted if you are replacing your exhaust or reed setup as well. On two stroke engines, replacing the carburetor is the best time to also upgrade your exhaust, as it assists in drawing out the combusted gasses out of the cylinder. Also, always remember that two cycle engines are sensitive to the type of oil you are using. Using premium unleaded gasoline and brand name, quality synthetic two cycle oil, such as Castrol 2-stroke oil, will make all the difference if you plan on keeping your pocket rocket for a long time!