I can not begin to tell you the number of times that I picked up lawnmowers and weedeaters that have been thrown out and had them running, often without spending any money. At the young age of 15 my first job was at a small engine repair shop. I would say that 90% of the small engines that came in had a carburetor problem, mostly due to old gas that has tarnished or foreign material introduced into the gas tank. If you have ever brought a lawnmower or weedeater to the repair shop, you likely know that having one repaired cost almost as much as it would to purchase a new one. For this reason, most homeowners change the spark plug and the air filter, hoping that it fixes the problem. When this does not fix the problem they kick it to the curb and purchase a new one. Before you throw out that good engine or bring it to a small engine repair shop, follow along and learn how to rebuild and clean a weedeater or lawnmower carburetor. Before you go any further, keep in mind that rebuilding one of these devices takes a little bit of time and a lot of patients. There are a few small parts involved and taking one apart and putting it back together properly can be a tedious task.
How Do You Know if You Have a Bad Carburetor?
Diagnosing a carburetor problem is fairly simple, yet many people jump the gun on this one and replace unnecessary parts. Simply stated, any gasoline engine needs four elements to run; fuel, air, compression and spark. The absence of any of these will cause your small engine not to run. Since the carburetor provides the air and fuel mixture, you will want to test these two components while assuming that the compression and spark are functioning properly. In order to test this you should remove the air filter and push your primer bulb 4 or 5 times. Doing so feeds fuel directly into the intake port. Try to start the engine. If it kicks over a few times and stops, you have a problem with your carburetor. If you engine does not kick over, try to spray a couple shots of carburetor cleaner or starting fluid directly into the air intake port. The air intake is located behind the air filter. Let’s say that the engine does not even attempt to start after priming it and spraying starting fluid. In this case, you problem is not in the carburetor.
How to Take Apart, Clean and Rebuild a Small Engine Carburetor
Let me start off by stating that every engine is going to be designed slightly different but the fundamentals of cleaning a carburetor are however the same. For instance, some carburetors will have floats while others have diaphragms. Some have a pickup tube that draws gas from the take while others have a fuel bowel.
Items that you Will Need:
- Phillips and Flathead Screwdriver
- Carburetor Cleaner
- Gasket Scraper
- Gasket Remover Spray
- Air Compressor
- Wire Tie
- ¼ inch socket set
- Carburetor rebuild kit
Remove the Air Filter and Unbolt the Carburetor
Remove the air filter in order to access the bolts that hold the carburetor on. In most 4 cycle engines you will have to remove two bolts that hold the carburetor to the intake port or 5 bolts that hold it to the gas tank. While removing the carburetor, take note of how the gaskets are positioned. You will also have to remove the carburetor linkage and the fuel supply lines. The fuel lines are usually held on by a hose clamp or simply stretched over a nipple. Take note of which hose goes where – this doesmatter. The linkage should detach if you maneuver the carburetor around. Before disconnecting any linkage, it is a good idea to take a digital picture so that you know how to put it back together. Also, it may be necessary to plug the fuel line to prevent all of the fuel from running out. You can plug the line, secure the hose above the fuel level in the tank, or simply drain the fuel tank. Any of these methods will save your gasoline.
Disassemble the Carburetor
While performing this step you will want to pay close attention to how everything comes apart. Your engine will not run if do something as simple as place the diaphragm on the wrong side of the gasket or leave a small part out. Once again, it is a good idea to take pictures of everything as you take it apart. Remove all screws in order to access the top and the bottom portions of the carburetor. Take out the carburetor float by removing the small pin that holds it in. Remove the needle valve and the needle valve seat gasket if applicable. Remove all of the old gaskets and diaphragms. Some engines use a soft rubber diaphragm as sort of a fuel pump. Scrape off any remaining gasket material with a gasket scraper and gasket remover spray if necessary. Before removing the air-fuel mixture and idle screws, take not of how they are positioned. Do this by screwing it in and counting the number of revolutions that it makes before the screw stops. Write down the number of turns for both screws. You will use this information while reassembling your carburetor.
Give Carburetor a Good Cleaning
Use a wire tie, better known as a bread tie, to clean out the small ports within the carburetor. Strip the paper or plastic off of the wire before using. Be sure to run this wire through all visible ports. I would estimate that most carbs have between 5 and 10 of these small ports. This is a great time to break out those reading glasses if you are over 40 years old. These ports are often well hidden so it may take a bit of searching to find them. Don’t forget to look inside the carburetor for the jets that spray gasoline into the engine. You will likely have to remove e clips, pins, or other devices which secure parts to the carburetor.
After cleaning out the ports with a wire tie, spray a bit of carburetor cleaner into each hole. I always use the attached straw because you get more of a concentrated spray which does a better job at cleaning out these small holes. Also, clean the outside of the carburetor. Here you will likely find a bit of gas, oil and organic matter. Once you carburetor has been cleaned, blow everything off with an air compressor with at least 90 PSI. Force air through the same holes once again and clean off the overall carburetor.
Assemble Carburetor and Mount
This is perhaps the most difficult part of the carburetor clean and rebuild job because there are so many small parts that must be reassembled. Closely examine your rebuild kit to ensure that you picked up the correct one. I often use lawnmowerpros.com or eBay to find the parts that I need at a reasonable price. When shopping for parts, it is a good idea to purchase factory original parts and not aftermarket. For instance, if you have a Walbro carburetor then purchase Walbro parts. There are literally hundreds of different rebuild kits to choose from so ensure that you have purchased the correct one. Begin the assembly process by inserting the fuel-air and idle screws, float, needle valve, valve seat, and additional valves. Assemble the remainder of the carburetor, paying close attention to gasket and diaphragm placement. Remember, one small mistake such as leaving out a diaphragm or placing one in backwards will prevent your engine from running. Ensure that every part finds a home and there are none left over.
Once the carburetor has been assembled, mount it to the engine block. Don’t forget to place a gasket between the engine block and the carburetor. Connect the fuel supply lines to the correct nipple and connect the throttle control cable. On most models you will have to connect additional linkages such as governor linkage. Screw all idle and fuel mixture screws all the way in and back them out according to the notes that were taken in step 2. You may have to fine-tune these once you get your engine running. Replace and tighten the carburetor mounting bolts and replace the air filter. Avoid over tightening these bolts because they will snap off and you will have a major problem on your hands. Use a ¼ inch ratchet to tighten it sung.
Start Your Engine!
If you have completed the rebuild properly, your engine should start right up. Push the primer bulb 4 to 5 times, engage the choke and give at a few pulls. Once it begins to run, open the choke and give it a little gas. From here you should listen closely to the engine and fine tune the carburetor. First, locate the air fuel mixture screw. Turn it one way and listen to how the engine reacts. If it begins to stumble, turn it the opposite way. It will take a few seconds for the engine to react once the carburetor has been adjusted. Turn the screw ¼ turn at a time and wait a few seconds to see how the engine reacts. Once the engine is running smoothly, adjust the idle screw so that it idles low but not to the point where it begins to stall out.
FAQ Rebuilding Lawnmower and Weed Wacker Carburetors
How do I determine which brand of carburetor that I have?
While ordering parts, you will need to know the manufacture and model number of your carburetor. The best way to find out this information is to Google your devices model number and search for engine specifications. A good parts website can also steer you in the right direction when you give them your model number.
What does a bad carburetor diaphragm look like?
A good carburetor diaphragm will be somewhat soft and pliable. It should return to its original position when pushed on. A bad diaphragm will be stiff, plastic like, and remain in position once pushed on.
How can I tell if my carburetor is bad?
Diagnosing a bad carburetor is fairly simple. As noted above, you should spray carburetor cleaner or starting fluid directly into the intake chamber. If the engine kicks over and then ceases to run, you likely have a carburetor problem. A further explanation can be found at the beginning of this article.
How do I set the carburetor air fuel mixture screw?
While taking your carburetor apart, you should have noted the preset travel of your air fuel mixture screw. If you didn’t take note of this, consult the engine manufacture for the specifications.
My engine does not run after rebuilding my carburetor, now what?
If you correctly cleaned and assembled your carburetor, you may have a linkage problem, incorrect fuel mixture, dirty fuel, or possibly an obstruction in your carburetor that was not cleared. Look over these possibilities for a solution to your problem.