Zama Carburetor Rebuilding
(My Experiences)

(Pictures will follow soon)

This page was created to demonstrate my experiences with rebuilding carburetors for small engines. Primarily 2-cycle but applies to some 4-cycle types.

First I want to quote the saying, "an ounce of prevention is worth a pound of cure". If you keep the air filter clean and use clean fresh fuel each time, your 2-cycle engine will run properly for many years without other maintenance. It's a clogged air filter and/or water contaminated gas mix that will limit the life of your implement.

What led me to creating this page was my experience with chain saws, weed trimmers, generators, and leaf blowers. When one uses small engines from year to year sooner or later the blasted thing won't start. In many cases, if it's older than a few years one just tosses it and gets another. This is because we now live in a throw-away society. No one repairs anything anymore. It's just not cost effective. Most times it's just cheaper to just buy a newer and more improved version of the implement. But sometimes the newer versions are lower quality and built cheaper because no one repairs them anymore.

Being recently retired, and a little more time on my hands I decided to do a little work on a few pieces of equipment that have been tossed aside for a number of years. I don't like throwing out defective stuff. I always try fixing things. I also have a box of pulled carburetors from chainsaws and weed trimmers that I just didn't have a heart to dispose of.

First, a bit of theory about how small engines work. Small engines refer to chain saws, weed trimmers, edgers, leaf blowers, hedge trimmers. Most are 2-cycle meaning they use a gas mixed with oil at a 30:1 to 50:1 ratio. Most if not all use one of two brands of carburetors. Zama and Walbro. They all work just about the same way. Just configured a little different from equipment to equipment. Zama has an excellent PDF of understanding, troubleshooting and repairing their carburetors. It can be downloaded here.

Most, close to 95% of the problems with these engines has to do with not starting or only running when heavily choked. Most implements are left over the winter with fuel still in the tank and carburetor. Most gas now contains 10% ethanol. Ethanol is hygroscopic. Meaning it will attract water. If water is introduced into the fuel and is left in over the winter the ethanol/water will separate from the gas and collect in the bottom of the tank. If water is exposed to aluminum such as the carburetor, it will corrode and leave a white powdery residue throughout the engine. The engine may not start or you may find that it will only run when it is heavily choked. This is because the main jet is clogged and choking the air intake has a tendency to richen the fuel to make it run, or the fuel filter in the tank is clogged with contaminants.

Ethanol will also dissolve the fuel lines and gaskets in the carburetor. Contaminated fuel in the tank will have a strong stale paint or varnish odor and the gas may have a dark brown appearance. Trying to start the engine will draw the bad fuel into the filter and clog it up too. I highly recommend using ethanol free gasoline which can be gotten almost anywhere in the U.S. You just have to look for it. Here's a website that will help.

Most cheap weed/grass trimmers aren't worth taking to a repair shop to have fixed. Just the labor charge could exceed the cost of buying a new one. Some people may get a year or two use out of a cheap trimmer before it quits. They throw it out and get a new one. Some people with a little knowledge that want to repair their engine just buy a new carburetor, which for the most part is easy. Just go to your small engine repair shop with the model number of your equipment and a carburetor can be had for about $30 or so. If you know the exact model of the carburetor you can find them on eBay for maybe half the price. All of them are made in China. Even the ones in high end Stihl equipment. There are two parts to the part number of the carburetor. The first is cast in the side of the carburetor with something like C1U or C3A. There is also a number stamped elsewhere like 53a. These numbers together make up the part number for ordering a new carburetor or a rebuild kit.

The following is what I do to repair a typical carburetor. In this example a Stihl MS170 chainsaw.

(Much of what follows can be applied to any small engine.) I cut over 18 cords of wood a year and use my saws almost all year long. Rarely one will stop working and I have to perform maintenance. Two things usually cause problems: 1) contaminated gas 2) inadequate cleaning.

Always use fresh ethanol free gas mixed with good quality oil mix. I always stay with name brand oil mix and mix only a gallon at a time. Inspect the fuel when being poured. It should be clear with a tinge of gold or blue look. This is the oil additive giving it it's color. When your fuel supply tank gets near the end, dispose of the last portion. You may have picked up some moisture and/or debris and you don't want anything bad getting into the fuel tank.

Periodically clean the saw of any mud and sawdust that has collected on the inside and outside of the case. Use high pressure air to blow out any collection of sawdust. Also inspect the air filter -  clean or replace. A clogged filter can suck sawdust into the carburetor and cause problems. I usually get a gallon of low VOC paint thinner and a small brush. Pour a portion in a small can and go over the whole exterior of the saw and get into all the little hiding places. Then blow everything dry with compressed air. Soak the air filter in solvent and blow dry being careful not to damage it.

Should you decide to rebuild or repair your carburetor, do the following:

Take a camera or use your cellphone camera and take pictures of different angles the way the carburetor is mounted, the linkage, etc. Remove the carburetor. There are usually two nuts that will hold the carburetor to the engine through the air filter housing. Note: all hardware is metric. There might be other screws to remove the linkage connecting the carburetor. Remember to take pictures here too. Disassemble the carburetor noting where all screws are and the position of the two covers. Carefully remove both covers noting the location and sequence of the gaskets. Unscrew any mixture screws. If there are two side-by-side, they usually have different thread sizes so you can't accidentally swap them. Remove the screw for the needle valve. Be careful noting the lever, spring and needle. With a small needle, remove the micro mesh filter in the carburetor. This is usually clogged. Clean thoroughly or replace.

There might be a Welch plug tapped or glued near the needle valve bracket. You may not need to remove this plug unless absolutely necessary. Remove it by puncturing it with a sharp point like an ice pick. Carefully remove before ultrasonic cleaning. Make sure you verify the little holes are clear by blowing air through them. By this time it should be completely disassembled.

If your planning on just cleaning the carburetor make sure you hold onto all the parts. Inspect the gaskets for rips and holes. Check the needle valve and make sure it has a good point on the end. At this point I immerse the main carburetor body and both covers in an ultrasonic cleaner and dish detergent. My cleaner has a heater so I can bring the temperature up in the tank. I usually run the cleaner for about a half hour. Once cleaned I remove the water by using compressed air from aerosol can of dust-off. Push air through all ports and orifices. Make every attempt to push air back through the tiny holes in the venturi throat. Make sure you push air all around until carburetor body is completely dry. Make sure the throttle and choke operate smoothly. If you are using a rebuild kit use the new parts at this time.

Carefully reassemble starting with the Welch plug. Take new one and place into position humped side up. Carefully tap the plug with a flat tipped punch the size of the plug. Don't tap too hard. Just enough to cause an interference fit. Some will carefully place fingernail polish around the plug to assure it is sealed. Carefully place the micro mesh filter into the carburetor body. Next, place the needle valve assembly making sure spring is correctly positioned (do not overtighten any screws. They should be firm only), then the mixture screw(s) (turn the screw gently till it bottoms out and open it up 1-1/4 turns.) Install both covers making sure the gaskets are correctly positioned and the covers are properly positioned. Install carburetor and connect fuel line(s). Verify final assembly with your pictures.

Exercise the choke and throttle to make sure they operate smoothly without binding. Fill tank with fuel mixture, set choke and give it a few pulls. It should start and run smoothly. You may have to adjust the fuel mixture and/or throttle speed screw once it is warmed up. Make sure air filter is in place and unit is warmed up before making adjustments.

Note: Some small engines may have a primer bulb to get the fuel circulating. Pushing the bulb a few times should cycle the fuel from the tank into the carburetor and the excess returned to the tank. If fuel is cycling properly you should see it fountain up in the bulb and some bubbles in the tank. The bulb should quickly expand to its normal size immediately after releasing. If it stays depressed or comes up slowly, there is a fuel restriction somewhere. Either the filter in the tank is clogged, the fuel lines have collapsed or the check valve(s) in the primer section have debris in it. The carburetor will need to be removed and cleaned or rebuilt.


Copyright 2015 Rick C. Ver 0.3 9/20/2015