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The first post to my “small engine spring tune-up” week has to do with carburetors. Probably THE most touched item at Powered Agriculture is the carburetor. Cleaning, adjusting, and rebuilding makes up a large portion of our spring workload.

What it is

The carburetor on small engine equipment (either two-stroke or four-stroke) does essentially the same thing: allows metered fuel to mix with filtered air and flow into the combustion chamber. Carburetors will look very different from each other depending on application, engine size, and type of fuel, but the job is to mix the correct proportions of fuel and air for the engine to run optimally.

Some manuals and mechanics will even breakdown categories of carburetors based on the direction of air flow from the inlet to the engine. If you hear names such as natural draft, up-draft, or down-draft carburetors it only refers to the direction of air flow.

Adjustments

Carburetors vary on the amount of adjustment(s) you can make to them. For this section, I will assume the carburetor has two adjustable jets, the high speed and low speed, as well as a one mechanical idle speed adjuster. I will address less adjustable or fixed carburetors later.

As I said, the carburetor is trying to mix the correct amount of fuel and air for the engine to run. Carburetors should be adjusted everytime there is a change in ambient temperature or altitude as air density will change with these factors and your engine may run too rich (too much fuel for the amount of air) or too lean (too little fuel for the amount of air). Note: plugged or restricted air filters can also cause carburetor problems so check your filter(s) regularly. Here are the steps to make adjustments to your carburetor:

First: while the engine is running, open the throttle all the way and turn the high speed jet in or out in small increments until you reach optimal maximum rpm’s. Turning the jet “in” allows less fuel for higher altitudes or lower air density and “opening” the jet allows more fuel for lower altitudes or higher air density.

Second: while the engine is running, return the throttle back to idle and set the idle speed screw so the engine idles a little higher than the recommended minimum. Next, turn the low speed or idle mixture jet in or out to obtain the highest rpm without touching the throttle. The low speed jet is an air bleed adjustment, so turning the jet in will richen the fuel mixture and turning it out will lean the fuel mixture.

Third: while the engine is still running, adjust the mechanical idle speed screw out until you reach the recommended idle speed.

Just to some up the actions. You adjusted the high speed jet to reach the recommended maximum rpm’s to get the power out of your engine. Then you adjusted the low speed or idle mixture to make sure the engine can throttle up smoothly to the maximum rpm’s. Finally, you adjusted the screw that moves the throttle stop when you aren’t engaging the throttle. It literally holds the throttle at whatever rpm’s you set the it to.

Other carburetors

Emission regulators continue to force manufacturers to make cleaner burning equipment. This often results in carburetor changes. Some manufactures put limiters on their carburetors. These are usually plastic caps that only allow you to make minor adjustments to the carburetor jets. Others have engineered single or fixed jet carburetors. That is, there are no jets to adjust only mechanical idle screws.

If you can make adjustments than you can follow the steps above. If not, than your carburetor can only be rebuilt or replaced.

How to tell when something is wrong

Based on what we know the carburetor is supposed to do, the problems that arise with carburetors are they let too much fuel into the mixture, too little fuel, or none at all. There are a few tell-tale signs that anyone of these is occuring, but it may still take some investigation as to the exact cause so as not to repeat the problem. What I mean is to not just treat the system but fix the problem.

If there is water in the carburetor and you drain it out but don’t check your fuel cans, then you’re just going to pour more water in the carburetor, right?

The biggest sign of a carburetor running too lean is having to run equipment with the choke partially or completely engaged. This tells you that the engine isn’t getting enough fuel so you are mechanically restricting the amount of air into the engine so that what little fuel is making it through the carburetor can now mix with the appropriate amount of air. This can be caused by water (which is heavier and displaces fuel) in the fuel system, or stale gasoline that has varnished the carburetor, or just good old fashioned dirt and debris plugging a jet or fuel inlet.

Carburetors that are running too rich are going to burn a lot of gas and black sooty smoke may appear from the exhaust. This is a common occurance from lawn mowers. A lot of people don’t realize that lawnmowers should only be tipped toward the side opposite the carburetor. That is, make sure the air filter and carburetor stay up in the air. Otherwise gravity pulls oil through the crankcase and it comes out through the carburetor and usually soaks the air filter. Paper filters are especially prone to this because the oil swells the fabric and chokes the engine’s air supply. Again, it is a good idea to check filters regularly.

The worst case scenario is that a carburetor does not allow any fuel into the air mixture. Something phsically blocks the fuel inlet tube or the internal parts are rusted or dried to the point that they can not move. Simply removing the spark plug after attempting to start the engine can help you tell if fuel is making it to the combustion chamber. If the spark plug is completely dry, than chances are no fuel is getting to the engine. Another method is using starter fluid. Remove the filter and spray just a little bit of starter fluid into the intake manifold. If the engine “fires” up on the starting fluid and immediately dies, then you know that it is stil not getting fuel through the carburetor.

Rebuilds or Remove and Replace

The three situations above are just the more common scenarios I’ve seen. All of them can be fixed and rebuilt but those are topics too large for the scope of this post. Contact your local service shop or send me an email if you would like more information on rebuilds or R&R’s.