Firefighter Chain Saw Basics: Two-Cycle Engines - Fire Engineering: Firefighter Training and Fire Service News, Rescue

2022-05-14 11:29:49 By : Mr. Shanghai Yiguang

When you took the oath to become a firefighter, you were also committing to be a professional power tool operator. Knowing how these powerful engines work and maintaining them will help keep you and your community safe.

Two-cycle engines are typically used on chain saws, rotary saws, and some outboard boat engines. The two-cycle engine also has no oil crankcase. Lubrication for the internal parts is applied through a mixture of special oil and gasoline or straight gas, with the oil being injected in the cylinder. If your two-cycle engine uses gas oil mix, check your owner’s manual for the proper fuel-to-oil ratio, such as 40:1, 50:1, and so on (e.g., 50 parts of gasoline to one part of oil). Two-cycle oil containers are sized to give you a correct ratio based on the size of the gas can you are using. Because they have no oil crankcase or sump, they can be operated in almost any position. Too much oil will cause the motor to smoke excessively and foul the spark plug quicker. Not enough oil will cause the internal engine parts to be damaged. Most saw manufacturers recommend 89-octane, without ethanol. Rubber parts such as fuel lines may not be ethanol alcohol resistant, and they will lose their shape and get soft.

(1) The fuel primer bulb and choke lever in full choke position. (Photos by author.)

Use quality two-cycle oil for your motors if you mix your own two-cycle fuel. Agitate mixed-fuel cans to ensure proper distribution of the oil/fuel, especially if it has been sitting for some time. If you use a synthetic mix oil, follow the directions on the container. The ratio of oil to gas can be quite different between synthetic and traditional two-cycle mix oil. An expensive, trustworthy, and foolproof alternative to mixing your own fuel is to use one of the many gas/oil premixtures available for purchase. These have no ethanol and will have correct oil-mix ratios, long shelf life, and correct octane. Purchase the same ratio as recommended for your tool. Because these motors run hotter and faster than four-cycle engines, they need their spark plugs changed more often and are more sensitive to old stale gas.

Before you start a gas engine, understand how the choke operates. Photos 2 and 3 detail the choke plate for a small engine, which could be a two- or four-cycle; it is the same. The choke (on the left) is in the off—or “open”—position for a hot/running engine, allowing a full flow of air in to mix with fuel and sending it to the engine cylinder to be burned (exploded). The image on the right shows the choke in the on or “closed” position. Placing the choke in the starting position (cold engine) limits the air that flows into the fuel/air mixture and creates a fuel-rich condition for easier starting, just like when using starting fluid. Engage a middle position for cold starting after the engine “chugs.” However, this will not keep running.

(2) The choke in the “open” or off position.

(3) The choke in the “closed” or on position.

If you are using a hand choke cable, closing the choke usually requires pulling the choke handle out and attempting to start the engine; the engine will then likely chug but not start. Push the choke in halfway, and the engine should start. Finish by pushing the choke in to the fully open position, and the engine should smoothly idle up. If equipped with a fuel primer bulb, pump the bulb about five or six times until it is full of fuel.

All small gas engines (two- and four-cycle) need at least four things to start and run, which follow:

If one of these four things is missing or out of adjustment, you can have problems starting your two- or four-cycle motor.

When attempting to start a cold two-cycle engine, do the following:

You must begin to develop a feel for the motor—that “chug” in a high-noise environment. You should know the sound well enough that you can do it in the dark. If the saw is still “warm” (running/working) after shutdown but you need to start it again, perhaps within five to 10 minutes, and it is not cold outside, you may not need full choke.

It’s easy to flood a two-cycle engine (“flood” meaning there is too much fuel in the carburetor after repeated failed attempts to start). Indications of a flooded engine include a strong smell of fuel or the engine will seemingly fire but not start with no other indication of a maintenance issue. The fuel-air mixture in the cylinder is too rich to ignite and start the engine. Therefore, you must remove the excess fuel and do the following:

If this does not work, do the following:

If you are working an emergency and someone floods the saw, you are likely better off just getting another saw.

When performing equipment checks, only run the saw long enough (one to two minutes) to check that it is working. Allowing the saw to sit idle only allows unburned fuel to collect in the cylinder, speeding up the fouling process inherent with two-cycle motors. The motors are tuned to run at working loads/speeds—not to sit at idle—similar to a generator. A chain saw needs to be used to stay in good operating condition.

(4) Some newer engines have a cylinder decompression valve. This valve looks like a button. By pushing it in, you allow some of the air in the cylinder to escape during the startup. When the engine starts up, the valve resets itself. This makes it easier to pull the starter cord.

(5) Washable air filters. The paper style gets thrown away. Gently clean and rinse with dish detergent and reinstall when dry.

(6) An exhaust spark arrestor screen.

(7) Many engines are fitted with “EPA limiter” caps on the carburetor adjustment screws; they are in place to prevent adjustment by just anyone. To adjust them, you will have to obtain the correct and corresponding screw; there are more than a half-dozen different styles.

Many two-cycle motors today have self-compensating carburetors that retune the carburetor as the air filter loads with debris. Once you confirm the engine is running properly, shut it off and do a hot restart without engaging the choke. This helps simulate fireground conditions and ensures the hot engine has no issues.

Check the physical condition of the saw and ensure it is ready for operation. Fuel the saw including bar oil (for the chain saw). Start the saw and let it run at idle. After a few minutes, briefly run it up to ¾ speed for no more than two seconds.

Allow it to run until it is out of gas; failure to do this can shorten the life of your saw. Following this procedure allows for proper wearing in/lubrication of the cylinder and rings.

Bad gasoline. If your engine has a rough idle, stalls frequently during acceleration, or fails to start at all, it’s likely your gas has gone bad. You can also tell if gasoline is bad by its appearance. If it is darker than usual or has a sour smell, it is probably bad. Another good reason to cut with your saw is it helps keep fresh gas in it. Fuel leaks or poor performance caused by the softening of the rubber fuel line occurs with an ethanol additive. Ethanol is often added to fuels without our knowing it. So, check with the gasoline station about the ethanol content.

If you are having trouble with the motor gasoline, one of the first things to check or change is the fuel. Simply dump it out and properly dispose of it. Gasoline straight out of the pump is typically good for three to five months. If treated with a fuel stabilizer (typically one ounce in 10 gallons of gasoline), you can expect six to eight months of good performance. Canned premixed two-cycle fuel has an estimated two-year shelf life after opening.

Gasoline goes bad because many of its volatile compounds evaporate over time, leaving less octane. Stabilizers can be used in two-cycle mix, straight gasoline, marine fuels, diesel, and E85. Stabilizers limit this evaporation but won’t help much if you have water in the gas, which is another common issue. Most gasoline also has ethanol added. Ethanol attracts and holds water and thins the fuel, corroding the metal parts of the engine; it can also freeze up. Dry gas or other similar products bind with the water molecules, lessening these negative effects. If you are also going to store the engine long term, run the engine until it’s dry with the treated fuel and then drain any remaining gasoline into an approved container and properly dispose of it.

Spark plug. The spark plug is the final element in the ignition system that creates the ignition source (spark) that fires the air-gas mixture. The timing for this action keys off the engine flywheel and pickup coil, sending an electric charge at the proper cycle to the plug firing it. The plug is replaceable. Take care so as not to cross-thread the spark plug, start it by hand, and finish tightening using a wrench. Also take care to not break the porcelain or knock the gap out of tolerance.

The gap is typically set right out of the box, but it pays to make sure. A “feeler gauge,” which is a measuring device with finely calibrated wires or thin metal strips with the width indicated on each filament, is used to correctly gap the plug to the size indicated in the tool’s manual. Bend the side electrode slightly so the feeler gauge just barely fits. Look up the spark plug specifications in the tool’s owner’s manual; do not waste time cleaning the plug—just replace it. When gapping the plug, check the manual for proper gap dimensions.

Air filter. Make sure the air filter is clean. When changing the air filter, close the butterfly valve/choke to prevent debris from falling into the carburetor during the filter change. There are a lot of wood chips and sawdust flying around, and the filters on chain saws can get very dirty very quickly.

Spark screen. The spark arresting screen inside the muffler can clog with carbon deposits, especially on smaller saws with dirty air filters. The clogged filter causes the saw to run too rich in fuel and unburned fuel contributes to the deposits. Excessively idling a saw can contribute to carbon buildup. Give it a good, hard run regularly—cutting, not just running at high idle. If the screen is fouled, it is easy to remove and clean with a degreaser/solvent.

Do not change the settings on your carburetor unless you know exactly what you’re doing and have the proper tools. The carburetor setting is seldom the problem; check to make sure everything else is working properly. Check for proper air and fuel flow through the system. If you compensate for poor maintenance (a sign of a dirty air filter) by changing the factory settings, then you may be chasing your tail trying to keep the engine running. If your saw has a clogged air filter and you change the carburetor settings to compensate, you will have to do it all over again when you put a clean new filter on the saw.

Keeping your two-cycle engines running at peak efficiency is vital to safe and fast fire/rescue operations. At idle, neither the chain nor the rotary blade should be moving. You will need a single cylinder tachometer and to know the high-/low-idle factory settings to adjust the carburetor.

STEVE SHUPERT is a 37-year veteran of public service. He retired from Miami Valley Fire District in Montgomery County, Ohio, and serves as a rescue team manager for Ohio Task Force #1 Federal Urban Search and Rescue. Shupert is also the chair of the FEMA Rescue Subgroup and the director of training for 501c3 Crash Course Village in Kettering, Ohio.