Tuning and Repairs
Carburetor Adjustment for Chainsaws
Carburetor adjustment is critical on chain saws and other two-cycle engines. An adjustment that is too rich will cause a saw to smoke, have insufficient power, result in additional carbon build up and may damage the engine. An adjustment that is too lean will also produce insufficient power and is more likely to damage the engine through lean seizure. The correct carburetor adjustment will allow the saw to produce maximum power, extend the duty cycle of the engine, produce minimal smoke, idle smoothly, rev up nicely and give trouble free service.
An over rich carburetor adjustment is when the proportion of fuel in the combustible air/fuel mixture is so high that the fuel does not burn well. The burn does not produce much heat, so the power stroke is weak. The partially burned mixture is expelled into the muffler and exits the saw as smoke. An over rich condition causes carbon buildup and will plug the fire screen and cylinder ports if run for a period of time. Outside of carburetor adjustments this can also be caused by having too much oil mixed with the fuel, and/or old stale gasoline.
A lean carburetor adjustment is when the proportion of fuel in the combustible air/fuel mixture is so low that there is not enough fuel to burn. This also makes a weak power stroke and causes the saw to have insufficient power. In addition to low power, a lean condition causes the cylinder temperature to rise, which often leads to seizure. A lean condition also allows for excessive RPM which often leads to big end rod bearing failure. Other causes for a lean running condition include lack of oil in the fuel mix, and when the fuel tank runs empty. Always stop a saw before the tank runs completely dry. Be sure to use the correct gasoline:oil mixture ratio recommended for your chainsaw.
Most Chain Saw Engines Have Three Carburetor Adjusting Screws:
1. Idle Speed/Throttle Stop – This is the adjustment that controls how much the throttle valve (butterfly) stays open when the throttle trigger is released. If this adjustment is set too low, the engine will die when the throttle trigger is released. The throttle valve (butterfly) simply cuts off the supply of combustible air/fuel and the engine stops. If this adjustment is set too high, the high idle speed will cause the centrifugal clutch to engage and the chain will run. This is a dangerous condition and should never be allowed.
2. Low Speed Fuel Adjustment (marked L on carb, this is the low speed jet)- This is the adjustment that controls the proportion of fuel in the combustible air/fuel mixture at idle speed. An adjustment that is set too rich will cause the engine to load up and die at idle speed. A mixture that is too lean will starve the engine and cause it to race or surge. An extremely lean adjustment will cause the engine to die, too.
3. High Speed Fuel Adjustment (marked H on carb, this is the high speed jet) – This is the adjustment that controls the proportion of fuel in the combustible air/fuel mixture at cutting speed. It would not be accurate to say that this is the most important setting, because all of these adjustments need to be accurate for a saw to perform its best, but this is the adjustment that determines how the saw runs in the cut. An adjustment that is set too rich will not allow the saw to reach the RPM level necessary to build maximum power. Throttle response may also be sluggish and the engine would smoke and perform poorly. A mixture that is too lean will allow the engine to reach an RPM level where bearing failure and cylinder seizure are likely. It will also lack power in the cut and tend to run very hot.
The preceding information briefly explains rich and lean running conditions. It also identifies the three adjustment screws and their function. It should be noted that some chain saws lack the high speed adjustment needle. These saws have what is called a “fixed jet” which is set from the factory. “Fixed jet” carburetors are used to prohibit the saw operator from setting the adjustment too lean and damaging the saw. Unfortunately, they also often prohibit the saw from achieving maximum performance.
Carburetor Adjustment Procedure
The following procedure should be followed to insure proper carburetor adjustment:
• (1) Check the air filter and clean it if necessary. Adjusting the carburetor with the air filter partially clogged is like adjusting the carburetor with the choke partially on. If you adjust your saw with a dirty air filter, the saw will run too lean when the filter is cleaned.
• (2) Check the muffler and exhaust port for carbon build up. Carbon clogging the exhaust port or spark arrestor screen can cause a saw to run as if the carburetor requires adjustment. Changing the carburetor settings with a plugged muffler may result in the saw running to lean when the carbon build up is cleaned out.
• (3) If carburetor adjustment is required due to a new or rebuilt carburetor, it is best to start with the fuel adjustment settings one turn out. This is accomplished by gently screwing the adjusting screws onto their seats and then backing them out one turn each. Care must be taken when screwing them onto their seats because the seats may be easily damaged if the adjustment screws are seated too hard. These seats are delicate and if they are damaged, the carburetor has to be replaced.
• (4) Check the fuel level. The tank should be over half full. If the carburetor is adjusted when the fuel tank is nearly empty, the carburetor may be adjusted too rich when the fuel tank is filled.
• (5) Start the engine and warm it up. If the saw will idle let it run for a few minutes. If it will not maintain idle without stalling warm it up by repeatedly squeezing the throttle, do not run the throttle to wide open. Carburetor adjustments made on a cold engine will be too rich when it reaches normal operating temperature.
• (5) Begin by setting the idle speed. Try to set the speed at about 2700 RPM. If you don’t have a tachometer, try to set the speed so the saw idles as high as possible without engaging the clutch. Never set the idle so the chain runs when the throttle is released. If the saw will not idle, go to the next step.
• (6) Set the low speed fuel adjustment. Slowly turn in the screw until the engine surges or starves for fuel. Make a mental note of the position of the screwdriver slot. Now slowly turn the adjustment screw out and the engine should run better. Keep turning the screw until the engine starts to load up. Make a note of the position of the screwdriver slot and compare it to the position of the lean adjustment. Now slowly turn in the screw to a position where it idles the best. It should be about midway between the rich and lean settings.
• Click on the link below and a WAV sound clip will be loaded into your browser. What you’ll hear is an idling saw. Slowly we lean the low speed adjuster by turning it in, till the idle climbs and it then starts to die. Then we go rich and hear the saw start to blubber and the RPM’s drop, almost to a point where it dies. The optimum setting is between these two extremes. If you’re adjusted too lean it’ll run out of gas and die, or die when accelerated. (Note that a saw adjusted too lean on the high can also die when accelerated.) If you think you’ve found just the right spot squeeze the throttle and the saw should rev up without any hesitation or stumbling.
• (7) Go back to step (5) and reset the idle speed. Chances are that the RPM setting has changed since you optimized the fuel mixture adjustment.
• (8) Set the high speed fuel adjustment. Hold the trigger wide open and check the RPM with the tachometer.
NOTE: If you do not have a tachometer, set the RPM level so that the saw “four cycles” or “blubbers” at wide open throttle. Do not attempt to set at maximum RPM levels without a tachometer. Setting the adjustment slightly rich will diminish performance, but reduce the possibility of damage to the saw engine.
• Click on the link below and a WAV sound clip will be loaded into your browser. What you’ll hear this time is a saw being adjusted for full rev. After revving it up to max RPM and holding, we adjust the high speed rich, causing it to slow down and blubber, developing little power. Then we adjust it lean causing it to sound smoother and then it starts to starve, also creating little power and creating a LOT of heat. As we back it off from lean to rich again, the sound becomes slightly rougher. This slightly rough sound (or 4-cycle sound, as it’s sometimes referred to) is where you want it to be. We like to say, as “close” to the smooth as you can get it, but still just slightly into the rough.
This one is larger than the first, and requires more time to load. Those of you with slow systems or connections may not want to wait!
• Check that your low speed adjustment still works as well as before you adjusted the high speed jet. Changing adjustments on one jet will often affect the other slightly. If the acceleration is not as before go back to step 6.
• Set the final idle speed. This time dial the idle speed screw back to the lowest rpm where the saw will idle smoothly without seeming as if it will stall out.
• Your saw is properly adjusted and ready for work.
Why Can’t Carbs Be Set When They Are New & Then Leave Them Alone?
Some people wonder why their saw can’t be adjusted when it’s new and then not need any other tuning. They don’t understand that saws can’t be set at the factory and stay correctly adjusted for the life of the saw. The fact is, some manufacturers think that way, too, and have tried “fixed jet” and “semi-fixed jet” carburetors. These types of carburetors have never worked well. The reason is that elevation, fuel type, fuel age, humidity, wearing components on the saw and a host of other factors cause carburetors to need periodic adjustment. It is also helpful to be able to adjust the carburetor for a variety of applications; chainsaws installed on chainsaw mills for example should have their high speed adjustments set rich because of the type of stress on the saw.