Carburetor Adjustment On Pro Saws
All saws occasionally need to have their carburetor adjusted. A saw with a properly adjusted carburetor will produce maximum power, minimal smoke, run and idle smoothly, and give trouble free service. A saw that is too rich will produce excessive smoke and have insufficient power. A saw that is adjusted too lean will also have insufficient power, but this condition can quickly cause engine damage. The following information outlines the process of adjusting a carburetor on a pro saw.
Most Pro Saw Engines Have Three Carburetor Adjusting Screws:
- 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. 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.
- Low Speed Fuel Adjustment - This is the adjustment that controls the proportion of fuel in the air/fuel mixture at idle speed. An adjustment that is 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.
- High Speed Fuel Adjustment - This is the adjustment that controls the proportion of fuel in the air/fuel mixture at cutting speed. It would not be accurate to say 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 will 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.
Rich Running Conditions
An over rich carburetor adjustment is when the proportion of fuel in the air/fuel mixture is too high. When this occurs, the fuel does not burn well. What does 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 also causes carbon buildup.
Lean Running Conditions
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 can lead to a seizure. A lean condition also allows for excessive RPM, causing rod bearing failure.
It is best to use a digital tachometer when adjusting the carburetor on a pro saw. These tachs are easy to use and require no wires. When the tach is placed near the spark plug, it picks up an electrical signal when the plug fires. On a two-cycle engine, this occurs every engine rotation, so the tachometer simply counts the pulses and displays the results.
Carburetor Adjustment Procedure
- Before you start the saw, locate a small flat-bladed
screwdriver needs to be small enough to fit through any adjustment
hole. Make sure the holes are free from debris and the ends of the
adjustment screws are visible. On some saws, these areas can pack with
chips, making adjustment difficult or impossible.
You're also going to need a tachometer. It is difficult (for even a trained ear) to adjust today's saws as accurately as is necessary without a tach. We have adjusted thousands of saws and we use a tachometer religiously.
- Begin by checking the saw's air filter. 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.
- 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.
- Start the engine and warm
it up. Carburetor adjustments made on a cold engine will be too rich
when it reaches normal operating temperature.
- 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 will idle with the chain stopped. Never set the idle so the chain runs when the throttle is released. If the saw will not idle, go to the next step.
- Set the low speed
adjustment. Slowly turn in the screw until the engine surges or starves
for fuel (lean). Make a mental note of the position of the screwdriver
Now slowly turn the adjustment screw out and the engine should run
better. Keep turning the screw until the engine starts to load up
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 positions.
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 until the idle climbs and it then starts to die. Then we go rich and the engine starts to blubber, the RPM's drop, and the engine almost dies. The optimum setting is between these two extremes.
These are approximately. 25 seconds long, but require time to load. Those of you with slow systems or connections may not want to wait! Click here for Saw Idle Adjustment Sound WAV
back to step (4) and
reset the idle speed. Chances are that the RPM setting has changed
since you optimized the fuel mixture adjustment.
the high speed fuel
adjustment. Hold the trigger wide open and check the RPM with the
tachometer. The listing below identifies the maximum RPM setting for
popular pro saws with bar & chain and no load:
- STIHL 044, MS440, 046, MS460 - 13,500 RPM
- STIHL 066, MS660 - 13,000 RPM
084, 088, MS880 - 11,500 RPM
- HUSQVARNA 272, 372 - 13,500 RPM
- HUSQVARNA 288, 385, 390, 394, 395 - 13,000 RPM
- HUSQVARNA 3120 - 11,500 RPM
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 speed. After revving it up to max
RPM and holding the trigger, we adjust the high speed rich, causing it
to slow down
and blubber. Then we adjust it lean, causing it
to sound smoother, but then it starts to starve for fuel. 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! Click here for Saw Full Rev Adjustment Sound WAV
- Go back to step (4) and fine tune the idle speed adjustment and low speed fuel adjustment. Movement of one adjusting screw often causes the other two to need readjustment.
- Your saw is properly adjusted and ready for work.
Why A Carburetor Has To Be Readjusted
If a carb is set right when a saw is new, why does it have to be readjusted? Some people wonder why their saw needs occasional tuning. They don't understand why saws can't be set at the factory and stay correctly adjusted for the life of the saw. The reason is there are many operational changes a user makes without realizing it. These changes include working at different elevations, changes in fuel, and a host of other factors cause saws to need periodic carburetor adjustment.
Today, sophisticated electronics can monitor and meter air/fuel needs. With this technology, saws will be able to "self adjust." This technology is well developed for automobiles and trucks. The process of miniaturizing an economical and durable system that will work on a saw engine is being developed. Until then, keep your tachometer and screwdriver handy.
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 on today's pro saws, most of these adjustments have "limiters" that limit the range of carburetor adjustment. In most cases, proper adjustment is within this range, but not always. If proper adjustment for your saw is outside the limited range, take your saw into a shop. Sometimes this is caused by a problem with your saw, and sometimes the range on the limiters just needs to be reset.
Got questions or comments? Call or stop in.