Shindaiwa 446S Carburetor Tuning (Step-by-Step)
Tune the 446S only after confirming its exact manual, fuel mix, and carburetor setup. Work outdoors with a tachometer, a clean air filter, and a sound fuel system. Adjust the idle screw first; touch mixture screws only if the manual allows it. Use the manual’s speed limits, not your ear, and stop if the saw surges or will not hold a safe idle.
If you have pulled the starter until your arm is tired and the saw still will not run, turning carburetor screws may feel like the fastest next step. It often is not. A dirty filter, stale fuel, or blocked spark arrestor can make a good carburetor seem faulty. Tuning before checking those parts can waste time and, with a lean mixture, damage the engine.
I use a simple order: identify the saw, rule out faults, warm it safely, then make only the adjustments the correct manual permits. The 446S is a 45.0 cc air-cooled two-stroke. Published figures commonly list 2,800 rpm at idle and 13,000 rpm maximum, but the serial-specific manual is the authority. Think of those figures as a starting reference, not permission to override the manual.
Confirm the saw and prepare it for tuning
Before adjusting anything, identify the 446S by its data plate and serial number, then find the matching operator or service manual. Carburetor versions, limiter settings, and adjustment limits can differ. This check prevents you from applying another model’s settings to your saw.
Check the carburetor’s adjustment screws and limiter caps against the manual. Do not remove caps or defeat emissions controls to gain extra adjustment. If the manual does not permit user adjustment of a screw, leave it alone and arrange service if the saw needs that circuit tuned.
Use fresh fuel mixed at 50:1 with approved two-cycle oil, which equals 2.6 US fluid ounces of oil per US gallon of gasoline. Do not use straight gasoline or guess at the ratio. Confirm that the air filter is clean and correctly fitted, the muffler and spark arrestor are clear, and the fuel lines and filter are sound.
Before starting, inspect the throttle interlock, stop switch, chain brake, chain catcher, handles, fasteners, bar, and sprocket. Correct chain tension and use a bar and chain approved for the saw. Common 446S combinations use 3/8-inch pitch and .050-inch gauge, but check the fitted bar and chain markings, including drive-link count. Wear chainsaw-rated protective gear, start outdoors on stable ground using the manual’s procedure, and keep the bar clear. Never drop-start the saw.
Takeaway: Confirm the exact manual and remove basic faults before reaching for a screw.
Rule out faults that carburetor adjustment cannot fix
A carburetor meters fuel and air. A screw adjustment can change how the engine runs, but it cannot repair a blocked passage, air leak, weak compression, or faulty ignition. Check the basics first so you do not mistake a failing part for a tuning problem.
Start with fresh, correctly mixed fuel and a clean air filter. Inspect the fuel filter and visible fuel lines for blockage, damage, or leaks. Check the muffler and spark arrestor for blockage using the manual’s service procedure. If the chain is dull or the bar and chain are mismatched, correct that too; poor cutting performance is not proof that the carburetor needs adjustment.
If the saw still will not start, diagnose spark, fuel delivery, and compression rather than turning settings at random. Use the manual’s test procedures and proper equipment. A weak or absent spark, a fuel delivery fault, or low compression calls for diagnosis of that system. Do not keep cranking or adjust the high-speed screw in an effort to mask the fault.
Look for warning signs while the engine runs: surging, a hanging idle, unusual heat, or sudden loss of power. These can point to an air or fuel problem, among other faults. A lean high-speed setting can overheat the engine and seize the piston and cylinder. Stop the saw if these symptoms appear; do not continue cutting to “see if it clears.”
Takeaway: If the saw fails a basic condition check, repair the fault before tuning.
Tune the 446S step by step
Carburetor tuning is a controlled adjustment, not a search for the highest or smoothest sound. Warm the engine as the manual directs, use a calibrated tachometer, and make small changes. Do not proceed if the applicable manual is unavailable or does not authorize the adjustment.
-
Set up the tachometer and saw. Follow the tachometer maker’s instructions for pickup placement and engine type. Use a clean air filter, correct fuel, and a sound muffler. Place the saw outdoors on stable footing, with the cutting attachment clear of people and objects. Keep the chain brake use and starting procedure consistent with the saw’s manual.
-
Start and warm the engine. Use the cold-start control as directed. Once it fires, promptly move the control to the run or warm-start position as instructed. Warm the saw at moderate speed. Do not leave it at fast idle with the brake engaged longer than necessary.
-
Set idle with the T/LA screw. The T or LA screw controls idle speed. Adjust it only enough to meet the serial-specific manual’s idle setting while the chain remains stationary. The commonly published 446S reference is 2,800 rpm, but use the manual’s figure for your saw. If the chain moves at idle, stop and correct the problem before use. A screw adjustment is not a reason to accept chain creep.
-
Adjust the low-speed mixture only if allowed. The L screw affects low-speed mixture and response. If the manual permits user adjustment, make a very small change, then check idle and throttle response. The engine should accelerate cleanly without hesitation or a lean stumble. Recheck idle afterward. Do not assume a direction of turn or a baseline setting from another model; follow the correct manual.
-
Leave the high-speed screw alone unless the manual permits adjustment. The H screw affects the high-speed mixture. If user adjustment is authorized, set maximum engine speed only with an accurate tachometer and the manual’s limit. The commonly published 446S reference is 13,000 rpm, but the serial-specific limit controls. Never tune H by ear or run the engine beyond its limit to improve cutting.
-
Recheck operation. Confirm stable idle, a stationary chain at idle, free throttle return, and clean acceleration. Verify the stop switch and chain brake function. If the saw cannot meet its specified limits, or the adjustment range seems abnormal, stop and seek qualified service.
| Adjustment or check | What it affects | Safe decision |
|---|---|---|
| T/LA screw | Idle speed | Set to manual specification; chain must stay still |
| L screw | Low-speed mixture and response | Adjust only if the manual permits |
| H screw | High-speed mixture and maximum rpm | Use the manual’s limit and a tachometer; otherwise leave it alone |
| Tachometer reading | Engine speed | Compare with the serial-specific manual, not sound |
| Chain movement at idle | Safe idle condition | Stop and correct before operating |
Takeaway: Change one permitted setting at a time, then measure and recheck.
Read the result before making another adjustment
A useful tune ends with a stable idle and clean acceleration, not simply a smoother sound. The symptoms below help decide whether to stop tuning and return to diagnosis. They are clues, not proof of a single failed part.
| What you observe | What to do next |
|---|---|
| Chain creeps at idle | Stop; check idle setting and investigate throttle or clutch faults if it continues |
| Engine hesitates on acceleration | Recheck fuel, filter, and intake condition; adjust L only if authorized |
| Surging or hanging idle | Stop; investigate fuel delivery or air leaks rather than leaning on the screws |
| Maximum rpm exceeds the manual limit | Stop and correct it with the approved procedure; do not keep running |
| Speed cannot meet the manual limit | Stop; check for underlying faults or refer for service |
| Saw cuts poorly but idles and accelerates normally | Check chain sharpness, tension, bar condition, and oiling |
For example, if a saw idles but bogs when you open the throttle, it is tempting to turn L right away. First confirm fresh fuel, a clear air filter, and an unobstructed exhaust. If those checks pass and the manual permits L adjustment, make a small change and reassess. If the problem remains, do not keep turning screws to force a result.
A second pattern is a saw that idles too fast and keeps the chain moving. Lowering the idle may help only when the cause is the T/LA setting. If the chain still creeps with the idle set to specification, stop and inspect the saw rather than accepting unsafe operation.
Takeaway: A failed result after a small, permitted adjustment is a reason to diagnose, not to turn farther.
Finish safely and keep the setting reliable
After adjustment, confirm that the chain stops at idle, the throttle returns freely, the stop switch works, and the chain brake functions. Check that the chain receives oil using the procedure in the manual. Use quality bar-and-chain oil; flow depends on the saw, bar, and conditions, so do not assume one fixed rate.
Do not compensate for poor cutting by leaning the mixture or forcing a dull chain through wood. Sharpen the chain to its maker’s filing angle and depth-gauge specification, using the correct gauge tool. A .025-inch (0.635 mm) depth-gauge setting is common for standard chain, but the fitted chain’s specification takes priority.
For shutdown, engage the brake, switch off, and let the saw cool in a clear area. Inspect for leaks, damage, and loose parts. Before storage, follow the manual’s fuel instructions; for extended storage, drain or stabilize fuel as directed. Recheck fuel freshness, chain condition, and oiling before the next use.
Takeaway: The tune is not complete until the saw idles safely, accelerates cleanly, and passes its safety checks.
Frequently asked questions
These short answers cover the adjustment decisions that most often stop a 446S owner from making progress. Use them with the manual for your exact saw; published reference values do not replace serial-specific instructions.
What is the idle speed for a Shindaiwa 446S?
A commonly published reference is 2,800 rpm. Confirm the correct value in the manual for your saw, and make sure the chain stays still at idle.
What is the maximum rpm for a 446S?
A commonly published reference is 13,000 rpm. Treat the serial-specific manual as the limit, and use a tachometer. Do not set maximum speed by ear.
What fuel mix does the 446S use?
The specified mix is 50:1 gasoline to approved two-cycle oil, or 2.6 US fluid ounces of oil per US gallon of gasoline. Check your manual and use fresh fuel.
Which screw sets idle speed?
The T or LA screw controls idle speed. Adjust it only to the manual’s setting, with the chain stationary.
Can I tune the H screw without a tachometer?
No. Do not tune the high-speed circuit without the correct manual limit and an accurate tachometer. If the manual does not permit user adjustment, leave it to an authorized technician.
Why does the chain move while the saw idles?
The idle may be set too high, or a throttle or clutch fault may be present. Stop the saw and correct the cause before operating.
Why does the saw bog when I accelerate?
Fuel, air, exhaust, or carburetor issues can cause hesitation. Check basic condition first; adjust L only if the manual allows it.
What if the saw surges or hangs at high idle?
Stop using it. Investigate fuel or air faults and do not try to cure the symptom by guessing at screw settings.
Can I use another Shindaiwa model’s carburetor settings?
No. Do not transfer adjustment limits or torque values between models. Use the 446S manual that matches your serial number and configuration.
When should I stop and get service?
Stop if the manual is unavailable, adjustment screws are sealed or restricted, the saw will not meet its limits, or it shows surging, abnormal heat, or power loss. These conditions need diagnosis before further use.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
