How To Adjust Carburetor On Craftsman Chainsaw: Tune Up Guide

Can I adjust the carburetor on my Craftsman chainsaw? Yes, you can adjust the carburetor on your Craftsman chainsaw to improve its performance, especially if you’re experiencing issues like chainsaw sputtering or a general lack of power. This guide will walk you through the process of carburetor tuning for your Craftsman chainsaw, focusing on common adjustments and troubleshooting tips. We’ll cover everything from locating the adjustment screws to fine-tuning for optimal engine performance tuning.

Why Carburetor Adjustment is Important

Your chainsaw’s carburetor is a crucial component that mixes air and fuel in the correct ratio for combustion. Over time, and with varying fuel quality or atmospheric conditions, this mixture can become less than ideal. This can lead to several problems:

Chainsaw sputtering: This is a classic symptom of an incorrect fuel-air mixture. The engine may run erratically, hesitate, or die under load.
Poor acceleration: The chainsaw might struggle to rev up quickly.
Loss of power: The saw may feel weak, even when cutting through soft wood.
Difficulty starting: The engine might be hard to start or stall easily.
Black smoke from the exhaust: This usually indicates too much fuel (rich mixture).
Engine overheating: Too little fuel (lean mixture) can cause the engine to run hot.

Regular carburetor tuning is a vital part of maintaining your Craftsman chainsaw, ensuring it runs efficiently, powerfully, and reliably. It’s a key step in engine performance tuning that you can do yourself.

What You’ll Need for Carburetor Adjustment

Before you start, gather the necessary tools and materials:

Chainsaw Tune-Up Kit: Many kits include a tachometer and specialized screwdrivers.
Screwdrivers: You’ll likely need a flat-head screwdriver, and possibly a small, specialized “pac-man” or “D-handle” screwdriver for accessing the fuel mixture screws.
Tachometer: Essential for accurately setting the idle and high-speed RPMs. A digital tachometer is generally easier to use.
Carburetor Cleaner: A can of carburetor cleaner is useful for cleaning any debris around the adjustment screws or on the carburetor itself before you begin.
Clean Rags: For wiping down parts and cleaning up any spilled fuel.
Gloves and Eye Protection: Safety first!
Owner’s Manual: Always refer to your specific Craftsman chainsaw model’s owner’s manual for recommended settings and any model-specific procedures. For example, the adjustment points might vary slightly on a Craftsman 35cc chainsaw compared to a larger model.

Locating the Carburetor and Adjustment Screws

The carburetor is typically located under a plastic cover or air filter housing. On most Craftsman chainsaws, you’ll need to remove the top engine cover to access it.

Once the cover is off, you’ll see the carburetor. It’s a metal or plastic component with fuel lines connected to it and an air filter attached to its opening. On the side or top of the carburetor, you will find the adjustment screws.

Idle Speed Screw: This is usually a larger screw, often with a spring behind it, that contacts a linkage on the throttle. Turning this screw adjusts the engine’s idle speed.
Fuel Mixture Screws (Low Speed and High Speed): These are typically smaller screws.
- The low speed jet adjustment screw controls the fuel mixture at idle and low RPMs.
- The high speed jet adjustment screw controls the fuel mixture at higher RPMs and under load.

On some newer or more basic carburetors, you might only have an idle speed screw and a single mixture screw, or even no external mixture screws at all (these are often electronically controlled). However, for many common Craftsman models, you will find both the idle speed screw and the fuel mixture screws.

The “Pac-Man” or “D-Handle” Screwdriver

Many modern carburetors use screws that are not a standard slotted or Phillips head. These often have a small “D” shape or a small circle with a cross shape inside. You’ll need a specialized screwdriver for these. These are commonly found in chainsaw tune-up kits. It’s crucial to use the correct screwdriver to avoid damaging the delicate adjustment screws.

The “Lean” vs. “Rich” Mixture Explained

Before you adjust anything, it’s important to grasp the concept of fuel mixture.

Lean Mixture: This means there is too much air relative to fuel. A lean mixture can cause the engine to run hot, hesitate, or seize.
Rich Mixture: This means there is too much fuel relative to air. A rich mixture often results in a smoky exhaust, poor acceleration, and fouled spark plugs.

The goal of carburetor tuning is to find the “sweet spot” where the engine receives the optimal amount of fuel for clean and efficient combustion across its entire operating range.

Step-by-Step Carburetor Adjustment Guide

This process aims to set both the idle speed and the fuel mixture for optimal performance. We’ll start with the initial settings and then fine-tune.

Step 1: Initial Setup and Safety Precautions

Ensure a Safe Environment: Work in a well-ventilated area, away from flammable materials.
Check the Spark Plug: A fouled or worn spark plug can mimic carburetor problems. Inspect it, clean it, or replace it if necessary. Ensure the gap is set correctly.
Clean the Air Filter: A clogged air filter restricts airflow, making the mixture run rich. Clean or replace it.
Check for Fuel Leaks: Inspect fuel lines and the carburetor for any leaks.
Ensure Fresh Fuel: Use fresh fuel mixed with the correct oil ratio. Old fuel can cause engine issues.
Locate the Adjustment Screws: As discussed, identify the idle speed screw and the fuel mixture screws.

Step 2: Setting the Carburetor Adjustment Screws (Initial “Out” Setting)

This is a standard starting point for most carburetors.

Gently turn each adjustment screw (both low and high speed) clockwise until it just seats. Be very careful not to overtighten, as this can damage the carburetor. Stop immediately when you feel resistance.
Back out each mixture screw counter-clockwise.
- Low Speed Jet: Typically, back it out 1 to 1.5 turns.
- High Speed Jet: Typically, back it out 1 to 1.5 turns.
- Refer to your owner’s manual for specific starting points for your model, as these can vary.

These “out” settings provide a baseline. Now, we’ll start the engine and begin the fine-tuning.

Step 3: Starting the Engine and Setting Idle Speed

Prime the Carburetor: If your chainsaw has a primer bulb, press it several times until you see fuel in it.
Start the Chainsaw: Engage the choke, start the engine, and let it run for about 10-15 seconds.
Disengage the Choke: Move the choke to the “run” position and let the engine warm up for a minute or two.
Locate the Idle Speed Screw: This is the screw that adjusts how fast the engine idles when you’re not pressing the throttle.
Adjust the Idle Speed: With the engine warm, slowly turn the idle speed screw clockwise to increase the idle speed or counter-clockwise to decrease it.
- Target Idle RPM: Your owner’s manual will specify the correct idle RPM. A common range is 2,800-3,200 RPM, but always check your manual.
- Listen and Observe: The engine should idle smoothly without stalling. If it dies when you take it off choke, you need a slightly higher idle speed. If it’s racing, you need to lower it. The chain should not be moving at idle.

Step 4: Adjusting the Fuel Mixture Screws (Low and High Speed)

This is where we fine-tune the fuel-air ratio for optimal performance. This is often the trickiest part and may require a tachometer for precision.

Adjusting the Low Speed Jet

The low speed jet affects the mixture from idle up to about half throttle.

Engine at Idle: Ensure the engine is idling at the correct RPM (from Step 3).
Slowly turn the low speed mixture screw clockwise (leaner) until the engine RPM starts to drop or sound rough.
Then, slowly turn the screw counter-clockwise (richer) until the engine RPM picks up smoothly.
Continue turning counter-clockwise until the engine starts to sputter or run rough again.
Find the Middle Ground: Back the screw clockwise until the engine runs smoothly and at its highest RPM within this range. This is often the point where the engine sounds the most responsive.
Test Acceleration: Gently blip the throttle. The engine should accelerate smoothly without hesitation or bogging down. If it hesitates and then catches up, you might need to richen the low speed mixture slightly (turn counter-clockwise). If it bogs down immediately, it might be too rich.

Adjusting the High Speed Jet

The high speed jet affects the mixture from half throttle to full throttle.

Prepare for High Speed: Ensure your chainsaw is capable of running at full throttle safely. Have a solid object to cut into (like a log) or be prepared to run it at full speed in the air briefly.
Apply Full Throttle: Slowly and smoothly bring the engine to full throttle.
Listen to the Engine:
- If the engine bogs down immediately: The high speed mixture is likely too rich. Turn the high speed screw clockwise (leaner) in small increments (1/8 to 1/4 turn at a time) and re-test full throttle.
- If the engine sounds “raspy,” “tinny,” or like it’s “four-stroking” (a choppy sound) at high RPM: This usually indicates a lean mixture. Turn the high speed screw counter-clockwise (richer) in small increments (1/8 to 1/4 turn at a time) and re-test.
- If the engine hesitates or struggles to reach full RPM: This could be lean or rich, or a restriction. Try slightly richening first.
Finding the Optimal High Speed: The goal is to find the point where the engine reaches its maximum, smooth RPM without sounding raspy. Once you find this point, you might richen it very slightly (about 1/8 turn counter-clockwise) to ensure it’s not too lean and to help prevent overheating. This is often referred to as “foul proofing” the high-speed setting.
Test Under Load: The best test is to use the chainsaw to cut wood. If it bogs down, you may need to adjust the high speed jet.

Step 5: Fine-Tuning the Idle Speed (After Mixture Adjustments)

After adjusting the fuel mixture screws, the idle speed may have changed.

Return to Idle: Let the engine return to idle.
Re-adjust Idle Speed Screw: If the idle speed is too high or too low, readjust the idle speed screw to the manufacturer’s specified RPM. The chain should not spin at idle.

Troubleshooting Common Issues

Here are some common problems and how they relate to carburetor adjustments:

Chainsaw Sputtering at Idle: This often indicates a slightly rich low speed mixture or an incorrect idle speed adjustment. Try leaning the low speed screw slightly (clockwise) or increasing the idle speed.
Chainsaw Sputtering Under Load: This is usually a high speed jet issue, often too lean. Try richening the high speed screw (counter-clockwise).
Hesitation on Acceleration: Can be either the low speed or high speed mixture. If it hesitates when you first hit the throttle, it’s often the low speed. If it hesitates mid-throttle, it could be the high speed.
Engine Dies When Putting Down Throttle: This could be an idle speed that is too low, or the low speed mixture is too lean.
Black Smoke from Exhaust: Too much fuel (rich mixture). Lean both the low and high speed screws slightly (clockwise).
Engine Overheats or Loses Power on Long Runs: Too little fuel (lean mixture). Enrich both the low and high speed screws slightly (counter-clockwise).

Specific Adjustments for a Craftsman 35cc Chainsaw

While the general principles apply, a Craftsman 35cc chainsaw might have specific recommendations. Always consult your owner’s manual for the exact settings. However, here’s a typical scenario for a 35cc model:

Starting Point: Back out the low speed and high speed mixture screws 1.25 turns from seated.
Idle RPM: Aim for around 3,000 RPM (check your manual).
Low Speed Adjustment: Adjust for a smooth idle and quick, clean acceleration from idle.
High Speed Adjustment: Adjust for maximum power at full throttle, ensuring a smooth, consistent sound without any “raspy” or “four-stroking” noise.

Advanced Carburetor Tuning and Considerations

Environmental Factors: Altitude and temperature can affect the ideal fuel mixture. In higher altitudes, you generally need a leaner mixture (turn screws clockwise). In colder temperatures, you might need a slightly richer mixture (turn screws counter-clockwise).
Spark Arrestor Screen: A clogged spark arrestor screen can restrict exhaust flow, making the engine run rich and perform poorly. Clean it regularly.
Carburetor Rebuild: If the chainsaw continues to run poorly after adjustments, the carburetor may need to be cleaned thoroughly or rebuilt. This involves disassembling the carburetor, cleaning all passages with carburetor cleaner, and replacing worn gaskets and diaphragms.
Fuel Pump Carburetors: Some newer chainsaws use carburetors with built-in fuel pumps. These are less common on older Craftsman models but are worth noting.

Table: Common Carburetor Symptoms and Adjustments

Symptom	Likely Cause	Adjustment
Chainsaw sputtering at idle	Idle too low; Low speed mixture too rich	Adjust idle speed screw; Slightly lean low speed mixture screw (clockwise).
Hesitation on acceleration	Low speed mixture too lean or rich; Idle speed too low	Adjust low speed mixture screw; Adjust idle speed screw.
Poor power at high RPM	High speed mixture too lean	Enrich high speed mixture screw (counter-clockwise).
Engine sounds “raspy” at high RPM	High speed mixture too lean	Enrich high speed mixture screw (counter-clockwise).
Engine dies when throttling down	Idle speed too low; Low speed mixture too lean	Adjust idle speed screw; Slightly enrich low speed mixture screw (counter-clockwise).
Black smoke from exhaust	Mixture too rich (low or high speed)	Lean mixture screws (clockwise) incrementally.
Engine overheats/loses power	Mixture too lean (low or high speed)	Enrich mixture screws (counter-clockwise) incrementally.
Chain spins at idle	Idle speed too high	Lower idle speed screw (counter-clockwise).

FAQs About Craftsman Chainsaw Carburetor Adjustment

Q1: How often should I adjust the carburetor on my Craftsman chainsaw?
A1: It’s a good idea to check and adjust the carburetor annually or if you notice a decline in performance. Also, if you’re changing fuel types or operating in significantly different environmental conditions, an adjustment may be needed.

Q2: What happens if I overtighten the fuel mixture screws?
A2: Overtightening can damage the needle valve seats inside the carburetor, leading to leaks and incorrect fuel delivery. Always turn them gently until they just seat.

Q3: My Craftsman chainsaw is sputtering, and I can’t get it right with the adjustments. What else could be wrong?
A3: If you’ve tried adjusting the carburetor and are still experiencing issues like chainsaw sputtering, other common culprits include:
* A dirty or clogged fuel filter.
* A weak or faulty ignition coil.
* Air leaks in the intake manifold or crankcase seals.
* A worn or damaged spark plug.
* A dirty or faulty spark arrestor screen.
* The carburetor itself may need a thorough cleaning or rebuilding.

Q4: Do I need a tachometer to adjust the carburetor?
A4: While you can make basic adjustments by ear, a tachometer is highly recommended for accurate engine performance tuning. It allows you to set the idle speed adjustment and high-speed RPMs to the manufacturer’s specifications, ensuring optimal performance and preventing engine damage.

Q5: What is the difference between the low speed jet and high speed jet?
A5: The low speed jet controls the fuel-air mixture when the engine is idling and at low throttle speeds (up to about half throttle). The high speed jet controls the fuel-air mixture from mid-throttle to full throttle. Both are critical for smooth operation across the entire RPM range.

Q6: Can I use regular gasoline in my Craftsman chainsaw?
A6: No, you should always use a fresh mixture of unleaded gasoline and the correct two-stroke engine oil at the ratio specified by the manufacturer (usually 40:1 or 50:1). Using straight gasoline will severely damage your engine.

By following these steps, you can effectively adjust the carburetor on your Craftsman chainsaw, leading to improved power, smoother operation, and a longer lifespan for your equipment. Remember to always prioritize safety and consult your owner’s manual for model-specific details.