Zama C1Q-S64/S68 Carburetor | Stihl Trimmer & Blower Repair

The Zama C1Q-S64 and C1Q-S68 carburetors power popular Stihl trimmers, blowers and other handheld equipment. These diaphragm carburetors are reliable but eventually need attention. This guide covers identification, common problems and complete rebuild procedures.

Table of Contents

• Equipment Applications
• C1Q-S64 vs C1Q-S68 Differences
• How Diaphragm Carburetors Work
• Failure Symptoms
• Rebuild Procedure
• Adjustment and Tuning

Equipment Applications

Zama supplies carburetors to Stihl for their handheld power equipment line.

C1Q-S64 Applications:

• Stihl FS38, FS45, FS46 trimmers
• Stihl FS55 trimmer
• Stihl HL45 hedge trimmer
• Stihl KM55 Kombi motor

C1Q-S68 Applications:

• Stihl BG45, BG46 blowers
• Stihl BG55, BG65, BG85 blowers
• Stihl SH55, SH85 shredder vacs
• Stihl BR45 backpack blower

[NEED REAL IMAGE: Zama C1Q carburetor on Stihl trimmer/blower]

Identification: The carburetor model is stamped on the carburetor body. Look for “C1Q-S64” or “C1Q-S68” on the casting.

C1Q-S64 vs C1Q-S68 Differences

These carbs share the same basic design with minor calibration differences.

Feature	C1Q-S64	C1Q-S68
Primary Use	Trimmers	Blowers
Main Jet	Slightly smaller	Slightly larger
Fuel Delivery	Same	Same
Rebuild Kit	Same kit works	Same kit works

Can you swap them? They’ll physically fit, but jetting may cause running issues. Best to match carb to application.

How Diaphragm Carburetors Work

Understanding the design helps with diagnosis.

Basic Operation

Unlike float-bowl carburetors, diaphragm carbs use a flexible membrane to pump and meter fuel.

Components:

1. Pump diaphragm - Creates pressure pulses from crankcase
2. Metering diaphragm - Controls fuel flow to jets
3. Inlet needle - Opens to allow fuel into metering chamber
4. Metering lever - Actuates inlet needle based on diaphragm position
5. Main jet - Meters fuel at high speed
6. Idle jet - Meters fuel at low speed

Fuel Flow Path

1. Engine crankcase pulses pump fuel from tank through filter
2. Fuel enters metering chamber controlled by inlet needle
3. Metering diaphragm senses vacuum and opens needle as needed
4. Fuel flows through jets into venturi
5. Air rushing through venturi atomizes fuel

Why They Fail

• Diaphragms stiffen - Age and ethanol harden rubber
• Gaskets leak - Seals deteriorate
• Passages clog - Varnish from old fuel
• Metering lever bends - Affects fuel delivery

Failure Symptoms

Won’t Start

Engine cranks but never fires or only pops.

Check:

1. Fuel reaching carburetor (disconnect line, should flow when priming)
2. Pulse line connected and not cracked
3. Fuel lines not deteriorated

Common cause: Hardened diaphragms not pumping fuel

Starts But Dies Immediately

Fires for a few seconds then quits.

Causes:

• Metering diaphragm stiff - not opening inlet needle
• Clogged fuel pickup in tank
• Air leak in fuel lines

Bogs When Throttle Opened

Idles okay but stumbles when accelerating.

Causes:

• Main jet clogged
• Metering lever set wrong
• Air leak at intake

Won’t Idle - Dies at Low Speed

Runs at high throttle but dies when released.

Causes:

• Idle jet clogged
• Idle mixture screw misadjusted
• Air leak at carburetor gasket

Runs Rich - Four-Strokes at High Speed

Engine “four-strokes” (sounds like it’s missing) under load.

Causes:

• Too much fuel (mixture too rich)
• Metering lever set too high
• Wrong main jet

Runs Lean - High RPM, No Power

Engine screams but produces no power. Dangerous - can cause seizure.

Causes:

• Fuel delivery restriction
• Metering lever too low
• Clogged fuel filter
• Air leak

Rebuild Procedure

A complete rebuild restores these carbs to like-new performance.

Rebuild Kit Contents

Zama RB-89 kit (fits both C1Q-S64 and S68):

• Metering diaphragm
• Pump diaphragm
• Gaskets
• Inlet needle and seat
• Welch plugs
• Primer bulb components

Cost: $8-15

Tools Needed

• Small Phillips and flathead screwdrivers
• Carburetor adjustment tool (Stihl specific)
• Pick or small hook
• Carb cleaner spray
• Compressed air
• Magnification helpful

Disassembly

1. Remove carburetor from equipment

Note air filter, throttle cable and fuel line positions.

2. Remove pump cover

Two screws. Lift off cover and pump diaphragm.

[NEED REAL IMAGE: Zama C1Q carburetor disassembled showing diaphragms]

3. Remove metering diaphragm cover

Usually four screws. Remove cover and metering diaphragm.

4. Remove metering lever pivot screw

The small screw holding the lever. Lift out lever and inlet needle.

5. Pop out welch plugs

Use a pick to pierce and pry out. These cover internal passages.

6. Remove mixture screws (if accessible)

Stihl limits access on newer carbs for EPA compliance. If you can access them, count turns before removal.

Cleaning

Soak small parts in carburetor cleaner. 30 minutes minimum.

Clear every passage with carb cleaner spray and air:

• Main jet passage
• Idle jet passage
• Impulse channel (connects to crankcase)
• Fuel inlet
• All small holes in the body

Inspect the bore for scoring or damage.

Reassembly

1. Install new welch plugs

Tap flat with a punch. Some rebuilders seal edges with nail polish.

2. Install inlet needle and seat

Clean seat area first. New needle from kit.

3. Install metering lever

Critical: Set lever height correctly (see adjustment section).

4. Install new metering diaphragm

Gasket first, then diaphragm (soft side toward lever).

5. Install cover

Snug screws evenly.

6. Install new pump diaphragm

Gasket, diaphragm, cover. Snug screws.

7. Set mixture screws to baseline

If accessible: Low speed 1 turn out, high speed 1 turn out from lightly seated.

Adjustment and Tuning

Metering Lever Height

This is the most critical adjustment. Wrong lever height causes rich or lean running.

Checking Height:

1. Remove metering diaphragm
2. Look at lever height relative to carburetor body
3. Lever should be flush with body or slightly below (0.020” below is typical)

Adjusting:

1. Carefully bend lever at pivot point
2. Up = more fuel (richer)
3. Down = less fuel (leaner)

[NEED REAL IMAGE: Metering lever height measurement on Zama carburetor]

Mixture Screw Adjustment

Important: Many newer Stihl units have limited-access mixture screws to comply with EPA regulations. Special tools or cap removal may be required.

Low Speed (L) Screw:

1. Start engine, let warm up
2. Set to idle
3. Turn L screw for highest, smoothest idle
4. Should accelerate cleanly from idle

High Speed (H) Screw:

1. Run at full throttle under load
2. Turn H screw for maximum power without four-stroking
3. Should run at full RPM without excessive smoke

Baseline Settings:

• L screw: 1 turn out from seated
• H screw: 1 turn out from seated (some models vary)

Idle Speed

Adjust the idle stop screw so:

• Trimmers: Head doesn’t spin at idle
• Blowers: Lowest stable RPM (typically 2800-3200)

Common Issues After Rebuild

Still Won’t Start

Check:

1. Pulse line connected - this pumps fuel
2. Fuel line in tank has pickup filter attached
3. Primer bulb fills with fuel (if equipped)
4. Spark present

Runs But Four-Strokes

Too rich.

Fix: Lower metering lever slightly or adjust H screw clockwise.

Runs Fast Then Dies

Lean condition causing overheating.

Fix: Raise metering lever or adjust H screw counterclockwise.

Fuel Leaks From Carburetor

Check: Metering lever height (too high floods engine), inlet needle seating.

Aftermarket vs OEM

Genuine Zama: $25-40 Aftermarket: $10-20 Rebuild kit: $8-15

For equipment you’ll keep long-term, genuine Zama or the rebuild kit is best. Aftermarket complete carbs vary in quality.

Frequently Asked Questions

Can I adjust the carb without special tools?

The L and H screws need a Stihl adjustment tool (looks like a flat screwdriver with a notch). Generic 2-cycle carb tools also work.

Why does Stihl limit mixture screw access?

EPA regulations require manufacturers to limit user adjustment to reduce emissions. Factory settings meet emissions standards.

My equipment is still sluggish after rebuild. Now what?

Check compression, exhaust screen (spark arrestor), and air filter. Also verify fuel lines aren’t deteriorated.

How often should I rebuild?

Every 2-3 years with heavy use, or when symptoms appear. Using quality fuel with stabilizer extends intervals.

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Zama carburetors are precision instruments. Work clean, take your time with the metering lever and these carbs will give years of reliable service.