When your Craftsman lawn mower primer bulb isn’t working, it’s frustrating—especially when you’ve pulled the cord a dozen times and the engine still won’t fire. The quick fix for many cases: inspect the bulb for cracks, make sure it fills with fuel when pressed, and replace the bulb or reconnect the fuel lines if you see air, cracks, or dry pressing. Many issues can be solved in under 15 minutes with a new primer bulb or by reconnecting a loose fuel line.
In this guide, you’ll learn the most common causes of a Craftsman primer bulb not working, how to diagnose whether it’s a bulb, line, or carburetor problem, and multiple step-by-step fixes from simple to advanced. By the end, you’ll be able to reliably prime your mower and get it started without the guesswork.
Common Causes of a Craftsman Lawn Mower Primer Bulb Not Working
- Cracked or hardened primer bulb: Age, ethanol fuel, and sun exposure can cause the bulb to stiffen or crack, letting air in and preventing suction.
- Disconnected or reversed fuel lines: On most Craftsman mowers with primer bulbs, one line feeds fuel from tank/carb to bulb and the other returns air/fuel to the carb. Reversed lines stop priming.
- Clogged carburetor check valve or primer port: Dirt or varnished fuel blocks the tiny passages the primer uses to push fuel.
- Split or deteriorated fuel lines: Small cracks allow air to enter, so the bulb never fills with fuel.
- Blocked fuel filter or tank vent: The bulb collapses or doesn’t refill if air can’t enter the tank or fuel can’t flow.
- Faulty primer base/carburetor gasket: A torn gasket leaks air, reducing priming pressure.
- Wrong replacement bulb or base: Incorrect bulb style or mismatched base won’t seal or align with the carburetor ports.
- Cold-start flooding or no fuel in bowl: If the carb bowl is empty or flooded, the primer’s effect is minimal or inconsistent.
How to Fix a Craftsman Primer Bulb Not Working
Before you start: Work in a ventilated area. Allow a hot engine to cool. Shut off the fuel valve if equipped or clamp the fuel line to prevent spills. Keep a rag handy.
Method 1: Quick Checks and Easy Fixes
Diagnosing the Issue
- Press the primer bulb 3–5 times and watch:
- Does it fill with fuel or stay mostly full of air?
- Does the bulb stay collapsed after pressing?
- Is fuel visible in the clear bulb or line?
- Look for:
- Cracks in the bulb or base
- Loose, brittle, or disconnected lines
- Fuel leaking around the bulb or carb
What the symptoms mean:
– Bulb stays collapsed: Tank vent blocked or outlet/line clogged.
– Bulb never fills with fuel: Reversed lines, air leak, cracked bulb, or carb port blockage.
– Fuel squirts out or wet around bulb: Torn gasket or cracked base.
Fix Steps
- Inspect the bulb
– If the bulb is cracked, hardened, or cloudy, replace it. Many Craftsman models use a snap-in bulb on the air filter cover or a bulb integrated into the carb body. - Check fuel lines
– Trace both lines: primer bulb to carb, and carb/tank to primer bulb. Ensure snug fit on barbs. If lines are brittle or darkened, replace them with ethanol-resistant 3/16″ or 1/4″ line (match original size). - Verify line routing
– Typical routing for primer systems:- Bulb “inlet” connects to the carburetor’s primer port (draws fuel from carb bowl).
- Bulb “outlet” returns fuel/air to the fuel tank or the carb bowl vent.
- If reversed, swap them and retest.
- Check fuel cap vent
– Loosen the cap slightly and press the bulb. If the bulb now refills normally, the cap vent is clogged—clean or replace the cap.
Testing
- Prime 5–7 times and try starting. If it fires or sputters more than before, you’ve improved fuel delivery.
- If still no fuel in bulb, move to Method 2.
Method 2: Replace the Primer Bulb and Gasket (Most Common Fix)
Many Craftsman walk-behind mowers (including those using Briggs & Stratton and some Honda/MTD engines) rely on a simple rubber bulb and a sealing gasket. Age is the #1 failure point.
Diagnosing the Issue
- Bulb cracked/hard or won’t rebound smoothly = replace.
- Fuel seepage at bulb base = failed gasket or base.
Fix Steps
- Identify your model
– Look for the Craftsman model number on the deck or engine shroud. Use it to order the exact bulb and gasket kit. Common terms: “primer bulb kit,” “carburetor primer bulb,” “primer base gasket.” - Remove the old bulb
– If the bulb is on the air filter cover: remove the cover screws, gently pry the bulb retainer, and pull the bulb out.
– If the bulb is part of the carb: remove the retaining screws around the bulb housing. - Inspect and replace gasket
– Clean the seating surface with carb cleaner and a lint-free rag. Install the new gasket, align with ports, then seat the new bulb. - Reassemble
– Tighten screws evenly to avoid warping. Reconnect lines firmly.
Time: 10–25 minutes.
Testing
- Prime 5–7 presses. The bulb should fill partially with fuel and rebound.
- Start engine. If no improvement, proceed to Method 3.
Method 3: Clean the Carburetor Primer Circuit and Jet Passages
If the primer bulb won’t fill but the bulb and lines are good, the carburetor’s primer port or check valve may be blocked with varnish.
Diagnosing the Issue
- Fuel lines are good and correctly routed, bulb is new, but pressing the primer does little or nothing.
Fix Steps
- Remove air filter and carb
– Shut off fuel or clamp line. Remove air filter housing and linkage carefully. Photograph linkage orientation for reassembly. - Disassemble carburetor
– Remove the fuel bowl (10mm bolt on many Briggs carbs). Note the bowl gasket and O-ring.
– Identify the primer port passages (small holes near the throat or side of carb). - Clean passages
– Use carb cleaner with straw to spray through:- Main jet and emulsion tube
- Primer port and the tiny check valve area
- Follow with compressed air. Do not poke with metal wire—use nylon bristle or a soft tag wire to avoid damage.
- Inspect diaphragm and gaskets (if diaphragm-style carb)
– On some Craftsman engines (especially with primer-based pulsing), a stiff diaphragm prevents proper priming. Replace diaphragm/gasket kit if stiff, wavy, or torn. - Reassemble
– Replace bowl gasket if swollen. Reinstall carb, linkages, and fuel line.
Time: 30–60 minutes.
Testing
- Prime 5–7 times. You should see fuel move through the clear line or partially fill the bulb. Start engine. If it starts, let it run 3–5 minutes, then shut off and confirm hot restart with minimal priming.
Method 4: Replace Deteriorated Fuel Lines and Filter; Verify Tank Venting
Air leaks in the lines or a blocked filter prevent the primer from creating pressure.
Diagnosing the Issue
- Lines feel sticky, brittle, or cracked.
- Bulb works intermittently.
- Bulb stays collapsed until you loosen fuel cap.
Fix Steps
- Replace fuel lines
– Use ethanol-resistant line in the original sizes (commonly 3/16″ or 1/4″). Cut clean square ends.
– Route lines to avoid kinks and hot surfaces. Secure with spring clips where used. - Replace in-tank fuel filter (if applicable)
– Some Craftsman mowers use an inline filter instead. Replace with the correct flow direction. - Verify cap vent
– Clean the cap vent hole; replace cap if vent is blocked or the seal is deformed. - Fresh fuel
– Drain old fuel and refill with fresh, ethanol-free fuel if possible, or E10 less than 30 days old. Add fuel stabilizer if storing.
Time: 20–40 minutes.
Testing
- Prime and observe consistent bulb rebound and visible fuel movement. Start and run under load for a few minutes to confirm.
How to Prevent Future Primer Bulb Problems
- Use ethanol-free fuel or add stabilizer: Ethanol degrades rubber parts and causes varnish.
- Replace air filter annually: Dirty filters cause rich running and can wet the primer area.
- Run the engine monthly during the season: Keeps passages clear and the carb wet with fresh fuel.
- Winterize properly: Run the carb dry or drain the bowl at season’s end.
- Keep mower covered: UV light hardens rubber bulbs and lines.
- Replace bulb and lines proactively every 2–3 seasons if you use E10 fuel.
Pro Tips
- Mark fuel lines before removal: Use tape or photos to prevent reversing them.
- Prime count matters: Most Craftsman engines start best with 3–5 primer presses in warm weather, 5–7 when cool.
- Don’t over-prime: Excess priming can flood the engine. If flooded, open throttle, hold blade disengaged, and pull with choke off.
- Use the right bulb: Bulbs vary by base depth and port spacing. Match by model number or OEM part to ensure a proper seal.
- Check the carb bowl O-ring: A flattened O-ring leaks air, reducing priming effectiveness and causing hard starts.
- Avoid metal probes in jets: You can enlarge or damage the jet—use carb spray and air instead.
- Replace gaskets as a set: If you remove the carb or primer base, new gaskets help ensure airtight priming.
When to Call a Professional
- Persistent no-prime after bulb/line replacement and carb cleaning
- Fuel leaks from carb body or primer base despite new gaskets
- Recoil pulls are excessively hard or backfires occur (could be timing/valve issues)
- Plastic carb body is warped or cracked
- You’re uncomfortable removing and rebuilding the carb
What to look for:
– Small engine technician experienced with Craftsman/Briggs & Stratton
– Flat diagnostic fee and clear parts/labor estimates
– Use of OEM-quality parts and a warranty on work (30–90 days common)
Typical costs:
– Primer bulb and line replacement: $30–$80
– Carburetor cleaning/rebuild: $70–$150
– New carburetor (if needed): $60–$140 for common models, plus labor
Warranty considerations:
– If your mower is under warranty, use authorized service to avoid coverage issues. Keep receipts for parts and fuel stabilizer if claiming fuel-related failures.
FAQ
Q: How do I know if my primer bulb is bad or if it’s a fuel line problem?
A: If the bulb is cracked, hard, or won’t rebound, it’s likely the bulb. If it collapses and stays in, suspect a tank vent or line blockage. If the bulb rebounds but never shows fuel, check line routing and carb primer passages.
Q: How many times should I press the primer on a Craftsman mower?
A: Typically 3–5 times in warm weather and 5–7 in cooler conditions. Check your engine’s decal or manual. Over-priming can flood the engine.
Q: Can I use any universal primer bulb?
A: Not always. Bulbs differ in mounting style, depth, and port spacing. Use your mower’s model or engine code to select the correct OEM or high-quality aftermarket match.
Q: Why does the primer bulb fill with air instead of fuel?
A: Air leaks from cracked lines or gaskets, reversed line routing, or blocked carb primer ports let the bulb move air but not fuel. Fix leaks, verify routing, and clean the carb passages.
Q: My bulb stays collapsed after I press it. What does that mean?
A: The tank isn’t venting or the outlet/filter is blocked. Loosen the cap and try again. If it rebounds with the cap loose, replace/clean the cap or check the in-tank filter and lines.
Q: Do I need to drain the fuel before replacing the primer bulb?
A: It’s safer to clamp the fuel line or shut off the valve if equipped. If the bulb is on the carb, fuel may drip during service—use a rag and work in a ventilated area.
Q: Will a dirty air filter affect the primer?
A: Indirectly. A severely clogged filter can cause rich running and starting problems, but it doesn’t usually prevent the primer from moving fuel. Replace the filter annually.
Alternative Paths if Priming Still Fails
If priming still doesn’t deliver fuel after bulb/line/gasket replacement and carb cleaning, consider these options:
| Solution | Pros | Cons | Best For |
|---|---|---|---|
| Replace entire carburetor | Fast, restores factory performance | Higher cost than cleaning | Severely varnished or warped carbs |
| Switch to non-ethanol fuel + stabilizer | Protects rubber parts, improves starting | Higher fuel cost, availability varies | Preventing repeat failures |
| Professional ultrasonic cleaning | Deep cleans internal passages | Shop visit and labor cost | Stubborn internal blockages |
Get Your Craftsman Mower Starting Again
By now, you’ve seen the main reasons a Craftsman lawn mower primer bulb isn’t working and how to fix them:
– Quick inspections for cracked bulbs, reversed lines, and venting issues
– Replacing the primer bulb and gaskets
– Cleaning carb primer passages and jets
– Renewing fuel lines, filter, and cap vent
– Using fresh, stabilized fuel to prevent future issues
Follow these steps in order, and you’ll restore reliable priming and easy starts. Don’t let a small rubber bulb sideline your mowing—most fixes take under an hour with simple tools.
Have you fixed your Craftsman lawn mower primer bulb? Share your model number and what worked for you to help other readers. Found this helpful? Bookmark it for your next tune-up.