Craftsman Lawn Mower Will Not Start When Hot: Proven Fixes – 2025

Nothing is more frustrating than mowing halfway through your yard only to find your Craftsman lawn mower will not start when hot. The quick fix to try first: open the fuel cap for 30 seconds to relieve vapor lock, then pull the starter with the choke off and throttle at fast. If it fires, you’re dealing with heat-related fuel delivery or venting issues.

In this comprehensive guide, you’ll learn every common cause of hot-start problems—vapor lock, weak ignition coils, carburetor flooding, and compression loss—and how to fix each one step by step. We’ll cover both push mowers and Craftsman riders with Briggs & Stratton, Kohler, and Honda engines, with simple and advanced solutions, testing tips, and prevention advice to get your mower running reliably again.

Common Causes of a Craftsman Lawn Mower Not Starting When Hot

• Vapor lock from a blocked fuel cap vent or fuel line routing near hot engine parts
• Failing ignition coil (magneto) that loses spark when hot
• Carburetor flooding or heat soak causing a rich condition
• Sticking choke or auto-choke misadjustment
• Faulty spark plug or heat range mismatch
• Valve lash out of spec causing low compression when hot
• Weak fuel pump (on riders) or degraded fuel line collapsing when warm
• Old ethanol fuel causing boiling, varnish, or gumming
• Dirty air filter restricting airflow when hot
• Safety switches (seat, brake, blade) failing as they warm up (riders)

How to Fix a Craftsman Lawn Mower That Won’t Start When Hot

Method 1: Quick Checks and Vapor Lock Relief

This is the fastest way to diagnose fuel vaporization and simple venting issues.

Diagnosing the Issue

• Symptoms: Mower runs fine cold, stalls or won’t restart after 10–30 minutes, then restarts after cooling.
• Often accompanied by a faint fuel smell or a “hollow” sound when pulling the starter.

Fix Steps

1. Relieve tank pressure
   – Turn off engine (if running). Open the fuel cap slowly. Listen for a hiss.
   – Leave cap loose for 30–60 seconds to vent.
2. Attempt a hot start
   – Set throttle to fast, choke OFF (hot engine doesn’t need choke).
   – Pull starter or turn key. If it starts, replace or clean the fuel cap.
3. Inspect the fuel cap vent
   – Blow through the cap vents. If blocked, clean with compressed air or replace the cap.
4. Reroute/insulate fuel line (if close to the muffler)
   – Keep lines away from hot surfaces. Add a small heat sleeve or reflective tape.
5. Use fresh fuel
   – Drain old gas and fill with fresh, ethanol-free fuel if available, or E10 with stabilizer.

Testing

• Mow for 20 minutes, shut off, and attempt a restart with choke off.
• If the problem disappears, the issue was venting/vapor lock.

Method 2: Restore Hot Spark – Ignition Coil and Plug

Ignition coils can fail when hot, losing spark until they cool.

Diagnosing the Issue

• When hot, engine cranks strongly but no fire. After 10–30 minutes, starts again.
• Spark is weak or absent when tested immediately after shutoff.

Fix Steps

1. Test for spark when hot
   – Remove spark plug, reconnect wire, ground plug threads to engine, and crank.
   – Look for a strong blue spark. Weak or no spark suggests a failing coil or plug.
2. Replace the spark plug
   – Use OEM spec: typically NGK BPR6ES, Champion RJ19LM, J19LM, or per engine manual.
   – Set gap to 0.028–0.030 in (0.7–0.76 mm).
3. Inspect the ignition coil (armature)
   – Check coil air gap: 0.010–0.014 in (0.25–0.36 mm) using a business card or feeler gauge.
   – Inspect coil wire for cracks; ensure kill wire terminal is clean and tight.
4. Perform hot coil test
   – With the engine hot and failing to start, disconnect the small kill wire from the coil and try to start. If it now starts, you have a shorting kill circuit/safety switch issue. If still no start and no spark, the coil is bad.
5. Replace the ignition coil if needed
   – Use OEM or high-quality aftermarket coil for your engine model (Briggs: 593872/590454; Kohler/Honda varies by model). Torque coil bolts per manual (~30–40 in-lb).

Testing

• Run 30 minutes, shut off, hot restart. Confirm strong spark and reliable starts.

Method 3: Fix Carburetor Flooding and Heat Soak

Heat can expand fuel and cause a rich/flooded condition, especially with dirty carbs or sticky needle valves.

Diagnosing the Issue

• Hot engine smells like fuel; plug is wet after failed hot start.
• Starts when you hold throttle open and air filter removed.
• Primer bulb may be soft or collapsing.

Fix Steps

1. Try the de-flood procedure
   – Choke off, throttle full, pull starter 5–6 times to clear excess fuel.
   – If it fires briefly, you’re likely flooded.
2. Clean or rebuild the carburetor
   – Shut fuel valve or clamp line; remove carb.
   – Disassemble bowl, float, needle, and jets. Clean all passages with carb cleaner and compressed air.
   – Replace needle/seat and bowl gasket; install a new main jet/emulsion tube if corroded.
3. Check float height and needle seating
   – Float should sit level when inverted; replace if fuel-logged.
4. Inspect choke plate and linkage
   – Ensure choke fully opens when hot. Adjust auto-choke thermostatic spring (if equipped) per manual.
5. Replace the air filter
   – A clogged filter can cause rich running when hot. Replace foam/paper elements as needed.
6. Add a fuel shutoff valve (if not present)
   – Helps prevent heat soak flooding after shutdown: close valve 30–60 seconds before stopping to run bowl low.

Testing

• Hot restart with choke off should be immediate. If it still floods, verify fuel level in bowl and needle sealing.

Method 4: Address Low Hot Compression (Valve Lash and Head Gasket)

As engines warm, valve clearance can tighten and compression can drop, making hot starts difficult.

Diagnosing the Issue

• Pull cord becomes unusually easy when hot; engine spins fast but won’t catch.
• Backfiring through carb or muffler.
• On OHV engines, valve lash is out of spec or auto-decompression isn’t working.

Fix Steps

1. Check and adjust valve lash (OHV engines)
   – Remove valve cover. Rotate engine to compression stroke TDC, then slightly past to close ACR.
   – Typical clearances: Intake 0.004–0.006 in, Exhaust 0.006–0.008 in (check your engine spec).
   – Loosen rocker nut, adjust with feeler gauge, and retighten.
   – Replace valve cover gasket if needed.
2. Inspect head gasket (Briggs Intek riders are prone)
   – Look for oil blow-by, hissing, or soot near head. Compression test can help (>90 psi typical).
   – Replace head gasket; torque head bolts in sequence to spec.
3. Check for worn starter or battery (riders)
   – Hot engine needs good cranking speed. Clean battery terminals; test battery (≥12.6V resting). Replace weak starter solenoid if sluggish.

Testing

• After adjustment, the engine should crank evenly and fire quickly when hot.

Method 5: Safety Switches and Electrical Shorts (Riding Mowers)

Heat and vibration can expose weak safety circuits.

Diagnosing the Issue

• Starts fine cold. After mowing, no crank or no spark until cooled.
• Wiggle seat, PTO, or brake pedals changes symptoms.

Fix Steps

1. Inspect safety switches
   – Seat, brake, clutch, and PTO switches: ensure firm activation and clean connectors.
2. Test kill circuit
   – With key on, measure continuity to ground on the coil kill wire when switches are in RUN. Should be open. If you see intermittent ground when hot, a switch or wire is shorting.
3. Check harness routing
   – Keep wires away from the muffler and moving parts; repair melted insulation.
4. Replace suspect switch
   – Many Craftsman riders use simple plunger or lever micro-switches—cheap and easy to swap.

Testing

• Run for 20–30 minutes, shut down, and attempt hot restarts repeatedly. Confirm consistent crank and spark.

Prevention Tips and Maintenance Advice

• Use fresh fuel and stabilizer; avoid fuel older than 30 days.
• Replace the fuel cap if venting is weak; clean it annually.
• Keep fuel lines away from the muffler; add heat sleeves if needed.
• Service the air filter every 25 hours; replace paper filters yearly.
• Replace spark plug every season or 100 hours.
• Clean cooling fins and shrouds so the engine runs cooler.
• Adjust valve lash annually on OHV engines.
• On riders, keep electrical connectors clean and protected with dielectric grease.

Pro Tips

• Hot-start technique: choke off, throttle fast, and crack the throttle slightly on manual controls.
• If you smell raw fuel after shutdown, open the throttle and pull with no choke to clear flooding.
• For ethanol fuel areas, use fuel stabilizer with vapor-lock inhibitors and store fuel in a shaded, cool place.
• Install an inline fuel shutoff valve and run the carb near-empty before parking on hot days to reduce heat soak.
• Keep debris away from the engine shroud and muffler—heat retention worsens hot-start issues.
• When replacing a coil, always set the air gap with a business card evenly across the flywheel magnets.
• Don’t overtighten the fuel cap; some designs need a slight vent clearance to function.

When to Call a Professional

• You have no spark when hot even after new plug and coil—may indicate flywheel magnet or wiring faults.
• Persistent flooding after a carb rebuild—possible warped carb body or intake leak.
• Low compression or suspected head gasket failure—requires torque sequence and precision.
• Electrical issues on riders where multiple safety switches and interlocks are involved.
• Repeated vapor lock on V-twin riders—may need rerouting fuel lines, adding heat shields, or replacing the fuel pump.

Typical costs:
– Spark plug: $4–$10
– Fuel cap: $10–$20
– Ignition coil: $25–$60 (single cylinder), $60–$120 (V-twin)
– Carburetor kit/replacement: $15–$90
– Valve adjustment service: $80–$150
– Head gasket replacement: $120–$300

Warranty notes:
– Newer Craftsman models may have engine warranties separately through Briggs & Stratton, Kohler, or Honda. Avoid modifications that could void coverage; document maintenance.

FAQ

Q: Why does my Craftsman mower start cold but not hot?

A: Heat reveals weak components and causes fuel to vaporize. The most common culprits are a failing ignition coil, a blocked fuel cap vent causing vapor lock, carburetor flooding from heat soak, or tight valve lash reducing hot compression.

Q: How can I tell if vapor lock is my problem?

A: Loosen the fuel cap when the hot no-start occurs. If it starts right away, the vent is blocked. Also check if the fuel line is routed near the muffler—rerouting or insulating the line often fixes it.

Q: Should I use the choke for hot starts?

A: No. Using choke on a hot engine often floods it. For hot starts, set throttle to fast, choke off, and pull/turn key. If flooded, open throttle fully and crank to clear.

Q: How do I test the ignition coil when it’s hot?

A: Immediately after the stall, remove the spark plug, ground it to the engine, and check for a strong blue spark while cranking. No spark when hot but spark when cool points to a failing coil. Disconnect the coil’s kill wire to rule out safety circuit shorts.

Q: Can old gasoline cause hot starting problems?

A: Yes. Ethanol-blended fuel absorbs water, boils more readily, and can varnish jets. Drain old fuel, clean the carb, and use fresh fuel with stabilizer. Ethanol-free gas often improves hot restarts.

Q: What’s the right spark plug for my Craftsman mower?

A: It depends on the engine. Common options include NGK BPR6ES, Champion RJ19LM/J19LM, or equivalent. Check your engine model and set gap to 0.028–0.030 in.

Q: My riding mower cranks slow when hot—what should I check?

A: Test battery voltage (≥12.6V rested), clean terminals, inspect the starter and solenoid, and verify valve lash. Tight exhaust valves or a failing starter can cause hot slow cranking.

Q: How often should I adjust valve lash?

A: Check annually or every 100 hours on OHV engines. Heat and wear can tighten lash, reducing compression and making hot starts difficult.

Alternative Solutions

If you continue to experience hot-start vapor lock or heat soak on a rider, consider these upgrades:

Get Your Craftsman Mower Working Again

By now, you have multiple proven fixes to solve hot starting issues on a Craftsman lawn mower:
– Quick vapor lock checks: vent the tank, replace the fuel cap, reroute fuel lines
– Restore hot spark: new spark plug, correct coil gap, replace failing ignition coil
– Stop carb heat soak and flooding: clean/rebuild carb, ensure choke opens, fresh fuel
– Recover hot compression: adjust valve lash and inspect head gasket
– Verify rider safety circuits and wiring when heat-related electrical faults occur

Follow these steps in order, and you’ll eliminate the most likely causes first with minimal tools and cost. Don’t let a heat-soaked engine derail your mowing—tackle the quick fixes today, and your Craftsman lawn mower will not start when hot issue should be gone for good.

Have you fixed your hot-start problem? Share what worked for your model in the comments to help other owners. Found this helpful? Bookmark it for future tune-ups and seasonal maintenance.