FAQ
TL;DR: Under 90 psi compression, most 50cc engines won’t start — “Under 90 psi, most engines won’t run.” Clear flooding, verify float height, test the auto‑choke, and check exhaust. This FAQ is for Yamaha Booster/BWS owners fixing flooded plugs, fuel‑spitting mufflers, and stuck pistons after carb work. ["49ccScoot Compression Test"]
Why it matters: It turns a no‑start after carb work into a simple, safe checklist that prevents damage.
Quick Facts
- Typical 50cc two‑stroke compression: 110–150 psi; below ~90 psi usually won’t start. ["49ccScoot Compression Test"]
- Too‑high carb fuel level or a leaking needle floods the cylinder; set float height per the carb manual. ["Mikuni VM Carburetor Tuning Manual"]
- Hydrolock from raw fuel can jam the piston; never force‑crank with the plug installed. ["Hydrolock - Wikipedia"]
- Fast clear: fuel off, spark plug out, kick through to expel fuel; avoid touching the HV lead. [Elektroda, wodzu_1, post #17099014]
- Blue smoke plus fuel from the muffler signals overfueling; also inspect for an obstructed muffler. [Elektroda, Staszek49, post #17098354]
My 1998 Yamaha Booster floods the spark plug after carb cleaning. What should I check first?
Clear the flood, then verify cylinder compression and fuel level in the carburetor bowl. A high fuel level chokes the engine and pushes raw fuel into the exhaust. Inspect the muffler for blockage, as restriction worsens overfueling symptoms. Fix the cause before further starts. [Elektroda, Staszek49, post #17098354]
How do I safely clear a flooded cylinder on a Yamaha Booster?
- Turn fuel OFF, remove the spark plug, and disconnect the ignition lead.
- Remove the exhaust at the cylinder.
- Kick the engine through several times to expel fuel, keeping hands clear of the plug wire. Refit parts, then try starting with fuel ON. [Elektroda, wodzu_1, post #17099014]
What compression should a 50cc two‑stroke have, and how do I test it?
Use a screw‑in compression tester on a warm engine with throttle wide open. Healthy engines show roughly 110–150 psi. Below ~90 psi, starting becomes unlikely. “Under 90 psi, most engines won’t run.” If low, investigate rings, cylinder, or crank seals before chasing fuel issues. ["49ccScoot Compression Test"]
The piston jams at top with the plug in but turns with it out. Why?
That points to hydrolock from liquid fuel in the cylinder. Liquids don’t compress, so the piston stops dead. Do not force it; you can bend a connecting rod in severe cases. Remove the plug, clear the fuel, and fix the cause before restarting. ["Hydrolock - Wikipedia"]
Could a clogged muffler cause fuel to spit out the exhaust?
Yes. A restricted two‑stroke exhaust traps unburned mixture, making the engine run overly rich and expel fuel‑oil from the outlet. Check for blockage and clean or replace the muffler. Also verify the carb’s fuel level to stop the overfueling. [Elektroda, Staszek49, post #17098354]
How do I check the automatic choke (auto‑enricher) on my scooter?
Unplug and remove the enricher. Apply 12 VDC to its leads for a few minutes. The plunger should extend around 3–5 mm; lack of movement means failure. A stuck‑open enricher enriches continuously and floods the engine. Replace it if it fails this test. ["49ccScoot Auto Enricher Testing"]
How do I set the carburetor float level correctly?
Remove the bowl and invert the carb so the float just touches the needle without compressing the spring. The float should sit parallel to the carb body. Bend the tang to adjust, and replace a worn needle/seat. An incorrect level will cause persistent flooding. ["Mikuni VM Carburetor Tuning Manual"]
Fuel is dripping from the muffler. What’s the usual cause?
Excess fuel is entering the engine from a high float level or a leaking needle/seat. The raw mix passes unburned into the exhaust, where it drips out. Correct the float height, renew the needle and seat, and re‑check starting. ["Mikuni VM Carburetor Tuning Manual"]
What spark plug and gap help avoid fouling?
Use the plug type and gap specified in the Yamaha service manual. Ensure strong spark and correct heat range. Wet fouling comes from an overly rich mixture or weak ignition. “Wet fouling is caused by an overly rich air‑fuel mixture.” ["NGK: Spark Plug Fouling"]
How do I test the vacuum fuel petcock on a Yamaha Booster/BWS?
Disconnect the fuel hose from the carb and the vacuum hose from the intake. Apply suction to the vacuum port; fuel should flow only with suction. If fuel flows without vacuum, the diaphragm leaks and can flood the engine. Replace the valve. ["Mikuni Vacuum Fuel Valve"]
It backfired and smoked blue before I cleaned the carb. What does that indicate?
That behavior points to overfueling and unburned oil. Two‑strokes smoke some, but excess blue smoke with fuel spitting suggests a high carb fuel level and a possibly obstructed muffler. Correct the carb level and inspect the exhaust. [Elektroda, Staszek49, post #17098354]
After I clear flooding, how do I stop it from returning?
Fix fuel control: reset float height, replace the needle/seat, verify the auto‑enricher works, and confirm the petcock shuts off. Reassemble, set idle and mixture by the book, and test for clean starts and revs. ["Mikuni VM Carburetor Tuning Manual"]
