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Pours Candles in 1998 Yamaha Boster Scooter - Spark Plug Floods & Carburetor Cleaned

Artur006 25860 16
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How do I stop a Yamaha Booster scooter from flooding the spark plug and not starting, even after cleaning the carburetor?

Check the cylinder compression and the fuel level in the carburetor; if the fuel level is too high, the engine can flood, suffocate, and push fuel out of the exhaust [#17096899] [#17098354] Also inspect the exhaust/muffler for blockage [#17098354] To clear a flooded cylinder, unscrew the exhaust from the cylinder, remove the spark plug, disconnect the ignition coil, and kick the starter several times so the fuel is pumped out [#17098879] [#17099014] If the piston is actually stuck at the top, the engine likely needs repair rather than just carburetor cleaning [#17098638] [#17098760]
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  • #1 17096846
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    I have a problem with the Yamaha Boster scooter from 1998. The problem is as follows, it floods the spark plug when I want to start it. I cleaned the carburetor and nothing, then the same nwm what to do.
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  • #2 17096857
    Pedros050
    Level 43  
    Posts: 17906
    Help: 2471
    Rate: 3901
    Hello. Have you checked the spark is?
  • #3 17096875
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    The spark is ok, I checked on a new candle.
  • #4 17096899
    1133
    Level 14  
    Posts: 47
    Help: 12
    Rate: 29
    Check compression.
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  • #5 17096906
    Pedros050
    Level 43  
    Posts: 17906
    Help: 2471
    Rate: 3901
    When did it start flooding it before cleaning the carburetor, did it go out while driving?
  • #6 17096937
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    He was shooting while driving, but he spat more of this fuel from the muffler before cleaning the carburetor
  • #7 17098354
    Staszek49
    Level 35  
    Posts: 2421
    Help: 228
    Rate: 1138
    If the spark is certain and floods the spark plug, you really need to check the cylinder pressure and the fuel level in the carburetor. I suspect that the level of fuel in the carburetor is too high, as a result of which the engine suffocates and is not able to burn through too much fuel. Hence fuel leakage from the exhaust pipe and the impossibility of increasing the engine speed.
    Assess whether the muffler is obstructed.
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  • #8 17098638
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    Today I tried to fire the same and the piston is jammed as it is up
  • #9 17098679
    Pedros050
    Level 43  
    Posts: 17906
    Help: 2471
    Rate: 3901
    The piston is up, have you rubbed the engine?
  • #10 17098719
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    When it is up, when I smoke it from a fireplace, the piston is cut
  • #11 17098760
    Pedros050
    Level 43  
    Posts: 17906
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    Rate: 3901
    Even how is the candle unscrewed? This is a renovation project.
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  • #12 17098847
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    If the candle is unscrewed, it is not
  • #13 17098879
    Pedros050
    Level 43  
    Posts: 17906
    Help: 2471
    Rate: 3901
    The engine is probably terribly flooded with this fuel, unscrew the exhaust pipe, remove the candles from the cap, disconnect the power to the ignition coil and pump this fuel out of the center as you have the starter, and if not, the digging remains and turn the tap so that the fuel does not fly back to the center of the cylinder.
  • #14 17099014
    wodzu_1
    Level 25  
    Posts: 622
    Help: 72
    Rate: 140
    Unscrew the exhaust from the cylinder, unscrew the spark plug and use a vigorous kick several times while gently touching the hole to the candle, Important, do not touch the cable. from the candle. Watch what comes out of the cylinder outlet and the spark plug socket. You may have too high a fuel level in your carburetor or a clogged exhaust. Before cleaning the carburetor, it fired at the carburetor or the exhaust, what was the color of the exhaust: it smoked black or more white and blue? Nobody will answer your questions without specifying details, unless his name is ... Maciej Sróżbita. A bit of humor. No offence.
  • #15 17099115
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    And how do I do it, put everything together and flood it again?

    Added after 5 [minutes]:

    He smoked blue

    Added after 2 [minutes]:

    And if the fuel level in the carburetor is too high, how to adjust it?
  • #16 17099149
    wodzu_1
    Level 25  
    Posts: 622
    Help: 72
    Rate: 140
    First, do what you say. Nobody will help you without it, not even the aforementioned fortune teller.
  • #17 17099165
    Artur006
    Level 7  
    Posts: 9
    Rate: 6
    I will act tomorrow and I will see. If I keep writing something
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Topic summary

✨ The discussion revolves around a 1998 Yamaha Boster scooter experiencing spark plug flooding issues during startup. The user has cleaned the carburetor but continues to face problems. Responses suggest checking the spark, cylinder compression, and fuel level in the carburetor, as a high fuel level may cause flooding and exhaust leakage. The user reports that the piston is jammed and has experienced blue smoke from the exhaust, indicating potential fuel flooding. Recommendations include unscrewing the exhaust pipe, removing the spark plug, and clearing excess fuel from the cylinder. Adjusting the fuel level in the carburetor is also advised.
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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?

  1. Turn fuel OFF, remove the spark plug, and disconnect the ignition lead.
  2. Remove the exhaust at the cylinder.
  3. 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"]
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