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I have an odd problem with a 50 cc air-cooled two-stroke scooter engine that keeps locking up at full throttle, then freeing up again.

I just bought the bike second hand with a new belt needed fitting. I have pre-mix oil/fuel in case the oil pump keeps failing.

It ticks over no problem and has plenty of pull, but when I apply full throttle for more than a minute the pitch of the engine drops suddenly, and the engine seizes and the back wheel locks up. I don't know why it is seizing up.

I had the head off and checked the piston and barrel and rings. All look good. I really have no clue now why this is happening.

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Likely it's overheating under full throttle load. There could be many causes:

  • Dirty or obstructed cooling fins
  • Wrong oil/gas ratio
  • Carb clogged or jetted wrong, causing a lean condition
  • Air leak, allowing fresh air in past the carb, causing a lean condition
  • Internal wear causing excess friction and heat

It's hard to diagnose this remotely on limited information, you may be able to get specific help by posting photos and some more details, or look for an online community dedicated to that make/model. Or take it to a local repair shop. The good news is, 50cc two strokes are usually very easy and cheap to work on.

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  • Thank you for your reply, It had a 70 kit on it and standard jetting. The problem was lean fuel/air ratio at full throttle. The main carburetor jet was not big enough and was not providing enough fuel at full throttle causing the fuel/air mix to go lean and the engine to seize.
    – user2521546
    Commented Jun 21, 2019 at 6:51

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