Mike,
I would suspect that the timing tape for speed control is damaged or otherwise defective. You'll need to remove he boiler and inspect the stripes for one of the following:
- Tearing of paper timing stripes would cause the engine to run faster than its displayed speed
- Fading of black stripes towards grey would cause the engine to run faster than its displayed speed
- If the stripes are painted, they could be overly porous, causing the engine to run fast.
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