Paul, Have you check the cylinder compression or had a look at the inlet & exhaust valve seats. It sounds like the inlet valve could be slightly damaged.
Let me know how you get on.
Regards, Bruce
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Commercial Outdoor Power Equipment P/L.
Ph: +61 2 9713 4699 Fax: +61 2 9713 4915
Regards,
Bruce
Please do not PM me asking for support. Post on the forums as it helps all members not just the individual.