I don't know what engne is fitted to your Stiga - but 4.5HP is 4.5HP - whoever built the engine. It sounds like an adjustment problem - you need to check the RPM. It should be around 3200 RPM at full speed - if not, then it needs adjusting, or the govenor spring has been damaged (stretched).
I got my optical tachometer off ebay - £10 imported from Hong Kong.
phil_saunders@bigfoot.com
Phil Saunders
August 2008