Hi Pete
What fuel are you using? This could affect idling speed. Otherwise it could be the carb. Unfortunately Otto's mods were not to factory standards so I am hoping the mods themselves haven't affected your engine.
I am currently out of the office (I'm in the USA for the IHobbyExpo). Could you please email a copy of this to
[email protected] and they should be able to help you out
Best regards
Laurenne