Perforated Radiator

[Moggy]

It sounds like you may have leaned your mixture too much and then throttled up. The rich position (according to the Hurri Mk.I pilot notes) is set back, if you push the mixture lever forward you will lean the mixture.
The exact words of the pilot notes are;

The mixture control lever is returned to the "Rich" position by the closing movement of the throttle control lever.

Try that and see if it helps.

Also, do you mean the lever up in the little gauges panel or do you mean the lever up on the radiator flap control (this needs to be down to be open) in the cockpit itself?