Thanks Simon
The engine has not got a power problem and has no problem with my lump but with trims out it is working hard. Saying that though i used to fly with full trims open and had no problems with engine. But as this was pushing the engine i started to fly at half trim so engine would not have to work so hard. After fitting temp sensors i realised that the throttle level required for half trim is actually the level the engine runs at the highest exaust temp. So my theory is that without the air dam enough air does not pass across the head add to this that i was cruising at a throttle level that produced the highest gas temp and i would have been doing this for long periods of time as much as 30mins.
To be honest i do not know why but the above seems to fit. One thing that i am sure is a problem is air flow as the air dam reduced head hemp by 40deg. Also there is no clear air inlet as my shoulder and harness are about 6 inches in front of the engine fins. I am not a aerodynamic expert but air behind a flat backed object passing through the air tends to be slow moving. Bit like tailgating a lorry.
Anyway it does seem like the air dam has done the job. I guess time will tell.