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Agreed, Colin. A polished/reflective surface would make far more sense if used in combination with an insulative layer, but the effecriveness of a single thickness of highly-conductive metal, however well polished, is questionable. Logic suggests it will reflect a high proportion of radiated heat, but the effectiveness will diminish as it approaches the block temperature.

Still think the best thing you can do is get the hot air out, you could try propping the bonnet open a tad to see if that helps before you go nuclear.  

Hey Craig site seems to be going exceptionally slowly again for me at least. Colin

Yes possibly, but theres not so much direct heat there. You'll get more vapourisation in the large volume of petrol at atmospheric pressure sitting in the large pots directly over the hot exhaust manifold. Even without any radiated heat that where the hottest air will be. Hence airflow.

Reading various forums on the effect of the carburettor shields suggests that they are not that effective in themselves, although adding some form of insulating material might, and the jury seems out on this, help a little. Based on what folks are indicating on the forms a super polished surface isn't the magic sauce ingredient in the heatshield, its the insulation. Going back to various 70s and 80s carburetted cars like my dad's Maxi, the Metro turbo, etc, these cars had carb heatshields made of (non-shiny) insulating fibre. As did late Triumph 2000 and 2500 saloons and US spec Spitfires. This makes sense. A metal heat shield will cut radiated heat but still get hot and re-radiate on the carb side. Whereas an insulated fibre one just stops any radiation getting to the carb at all. Neither of course deal with convection from hot air under the bonnet. So... The main thing though being airflow.

Thank you all for the feedback. This actually brings me back to an earlier line of attack on cooling. When the head was replaced some eight years ago, the engine came out and was thoroughly flushed, since when it has maintained reasonable(ish) temperature with the aid of the mechanical fan, even in slow/standing traffic in 35degC. However, I've never got round to ditching the K&N pancake filters and reinstating the original front-feed filter box, and the air clearly gets too hot for efficient combustion. My plan a couple of years ago was to acquire an original filter box and use it as a filter-less plenum, feeding it with air from ahead of the shroud via a 70mm dia flexible pipe (equivalent to the combined area of two HS6 carb throats) and a cone filter mounted between the shroud and the grille. But this came acropper because there is insufficient room to get a pipe of that diameter from the back of the shroud to the box. So the current arrangement is that there is a hole on the right side of the shroud feeding fresh air roughly in that direction. I have the shiny stainless steel heat shields, but they are currently incorrectly mounted (fore and aft instead of underneath) because the shield design fouls the rod accelerator linkage between the carbs... However, once the temperature rises marginally north of brass monkeys and the garage is tolerable for work, oh and I've dealt with the leaking radiator on the Jaaag (can two people extract a radiator from an XJ6 without removing the bonnet..?), I'll return to finding a position for the pipe to feed air to just ahead of the front filter. Looping back to the original question, I had assumed that vaporisation was occurring in the (low pressure) feed pipe as it wended its way from the pump to the carb bowls, on the basis that it might spend (by extraordinarily crude calculation) approximately six seconds in that transit at tick-over. Could this assumption be... flawed?

So it is to the carbs themselves!    Pancake air filters in spiffy chromed holders look great, but draw in hot air from the engine compartment.     A supply from in friont of the rad is better.   There may be space for a similar to cool the carbs/heat shields. But a  mirror finish to the shields is also better, less radiation to the carbs and reflection back to the manifold. John

Yes, I think the airflow is the key bit here.

Did similar heat shields with my carbs, they really need some air flow over them if you can.

Nick's point on low vs. high pressure systems is very true. And Colin's on how carbs gain heat from the exhaust - are heat deflectors in place between the exhaust and the float chambers, thusly?  Random picture of such heat shields, from the net.