RE the OS 91 bubbles in the fuel line.

My thoughts on this topic are that engine heat is being conducted through the remote NVA and causing the methanol to vapourise. The bubles are not air but methanol vapour. This, combined with the (relatively) lower pressure on the carb side of the NVA caused by engine suction assists the vapourisation - the pressure drop is likely to be the final factor needed to vapourise an already warm fuel supply. This could account for the reports of high engine temperatures - if the engine heats up and runs on partial vapour the resulting weaker mixture would cause higher engine temperatures.

The reason a pump helps is because it will tend to reduce pressure fluctuations across the NVA: it addresses the symptoms, however, and is not the cure. All engines have a pressure drop across the needle valve - not all engines have a warm fuel supply AND a remote NVA. Move the NVA to a bulkhead or change the carb for an on-carb needle valve assembly and the problem will not occur.

With an on-carb NVA it makes little difference if the fuel reaching it is hot and ready to vapourise - the carb is about to do that anyway.

OS scored a bit of an own goal with this one...

Mick.