Mmmm thats very useful info, I was actually on their site and asked myself about the risk, which you've answered...

I'll take another look - Thank you!

 

Its only right to follow-up your post with the solution to my problem, not that it will be of much use to ordinary gasoline uses only those with LPG (Liquified Petroleum Gas) conversions.

It turned out the problem was with my vaporizer control valve which was defective. For those with interest in LPG this valve is heated by the coolant system and automatically switches from gasoline to LPG when its warm enough, usually within a couple of minutes. Engines cannot start on LPG alone and so must start on gasoline, in my case the valve was leaking LPG which was diluting the gasoline hence it made starting difficult.

Its a good job I didnt replace the fuel-pump as it would not have helped, nevertheless it was good advice and to know this was a possibility.

So another one for the Hummer knowledge base.