fuel tank vacuum

[TrmpetDave@xxxxxxx]

In a message dated 5/6/00 9:56:21 AM, owner-st-digest@xxxxxxxxxx writes:

<< I've just noticed that on my '00 ST this week.  Not as strong as you
described, but a high vacuum built up none the less, you hear the pressure
release easily wearing the helmet.  This is the first time I've noticed any
kind of vacuum being exerted against the cap.
Does temp and/or humidity play a factor here?
 >>

I had this problem on my 99 ST. Mark at Rocket motorcycles in San Diego sai 
it was due to a malfunctioning check valve in the vent tube, that is supposed 
to let air in as the tank empties, but not let fuel out in the event of a 
tip-over. He said the Triumph item is cheapo, and replaced it with a Ducati 
one, which works fine. The reason such a strong vacuum can build up is 
because of the fuel pump required by the fuel injection system. With a 
carbureted gravity feed system, you would have fuel starvation with such a 
vacuum.

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