Sprinter Petrol Smell Problem

I have a Petrol Sprinter SWB High Top minibus that has been LPG converted with a Tartarini system.
Iím getting a strong smell of petrol through the front windows when
opened and the vehicle is moving. This happens whether driving on petrol and LPG.
When stationary with the bonnet up, there is no evidence of leakage visble and no smell either... as I canít persuade anyone to hide under the bonnet when Iím driving, it is proving very difficult to track this down - even my local Merc Commercial dealer seems to have given up!!!
has anybody got any ideas?
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Surely you need to take it to someone who is an expert in the Tartarini system?
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