Dual system with problems

Hello friends, I have a fault that often occurs in vehicles with dual system (gas and gasoline), and I have not found how to solve them satiasfactoria.Asi as I do not have a technical explanation of why it happens. A last case: Nissan Sentra 0 km, Ga16 engine, converted to Dual by consequence of Nissan, with the last generation gas system. To have cold starting difficulties, the gasoline of the tank takes another smell. She says one day and no longer starts. The car has had its respective maintenance every 5000km. At and of the entire gasoline system. A while in perfect condition. It works for a month and the failure returns. In other vehicles the failure returns, especially if it is not used in gasoline but in much longer 2 or 3 years. This second time I have disarmed cylinder head I have noticed around the gas injector quite a lot of the sticky and crystallized material.

The sticky waste is in admission or escape? An employee of a gas plant told me that the gas problem is that it does not lubricate like gasoline, it is too dry and the problem it gives is to spend the admission valves and the seats. The problem with gasoline I imagine that it is when using special non -regular gasoline in some countries I think they add a dye to special gasoline to differentiate it from the regular and this when drying gasoline leaves a sticky substance and reddish the bad smell is when gasoline is too old it takes a smell of rancid.

Thanks for the answer. Correct in everything he tells me. The mace is formed on the admission side around the valves, on the valve head. The gas injectors are complete of that mace around them and as following the gas flow. The gasoline injectors are clean.