Why does my Toyota hi ace 1986 3y duel fuel keeps cutting out losing power and stalling while driving then back fires replaced ignition switch and still happening?

Check the following:

PCV valve (positive crank case ventilator valve) (refer to "do it yourself" section at www.Toyota.com - owners section)

Fuel pump

Gas valve (if by duel fuel you mean LPG and petrol)

Gas valve at engine

Rocker cover gasket

Spark plugs

Plug leads

Distributor

Points

Ignition coil and ignition coil connections.

Battery leads and terminals.

Any of the above can cause the problems and more such as fuel filters, but these are less likely. For a Toyota 3Y engine a set of standard plugs is around $8 AUD and about 25 for a set of leads. I would check this first - cant hurt anyway especially is they havn't been done for a while.

Courtesy of Www.Dranoweb.com

Verify proper operation of petrol tank vapour recovery system. If faulty, upon braking in particular, liquid petrol may be drawn up the vapour line, into the intake manifold, causing the engine to run very rich on petrol and LPG until the line clears of liquid petrol. Engine may idle smoothly while stationary on LPG but run rough and stall after braking at stoplights. Test by plugging petrol vapour lines and run on LPG. Absent petrol system leaks, a telltale of a vapour recovery fault is a dropping petrol level when only LPG has been used. The same fault in petrol-only YH51 HiAce vans will cause poor fuel economy and petrol smell in the passenger compartment.