I have converted the fuel system to CNG from Octane when I drive with octane its perfectly okay but while driving with CNG r.p.m falls and it shuts down.I have tuned it several times but the tuning does not last for two days.I have now changed the butterfly but no use.It could be that the butterfly that I am using now is also defected in that case I can change it again. When I turn on the A/C the r.p.m is suppose to rise slightly but rather it falls down causing the engine to shut down.