FORM-2 THE PATENTS ACT, 1970 The Patents Rules, 2003 COMPLETE SPECIFICATION (SECTION 10 and RULE 13) TITLE AN IMPROVED REGULATOR FOR AUTOMOTIVE VEHICLE USING GASEOUS FUELS APPLICANT We, Bajaj Auto Limited, an Indian Company, having its principal place of business at Akurdi, Pune - 411 035, State of Maharashtra, India, an Indian Company. The following specification describes and ascertains the nature of this invention arid, the manner in which it is to be performed:- This invention relates to a gas regulator and power screw for a vehicle operated using a gaseous fuel. BACKGROUND OF THE INVENTION Vehicles operating on gaseous fuels are becoming increasingly popular in India as well as in other countries. These vehicles primarily use gases such as CNG or LPG as fuels. However, and alternatively, vehicles may run on conventional fuels, such as petrol in a limp home mode of operation. Engines are tuned to provide adequate performance and also to meet statutory emission requirements, if mandated. A gas regulator is used in a gas fuel system to regulate the supply of gas to the engine. The regulator plays a predominant role in control of air fuel ratio. Vehicle emissions depend upon the air fuel ratio and, particularly, on the quantity of gas being fed to the engine. In such vehicles, the supply of pressurized gas from the gas cylinder is progressively reduced to near atmospheric pressure. As an engine runs, such gases are drawn through the regulator and mixed with air in a mixer prior to induction to the engine. In known regulators, a screw - commonly called a power screw - is provided, at the outlet. This screw may be provided with a flat end, a spherical end or a conical end tapering, generally say 9 degrees, into the gas passage. Power screws with flat ends are normally used in closed loop systems where control on mass emissions is maintained through complicated lambda measurement. The end portion of the screw either partially blocks the gas passage enabling reduced supply of the gas or opens the passage for increased supply of the gas as the screw is respectively turned in or out. The quantity of gas therefore varies with rotation of the screw. For proper adjustment of the screw, the threading of the screw is maintained as fine threads to control axial movement of the screw end as it is turned. There is also provided a lock nut for the screw so that the screw does not change its position due to vibrations of the vehicle during operation. Such vehicles are tuned by rotating and locking of the screw during assembly. The axial movement of the power screw increases or decreases the dimension of a gap between the tapered portion of the screw and 2 corresponding inner periphery of a power screw housing portion of the regulator. The power screw may be located in an elbow of the regulator. Variation in the dimension of the gap and the flow area increases or decreases the flow of gas to the mixer and the engine. During tuning, such axial movement is measured in terms of the number of turns out from the fully closed position of the power screw. One or combination of measurements such as engine power, maximum speed of the vehicle, road load CO, Wide Open Throttle (WOT) CO etc. is done during such tuning, for obtaining an optimized gas supply to achieve desired operational performance of the vehicle. Difficulties arise during tuning of such regulation screws. One difficulty arises in adjustment of the power screw. For a typical engine, the screw is required to be set at a particular position to enable a fixed quantity of gas to flow to the engine for achieving desired vehicle performance. In spite of provision of fine threads, a conical configuration of the screw end causes gas flow to either increase or decrease to a large extent. Problematically, decrease in flow may be such as to cause the engine to stop, particularly under idling conditions. On the other hand, increased gas flow results in increased emissions. The substantial flow variation is undesirable. SUMMARY OF THE INVENTION An object of the present invention is to provide an improved gas regulator allowing more precise control over gas flow to the engine through a power screw. With the above object in view, the gas regulator is now provided with a power screw located in a power screw housing wherein said power screw is configured to control gas flow through a plurality of flow paths between a gas supply and a mixer prior to delivery of gas to the engine. 3 In one embodiment, the power screw may control gas flow through two gas passages communicating gas supply with mixer. The mixer enables controlled mixing of air and gas prior to the delivery of a fuel gas charge to the engine. At least one first passage may be configured to provide a fixed quantity of gas to the engine. This first passage may pass through the screw, and in particular a tapered flow control portion of the screw. It may be of fixed dimension or otherwise designed to deliver a fixed quantity of fuel at a given regulator pressure setting. A number of first passages may be provided that deliver, either alone or together, a fixed quantity of gaseous fuel to the mixer. The fixed quantity may vary with gas regulator pressure setting. A second passage or gap may be provided between an external surface of the power screw, again with location of the surface on a tapered flow control portion of the power screw being preferred, and a corresponding inner surface of the power screw housing. However, this configuration of this passage need not be limited to a taper; it can be of different configurations, such as stepped configurations to allow variation of flow area or volume of space for passage of gaseous fuel with rotation of the screw. This passage or gap may be of variable dimension, or cross sectional area, such variation being dependent on the rotation of the power screw. Such rotation may be conducted, during tuning, in precise manner with less risk of engine stalling or unacceptable emissions. As the second passage or gap is of variable dimension, the quantity of gas flowing through the second passage may be varied. Fuel quantity may thus be controlled in accordance with engine operating conditions, greater quantities of fuel being delivered at higher engine speeds and loads, for example. A power screw tapered portion, if adopted, may be configured to have a taper angle of degrees selected as a function of the fixed quantity of fuel delivered through each first passage. 14. The regulator of claim 13 wherein said inner surface of the power screw housing, is provided with a corresponding angle of taper to the flow control portion. 15. The regulator of any one of claims 2 to 14 wherein a first passage for gaseous fuel is configured to provide such quantity of fuel that stalling during idling is prevented. 16. The regulator of claim 15 wherein said first passage delivers no less than the minimum gaseous fuel requirement to maintain idling. 17. The regulator of any one of claims 5 to 16 wherein said first passages comprise axial and corresponding traverse ports provided in the tapered flow control portion of the screw for delivering a fixed quantity of the gas to the mixer. 18. A power screw for a gas regulator comprising a flow control portion for controlling gas flow through a regulator to an engine wherein the power screw is provided with a flow control portion having an internal passage for 11 delivering a fixed quantity of fuel to the engine and an external surface over which, in use, a quantity of fuel variable with a power screw setting flows. 19. The power screw of claim 18 wherein the flow control portion is tapered. 20. The power screw of claim 18 wherein the flow control portion is of non tapered configuration. 21. The power screw of claim 19 wherein the angle of taper, O, is selected as a function of the fixed quantity of fuel to be delivered through the power screw to the engine. 22. The power screw of claim 19 wherein the angle of taper is selected as a function of a dimension of the internal passage. 23. The power screw of claim 19 wherein a plurality of internal passages are provided and the angle, O, is selected as a function of the dimension of at least one of the internal passages. 24. The power screw of claim 18 wherein the dimension of one or more of the internal passages or the fixed quantity of fuel is selected as a function of the angle of taper O. 25. A power screw assembly for controlling gas flow through a regulator to an engine comprising a power valve body, a power screw and setting means for setting flow through said power valve body wherein the power screw includes means for delivering at least a minimum quantity of fuel to the engine to sustain idling irrespective of the power screw setting. 26. A gaseous fuel operated engine comprising a regulator as claimed in any one of claims 1 to 17. 12 27. A gaseous fuel operated engine comprising a power screw or power screw assembly as claimed in any one of claims 18 to 25. 28. A vehicle comprising an engine as claimed in claim 26 or 27. 29. A gas regulator substantially as herein described with reference to Figures 4 to 6 of the drawings. 30. A power screw substantially as herein described with reference to Figures 4 to 6 of the drawings. . 13