in the following , the present invention will be described in conjunction with a preferred embodiment thereof which is incarnated as the weft insertion control apparatus for a two - color jet loom , by way of example . referring to fig1 a weft y 1 of a small count is ejected from a weft inserting main nozzle 1 , being carried by a pressurized air jet therefrom , while a weft y 2 of a large count is expelled from a weft inserting main nozzle 2 by a pressurized air jet , whereon the wefts y 1 and y 2 are transferred to relaying transportation effected by jets produced sequentially by a plurality of weft inserting auxiliary nozzles set in groups 3 , 4 , 5 , 6 , 7 , 8 , 9 and 10 . when the weft inserting has been performed in a satisfactory manner , the inserted weft is detected by a weft detector 17 at a rotation angle of the loom within a predetermined range , whereby loom operation is allowed to continue . reversely , when the presence of the weft is not detected within the predetermined range of loom rotation angle , the loom operation is then stopped . the ejections of pressurized air jets from the corresponding weft inserting main nozzles 1 and 2 are controlled by turning on / off solenoid valves v 1 and v 2 provided in association with the main nozzles 1 and 2 , respectively . on the other hand , the ejections of pressurized air jets from the associated weft inserting auxiliary nozzles of the groups 3 to 10 are controlled by turning on / off the solenoid valves v 3 , v 4 , v 5 , v 6 , v 7 , v 8 , v 9 and v 10 provided in association with these nozzle groups , respectively . the solenoid valves v 1 and v 2 are connected to pressurized air supply tanks 11 and 12 provided independent of each other , while the solenoid valves v 3 to v 10 are connected to a single pressurized air supply tank 13 . the on / off ( opening / closing ) control of the solenoid valves v 1 , v 2 and v 3 to v 10 are performed under the commands of a loom control computer c which is so programmed as to issue the on / off commands to the individual solenoid valves v 1 , v 2 and v 3 to v 10 in response to a loom rotation angle detection signal produced by a rotary encoder 14 . selective insertion of the wefts y 1 and y 2 is conducted in accordance with a preset weft color pattern loaded previously in a memory c 1 from a weft color selecting pattern input unit 15 through a central processing unit cpu constituting a main part of the loom control computer c . more specifically , the central processing unit cpu reads out the weft color selecting pattern information from the memory c 1 at a predetermined angle θ of loom rotation which can be derived from the detection signal of the rotary encoder 14 , whereby commands for turning on or off the solenoid valves v 1 or v 2 which correspond to the weft to be inserted next are generated on the basis of the information as read out . the loom control computer further incorporates a memory c 2 in which on / off timing information for the solenoid valves v 1 , v 2 and v 3 to v 10 ( hereinafter v 3 to v 10 will be denoted collectively by v 2 + i , where i represents 1 to 8 ) are previously loaded from an on / off timing setting unit 16 through the central processing unit cpu . thus , the central processing unit cpu can command the on / off operation of the solenoid valves v 1 , v 2 and v 2 + i in accordance with the on / off timing information stored in the memory c 2 . for both the solenoid valves v 1 and v 2 , the same on / off timing is set so that the pressurized air jets are ejected at the same time point from both the weft inserting main nozzles 1 and 2 . the jet timing for the weft inserting auxiliary nozzle groups 3 to 10 effectuated by on / off control of the solenoid valves v 2 + i is set as [ α 1 , β 1i ] ( i = 1 to 8 ) expressed in terms of the loom rotation angle range for the weft y 1 of a small count , as shown in fig2 a , while being set as [ α 1 , β 2i ] ( i = 1 to 8 ) for the weft y 2 of a large count , as shown in fig2 b . now , the weft insertion control will be explained by reference to fig3 which shows in a flow chart a weft insertion control procedure , by way of example . referring to the figure , at the loom rotation angle of θ , the central processing unit cpu reads out the weft color selecting pattern from the memory c 1 to thereby determine the type of the weft to be next inserted . when the weft y 1 is the one to be next inserted , the central processing unit cpu reads out selectively from the memory c 2 the on / off timing [ α i - δ , β 1i - δ ] ( where δ represents a delay in response in terms of angle ) for the solenoid valves v 2 + i to thereby command the turn - on ( opening ) of the solenoid valves v 2 + i at the loom rotation angles corresponding to ( α i - δ ), while commanding the turn - off ( closing ) of the solenoid valves v 2 + i at the loom rotation angles ( β 1i - δ ), respectively . thus , the weft inserting auxiliary nozzle groups connected to the solenoid valves v 2 + i are correspondingly actuated to produce air jet streams within the range of loom rotation angles of α i to β 1i , as the result of which the weft y 1 is caused to run as indicated by a curve d 1 shown in fig2 a . fig3 uses the conventional computer flow chart symbols , in which a diamond - shaped box represents a decision and arrows represent paths and sequences of instruction execution . each diamond - shaped decision box represents an instruction . the upwardly - directed &# 34 ; no &# 34 ; arrows in fig3 show that the next process step of the sequence is not to be undertaken . thus , upon detection of loom rotation , the rotary encoder 14 shown in fig1 transmits a signal representing an angle of loom rotation to the cpu . the cpu monitors the detected signal and reads out the weft color selecting pattern stored in memory c 1 , only if the angle of loom rotation reaches a predetermined value θ . that is , the uppermost &# 34 ; no &# 34 ; arrow of fig3 means that the cpu continues to monitor the loom rotation angle until that angle matches angle θ , at which point the decision changes to &# 34 ; yes &# 34 ; and the sequence progresses . the other subsequent upwards - directed &# 34 ; no &# 34 ; arrows have similar meanings for the subsequent loom rotation values as shown in fig3 . when the weft y 2 is to be next inserted , the central processing unit cpu reads out selectively from the memory c 2 the on / off timing [ α i - δ , β 2i - δ ] for the solenoid valves v 2 + i to command the turn - on of the solenoid valves v 2 + i at the loom rotation angles ( α i - δ ) while commanding the turn - off of the solenoid valves v 2 + i at the loom rotation angles ( β 2i - δ ), respectively . in this way , the weft inserting auxiliary nozzles connected groupwise to the valves v 2 + i eject the air jets within the range of loom rotation angles from α i to β 2i , as the result of which the weft y 2 is forced to run in such a manner as indicated by a curve d 2 in fig2 b . through the weft insertion control according to which the jet timing [ α i , β 1i ] is set for the weft y 1 to be inserted with the jet timing [ α i , β 2i ] being set to the weft y 2 to be inserted , the wefts y 1 and y 2 are inserted in a relaying manner in optimal jet timing . in other words , the weft y 1 of a small count which is intrinsically of a high running speed is prevented from running at an abnormally high speed while the weft y 2 of a large count which is intrinsically low in running speed is protected against flying at an abnormally low speed . thus , weft breakage due to the running of the thin weft y 1 at an excessively high speed as well as insertion failure of a thick weft y 2 due to the running thereof at an excessively low speed can be positively prevented , while the amount of air consumption can be reduced . by virtue of the selective jet ejection timing control described above , a satisfactory weft insertion can be achieved without the need for additionally providing a pressurized air supply tank for the weft inserting auxiliary nozzle groups 3 to 10 as well as additional piping therefor , whereby the problems of increase in installation space and cost can be avoided . the present invention is not to be limited to the embodiment described above , but may be modified or varied without departing from the spirit and scope of the invention and equivalents thereto . for example , although it has been described that the memories c 1 and c 2 are provided separately , it is obvious that they may be implemented in a single memory . further , it goes without saying that the timing at which the solenoid valves v 2 + i are turned on may be changed in conformance with the types of wefts . further , the invention can be equally applied to a jet loom in which the pressurized air supply tank for the weft inserting auxiliary nozzle group 10 is provided separately from that for the other auxiliary nozzle groups 3 to 9 , wherein the ejection pressure of the weft inserting auxiliary nozzle group 10 is increased as compared with that of the other auxiliary nozzle groups 3 to 9 . as will now be appreciated from the foregoing description , according to the teaching of the present invention that the jet timing of the weft inserting auxiliary nozzles is selectively changed in conformance with the types of wefts to be inserted , the ejection or jet timing can be optimally set for the types of wefts to allow satisfactory weft insertion to be achieved without the need for expanding the pressurized fluid supply system .