Firing sequence is 1,3,4,2
Correct spark plug gap is 1.0-1.1 mm
For tappets
Intake 0.006-0.007 inches
Exhaust 0.010-0.012 inches
Note settings are to be performed on a cold engine
Well I would recommend you to go to your old and trusted mechanic and take him in confidence that whatever wrong happens will be your responsibility( Believe me nothing will go wrong as these are factory standards and even some honda guys dont follow them) just tell him the setting parameters and get tappets adjusted. In normal engines its difficult to set tappets as you have see when valves are closing and opening but in IDSI if you look closely on the extreme left side wheel on which timing chain and camshaft is mounted. If you watch from standing on left side of vehicle as you move the engine clockwise you will see a down arrow appear if you rotate it further 1 and 2,3 so on will appear so he can easily adjust the settings. These steps were from last tuneup and sorry if I posted something wrong because I may have forgotten some steps. Its always good to have an expert accompany you for the first time, observe all procedures and from than on do the procedure yourself. P.S the valve clearance and spark plug gaps are from service manual and are standards
@ What average are you getting on CNG and Petrol in both city and highway driving
