This idle problem actually occurs when our nincompoop CNG mechs try to adjust the RPM on cng by fiddling with throttle stop screw (the small one below where throttle cable attaches to the pulley on throttle body)
Never ever fiddle with the throttle stop screw. Now by changing throttle bodies they r just avoiding the procedure to undo the f***kup of throttle stop screw & calibration of TPS.
Here is what to be done:
1. Clean IACV, get valve clearance done (aka tappet setting)
, also set plug gap to 1mm or install new plugs without changing gap
2. Good CNG mixture settings
NOW THE IMPORTANT PART
3. Check that ur throttle cable has a play/ slack of 1/2 inch, if not then adjust it
4. Remove air Filter housing & check that ur BUTTERFLY is fully closed at idle or when engine not running, if not then either it is stuck or somebody has fiddled with the throttle body.
5. Adjust ur throttle cable slack to fully close the butterfly. if it doesnt close fully with 1/2 inch slack then adjust ur throttle stop screw (anticlock wise/ open it) a little & stop at the setting when it just closes. dont go any further.
ALL THE SETTINGS SHOULD BE DONE WITH ENGINE RUNNING ON PETROL, NOT ON CNG
6. Switch ON the ignition but engine OFF
7. Get/borrow a voltmeter & connect its black wire to chasis/ good ground
8. Connect its RED wire to TPS sensors centre wire by back probing (TPS has 3 wires)
9. now set the voltmeter to 20 volts & note the reading. It should be at 0.5 Volts. If its not then loosen the 2 screws on TPS & slightly move it. see the voltmeter. when the reading becomes 0.5 volts, tighten the screws.
10. Now start ur engine on PETROL & keep the rpm by accelerator at 2000 until engine reaches its normal operating temperature/radiator fan comes ON once (VERY IMPORTANT)
11. With the air cleaner box removed set the RPM with IDLE SETTINGS SCREW (can b found on the mouth of throttle body, it may be covered by a plastic cap). Set the RPM to 600. Dont go above it.
12. Now assemble everything back & start ur engine. the RPM should be approx 800.
13. Look for any vacuum leaks by hearing closely. The main culprit for leakage is usually the place where the CNG pipe enters the air cleaner housing.
14. Get a silicone sealant tube of Rs 30 & apply in liberally Because most low CNG mileage/idle problems occur because of vacuum leak/ broken housings.
15. NOW CONVERT TO CNG
16. adjust CNG rpm by moving the small Knob on CNG regulator. The best setting can be felt only. ideal is when engine vibration is minimum.
PHEW...... Now clean ur sweat & after saying BISMILLAH go for a test drive.
ALL This is to be done on ur own risk. dont blame me afterwards for any problems