I drive a Cultus which came fitted with a Landi Renzo cng kit from the factory. This car has the Suzuki 3 inline, G10B engine with hydraulic lifters (tappets) and an AISAN carburettor. No tappet adjustment is ever required with this engine. When the tappets wear out they must be replaced to avoid misfiring and loss of power due to valve float.
I used to take my car to enercon for tuneup but decided to do it myself later on. It is worthwhile to note that Air and Fuel must be mixed in just the right amount to keep the engine happy. Its false to believe that more fuel means more power and less fuel means more economy. To an extent its true but you get the max. power with max. economy with long life for engine, when your fuel mixture has the right amount of fuel mixed with air. The mixture ratio for petrol is 14.7 parts air to 1 part fuel. In the case of CNG it is closer to 17-18 parts air to 1 part fuel. At enercon thats what they analyze and adjust the setting screws on the kit to get the AFR right. Petrol and CNG are different fuels therefore the ignition timing must also be adjusted.
I assume that you already have a Timing Advance Processor (TAP) installed in your car to switch the timing when you run on petrol. Usually the distributor is set to full advance for CNG and when you switch over to petrol the TAP kicks in and retards the timing to suit running the engine on petrol. If you don't have a TAP installed your car will knock badly when you trying running on petrol.
For the Cultus, ignition timing is around 16 - 19 degrees BTDC for CNG and 6 - 8 degrees BTDC for petrol. You won't regret investing around Rs. 1200 for a cheap timing light, if you are an engine junkie like myself :o)
Here is how I do it:
1. The air filter is taken out and inspected. If it is slightly dirty then it must be cleaned with an air blower. If it is more than slightly dirty then it must be replaced.
2. Take out the spark plugs and make sure the tips are clean and sound. On a good engine there should be no oily deposits and the spark plug tip should look dry with a brownish color. The gap should be 0.60 - 0.70mm. Check this gap with a feeler gauge (i bought one for Rs. 250). For engines other than the G10B, ask your friendly mistri for the gap setting. While you are at it, it is a good idea to check the plug cables for cracks or other damage.
3. Drain any accumulated oil from the CNG kit by opening the drain plug, usually found on the bottom of the kit. Unscrew it completely to let the oil drain out. Don't forget to put back the drain plug and tighten after the oil is completely drained.
4. Locate the T shaped valve with a screw usually attached between the CNG kit and the Carburettor. Unscrew the locking nut and loosen the screw. Turn the engine on and open the throttle to make the engine turn at a fairly high RPM. I do mine at 4000 RPM but it is not critical if you don't have a tachometer. Start tightening the screw until you notice the RPM starting to drop. Loosen back to the point where the RPM started dropping and keeping the screw firmly in place, tighten the locking nut.
5. Adjust the screw on the CNG kit to make the idle smooth and without misfires.
6. Take the car for a drive and accelerate hard on a straight piece of road. If you feel the engine losing torque on high RPMs then try increasing CNG flow by loosening the big screw on the T shaped valve by a 1/4 turn. Keep repeating until you get decent acceleration and a smooth idle.
7. Top up the CNG cylinder and reset the trip meter to 0. Note the kms you do on a full tank. On my engine I get 140 - 150 kms in the city and 190 - 200 kms on the Motorway without A/C and 160-170 kms with A/C. I can do a top speed of 140-150 km/h on CNG. I easily go upto 170 km/h on petrol if the wind is right LOLZ!! Not bad for the lil 3 banger.
Driving my cultus I routinely beat badly tuned Civics and Corollas on up-slopes on the Motorway. They think that a 3 inch tail pipe, blue neon lights and high intensity head lamps make their cars actually fast!!! Poor souls.
I have grown to love this gem of a car from Suzuki even if the production quality is not at par with a Cultus produced elsewhere.
Sorry for the length of all that and happy tuning!!!