This particular post applies to standard engines, and does not cover electrically controlled, VVTI, VTec engines etc.

To get your car running smooth on CNG first of all you need to get the basics right, which are

1. Engine is in good condition.
2. The timing mechanism works fine.
3. check the advancer mechanism, it should be very smooth.
4. The electrics and ignition is good.

Once you are satisfied with this, you can proceede.

You will have to get your car atleast running on patrol.
Try to achieve maximum performance on that first THIS IS IMPORTANT as this will serve as a bench mark.

Once this is done, convert your car to CNG.

Choke the air intake - This can be achieved by blocking the air intake by as much as 70 %. I have found cold drink cans to be excellent at this. Cut one open and use the metal. DO NOT use card as that will tear and will not do a good job.

Advance the timing - This can be done by rotating the distribution mould after you have loosened the bolts.
If you have a timing gun then you should increase it by as much as 30-40 degrees depending on the make of your car.

Once you have choked the air intake your car's RPM will go high, which is fine.

Pay special attention to the vaccum lining of the car. If you have a leak in the vaccum lining, try all you want the car WILL NOT perform. make sure it is leak free and a VACCUM !

Now get set on the only two CNG controls, INPUT and MIXTURE.

a. You will turn the input bolt till the car's RPM comes to reasonable rate,
b. adjust the mixture screw so the RPM increases.
c. turn the input screw till the car comes to normal again.
d. go to step a.

Repeat a-d till no further improvement can be gained.

Remember tightening the mixture screw will make it lean, the car will loose accelaration and power, and tend to drive heavy.
If you loosed the screw the mixture will get rich, the car will generally become lighter but the milleage will deteriorate.

Once all is done, shutdown and start the car again, if the starting is too long, loosen the mixture screw a little bit, and compensate for it from the input screw or the carbeurator's RPM screw.

There if all worked fine, then you should have a nicely running engine on your hand.

Enjoy !  

Ok understand the concept first,
Understand this EFI is electronic fuel injection.

That means contrary to the dumb carbeurator the fuel is inteligently injected in the right proportion.

In a carbeurator fuel is sucked in due to the air flow and the resultant vaccum. So you have little control over the quality of the mixture AND at different running conditions it is impossible to dynamically adjust becasue the screws are in the carbeurator.

EFI takes care of this, as it will intelligently inject the right amount of fuel in each cylinder, the microprocessor will
keep on monitoring your engine and adjust the amount of fuel / mixture. So now the problem of dynamically adjusting fuel is solved.

Here's the sad part, All the CNG conversion I have seen for electronic cars actually disconnect the EFI.
Some wires are connected only to fool the microprocessor so it doesnt shut the car down; but operationally the EFI is DEAD !.

Here's what I suggest you can do.
Fist of all DO NOT believe the CNG people, thats the worst you can do to yourself.

U know the emulator, please choose it very very carefully.

Give the Landi renzo office a call and ask them.

Look around and find an emulator which is the closest match to your engine type / capacity and your lambda sensor.

By engine type please understand VVTI is different from VTI, and EFI. all three are different.

Cars like Santro allow you to adjust the properties with a serial cable. The port is under the dash. 

(Added 11-30-09)

 
How good the car will perform on CNG or any fuel for that matter depends on the following attributes
1. Engine Timing
2. Spark Plugs and High Tension wires
3. Distributor rotor and point / contact condition
4. Carburetor condition
5. Air fuel mixture

Now I will go step by step in the order that I like to tune my cars.
By the way, I am more accustomed to Toyota Corolla so some times my advice will be specific to Corolla only. Please point those out and I will fix those.

Tools:
WD 40. Sand paper (very fine, the one used by car painters)
Gauge for thickness measurement ( buy a good stainless steel gauge, German is best)
Set of pliers, screw drivers, ratchet etc...

Always invest in good tools do not buy cheap knockouts !

Step 1 - Spark Plugs.

Open the spark plugs, and check their condition. you must make sure that they are clean not just on the sparking end, but also on the end where the HT wire connects.
Use WD40 for a good clean and use the fine sand paper to remove any deposits.
Remember to clean not just the top, but also from the inside of the sparking end.

AFTER all of this is done, measure the Resistance and any current leakage. You will need specialized equipment for this.


Adjust the gap according to engine specification do this accurately. If the car has to run on CNG, add 0.5 to 0.7 mm to the specifications.

Observe the spark plug, if the sparking end seems to be worn out, OR if the top node is bent down, CHANGE the spark plug.

Remember Changing the spark plug is the simplest and most effective favor you can do your self.

Step 2 Ignition system
Next up is the ignition system, the source of many problem and the most difficult to find out.
Carefully lay down all the HT wires (Plug wires) and observe the wires.
If the wires are damaged any where, if they have been burnt or even heated up
CHANGE THEM ! that is it ... do not try to fix those wires; you MUST change them.
Check if the resistance value is according to the ones printed on the wire and according to engine specification. Make sure that resistance value is the same for all wires very very accurately.
Make sure that the wire contacts are clean, use WD40 and wipe with soft cloth.

Next observe the distributor cup. Make sure that the Contacts are clean, and that there are no breakages cracks in the distributor cup.
Make sure that the contacts are clean from outside ( where the wires connect)
A generous spray of WD40 is very helpful.
Use patrol if u don't have WD40.

Repeat the process with distributor rotor. DO NOT rub it with sand paper !
just clean it with liquid and cloth.

Now if your car still has the old point contact system, get it out and throw it away !
Get your self a magneto system and earn yourself some respect !

Magneto systems are very reliable and hardly ever go wrong.

Next important step is to check the vacuum diaphragm. attach a pipe to the lower end of the diaphragm and suck some air and hold it.
observe if the vacuum holds or not. if it does not hold, then change the vacuum diaphragm.

The above step is a MUST ! if you get this wrong your car will always be shaky, never accelerate and just be a real suck up !

Step 3. Engine Timings

This step needs to be done just once, and will last u at least some time ! so do it
very carefully very slowly and very accurately.

This step is not easy and U will NOT be able to do it. Get it done by a professional.
But be careful and observe.
I will only list here things that u need to check.
99 out of 100 mechanics do this wrong !

1. check the rocker arm gaps. remember that the gap is different for inlet and exhaust. Please check the exact GAP for your engine. IF you are using CNG, u need to add 0.7mm to the manufacturer values.

2. Get the timing belt adjusted / replaced.
100 out of 100 mechanics do this wrong !
They adjust every thing according to the marking on the top pulley and gears.
Please understand that there are guide marks on other gears and pulleys also.
Pay special attention to the belt adjustment pulley position.
It must be at natural zero.
Also note the position of the crank pulley, it should be exactly zero.
They all have to be aligned with marks on the engine block.
Once all the gears are marked and the belt is about to be put on, check if the over head CAM and distributor guide mark are aligned or not. If they are not aligned u need to turn the engine one full revolution.
This step is if UTMOST importance.
Spend all the time you want on this, but do it VERY accurately.

If you can get this right, You have done 70% of the work and u can be proud.
The engine is not at TDC = Timing dead center.
This is absolute neutral and this is where u need to start every thing from.

Step 3. Air fuel mixture.

Step 2 is done, once after a very long time. so once that is done you can be in peace.

I will not say too much about it, because this can not be done by novice, so I shall not encourage you to even try this.
Step 3 has two parts.
Part 1.
Get the carburetor calibrated. This step needs to be done by a professional and you can not do it your self.
Once this is done right, it will last a life time !
Part 2.
Get the fuel pump and vacuum lining adjusted.
Get the car running and idling on patrol.
The best way to do this, is by a smoke analyzer, if you do not have it then
1. Adjust the mixture screw till the car's RPM max out.
2. Adjust the speed screw for normal rpms
go to step 1.
repeat until there is no further gain.

Points to be very careful about:
NEVER ever remove the little black pipes with the carburetor and exhaust system.
NEVER let the mechanic block them or choke them (kill the b@stard if he does it)
NEVER ever remove the heat compensator with the carburetor.
NEVER change the fuel JET size.
NEVER let the mechanic put any pointed tools in the carburetor
NEVER let the mechanic sand paper the fuel jets. One stroke then they are gone !
In engines 2E + toyota had heat compensator and auto chokes, NEVER EVER disconnect them.

if your mechanic has second opinion about any of the above, you need to kill the B@stard.

Once you have done all this. You have almost a perfect engine working on patrol for you.
If you do not use CNG, then all u need to do is change the air and fuel filters and you are done !

if you are using CNG then start on phase 2.

I can cover the adjustment and servicing of the CNG kit if you guys want me to,
but that is better suited in it's own section. Let me know what you guys need.

For now I will assume that you have a working CNG kit, with no problems.

The first and most important question you need to ask your self is HOW will CNG be inserted in the engine.

The most popular way of doing this is by making a hole in the carburetor and inserting a Nozzle.
This is a very very bad technique and you should NEVER get this done.

Even if your car is EFI, and has the latest air intake type, DO NOT let them puncture any thing and insert a nozzle

The best solution to this is a VENTURY.
There are two type of venturies available, There is one type which is basically a ring with nozzle in it, and second type which has holes all around the circumference.
The second type is best, and you have to get that !

Make sure that the internal diameter of the venture is equal to the air intake diameter of the carburetor.
If the ventury's diameter is smaller than the carburetor's diameter ( or the air intake pipe for corolla 2000 + ) the car will not run on patrol.

U need to spend some time on getting your self the right ventury.
I had to get one custom made for my self.

Please do not hesitate to spend time and money on the right ventury.

Advantages of the right ventury.
1. Car starts on gas in winter
2. Car mileage is TOO good.
3. Car power is just superb.
4. You can easily convert back to patrol without trouble.
5. no pipes and vacuum diaphragms need to be disconnected.
6. The car will not shiver on AC, or headlights.
7. The car will perform equally good on low speeds and high speeds.

Lastly make sure that there are no leaks etc in the whole system,

You need to follow the same steps for tuning CNG as i listed b4.

Voila !

Let me know if u need any other information.
Feel free to copy this information but please link credit to author and original forum.
If there an error in this information, please point out.

Do let me know the results.