Steve Zack Article:
Henry Olsen Articles:

Tuning a Carbureted Street Rod Engine - Part 1 of 2

Tim Wusz Articles:
spacer Drivability

By: Henry P. Olsen

The definition of drivability: the general qualitative evaluation of a powertrain's operating qualities, including idle smoothness, cold and hot starting, throttle response, power delivery, and tolerance to altitude and weather changes.

To me drivability is the result of tuning an engine to have both the proper air/fuel mixture and the correct ignition spark timing in order to get the most power out of the engine in the rpm range being used. Many tuners will only tune to get the maximum power out of an engine on a dyno and do not even think about tuning the engine for the way it is driven. A drag racing engine should produce its power at wide open throttle but if you have to lift or feather the throttle the engine still must be supplied with the correct air/fuel mixture and the proper amount of ignition timing so the driver can actually drive the car thru a problem with out a stumble or miss. A race engine used on an oval or road race situation also needs to produce power at part and full throttle and the driver that has an engine that not only produces good power but is drivable at any throttle position can often drive around another car that has a engine that has not been tuned to produce power at all engine rpm and load conditions that the engine may see. A well-tuned drivable engine can help a good driver win races especially on a loose or slick track. T his drivability does not only help win you races, it can help a engine run cooler, get better fuel mileage, and help in the production of maximum power at all engine rpm's.

What we are going to try to show our readers is how to tune a engine to get the most power out of a engine at all the load conditions the engine will be used at, and show the tools we are using to allow you to see how we are achieving great results.

Before any tuning is done the engine must be in good shape and have been designed for the type of use it is intended for, a high rpm race engine will not make a good engine for a vehicle used as a every day commute vehicle (always pick or build a engine designed for the way it will be used). Fuel pressure, spark plug gap and all tune up related items must first be checked and set, and then the initial timing and advance curve must be checked and set for the type of fuel being used.

The tools used for checking ignition advance curve are both a dial-back or advance timing light such as the unit we use from OTC/SPX and a distributor machine are very necessary to obtain good results. The distributor machine we use is a Sun model 506 which we have modified to check the advance curves on electronic distributors.

The air/fuel mixture curve may now be addressed; the method of preference for me is by using an infrared exhaust gas analyzer such as the unit that has made it possible for us to obtain great results, the Performance Gas and MicroGas exhaust gas analyzers from O T C/SPX. These exhaust gas analyzers can accurately supply readings that can indicate the air/fuel ratio by looking at the C.O. (carbon monoxide), the amount of engine misfire by looking at the H.C. (hydrocarbons) reading, and indicate if the engine has to much ignition timing advance by looking at the NOx (nitrous oxide) reading and/or high H.C. readings.

Checking the Air/Fuel Mixture �������������������������������������������������������������

A lean fuel mixture (too little fuel for the amount of air in the cylinder) can cause an engine to stumble or give a rough idle as well as to run too hot, overheat, and cause a lack of power. A rich fuel mixture (too much fuel for the amount of air in the cylinder) can cause an engine to "load up" at idle, foul the spark plugs, and also run sluggish. There are several different methods to determine if the air/fuel mixture is correct, among them are:

��High-energy ignition systems have made this method much harder because very little color is seen on the spark plug and thus a job for an expert.

��The second method is by using timed acceleration runs or top speed for the power system, this involves using trial and error jetting changes to obtain the best results. Obtaining the correct cruise mixture is not as easy, since this involves jetting the carburetor to get the highest vacuum, then trial and error to get the best engine feel. When setting the power and cruise mixtures, it's always advisable to stay a little rich in order to avoid engine damage. The idle mixture is set using a tachometer to get the max speed from each idle screw and the go leaner to get a 20-rpm drop in speed, this is know as the lean drop method.

The easiest and most accurate method that we have found is by using an infrared exhaust gas analyzer like the unit we use from OTC/SPX Tool Company called a MicroGas to "read" the exhaust gases. By using the MicroGas the carburetor's jetting (air/fuel mixture) curve can be checked at idle, cruise, or power loads and then be tailored to what your engine needs to run at its best at all driving conditions.


On Car Testing

After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, as well as checking to be sure there are no vacuum leaks, the next step is determine what the air/fuel mixture is at idle thru 3000rpm. If the cruise mixture is off, first change the jets in order to get the air/fuel mixture correct at the 2500-3000 cruise rpm range. Then check and set the idle mixture. If the air/fuel mixture is too lean at idle or part throttle and the idle mixture screws do not provide enough adjustment, the correction may involve enlarging the idle "jet". If the mixture is still lean at 1000 thru 1800 rpm, the idle channel restriction, if used, may have to be slightly enlarged to allow more fuel to be delivered at part throttle.

This lean condition at part throttle condition will cause the engine to miss or stumble at part throttle due to a lean air/fuel mixture. This problem is very common on many carbs. ( The notable exception is the Demon carburetors, which have used the MicroGas to do exhaust gas analysis along with dyno, track and drivability testing to establish the factory fuel curve that allows them to come with a stumble-free guarantee.) If the air/fuel mixture is too rich at idle and part throttle, the idle jet/restriction may be too big and may need to be replaced with a smaller one.

The next step is a road test using a portable infrared gas analyzer to check the cruise speed air/fuel mixture-main jetting, followed by checking the power air/fuel mixture under load. During a road test you are able to read and correct the jetting to have the correct mixtures at idle, cruise and power/wide open throttle.

The starting point for air/fuel mixtures that we look for from a stock engine is:

Idle: 1 to 3 % CO or a 14.1-13.4 to 1 air/fuel mixture

Cruise rpm: 0.75 to 1.25- % CO or a 14.2 -14 to 1 air/fuel mixture (hot cam engine 1 to 3% CO)

Power mixture and acceleration 6.6% CO or a 12-1 air/fuel mixture for a "normal" engine, a high performance engine with improved combustion chamber design such as a Pro-Stock or a Winston Cup engine can use a power mixture of 4% CO or a 13 to 1 air/fuel ratio.

The CO reading from an infrared gas analyzer is the reading that we use to determine the air to fuel ratio. (Note: CO is partially burned fuel)

The other readings that exhaust analyzers provide are:

HC (hydrocarbons): The amount of unburned fuel or an indicator of an engine misfire, the best mixture gives you the lowest HC.

CO2 (carbon dioxide): The product of complete combustion, the best mixture gives you the highest Co2 reading

O2 (oxygen): A high O2 reading indicates a lean mixture or an exhaust leak.

Note: if O2 is above 2 to 3% any CO readings will not be accurate.

NOx a high reading can indicate detonation from excessive ignition advance or the gasoline does not have enough octane for the engines needs.

The best power and cruise air/fuel mixtures (CO) will burn all the o2 in the cylinder and create the lowest HC reading (misfire) and will also cause the co2 reading to be its highest.

Note: all infrared readings assume there is no air pump or catalytic converter. If used, the air pump must be blocked off and any readings taken must be in front of the catalytic converter.

A properly tuned fuel and ignition system will allow your hot rod perform up to it's potential and give you a better running, more reliable ride!


Ole's Carburetor & Electric Inc.

120 El Camino Real

San Bruno , CA 94066


John Bishop

Hot Rod Tuning

808 Burlway, #2

Burlingame , CA 94010 �����������

650.343.4860 ���

OTC/SPX Corporation

655 Eisenhower Dr .

Owatonna , MN 55060 ����������������������

800.533.6127 ��� �������

Barry Grant Inc.

1450 McDonald Road

Dahlonega , GA � 30533 ����������������� ������������