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Steve Zack Article:
Henry Olsen Articles:

Tuning a Carbureted Street Rod Engine – Part 1 of 2

Tim Wusz Articles:
Jetting a Carburetor

By: Henry P. Olsen

Jetting an engine with plm and microgas

Most carburetors sold today have a generic “tune-up” or jetting unless the carburetor is built for a specific engine package and fuel. A carburetor not built for a specific engine and fuel should supply an air/fuel mixture rich enough for a variety of engines since engine damage could result if the carburetor is too lean.


Regardless of the fuel you use (race or pump), the air density, barometric pressure, air temperature, humidity, compression ratio, camshaft, exhaust system, ignition timing curves, engine condition, fuel pressure, etc will all affect the carburetor “tune-up” needed to get the correct fuel mixture for your engine. The first order of business is to get the correct ignition advance curve for the engine and fuel being used. Once the ignition advance curve has been confirmed to be correct, many of the problems that we see can be traced to an incorrect fuel mixture for the engine.


Many of the problems seen on original equipment carburetors are caused because the fuel sold today is formulated in a totally different manner than the gasoline sold in the 60’s and 70’s and may shift the air/fuel mixture leaner by as much as 2.7%. Because of the changes in the fuel formulation, the distributor advance curves and vacuum advance curves, as well as the jetting-air/fuel mixture curve may need to be changed to compensate for the fuel sold today and as the fuel formulations change in the future.

Ignition Advance Curve

Before checking the air/fuel mixture, the ignition timing and advance curve must be checked for correctness first. Any distributor, performance replacement or original equipment must have the mechanical and vacuum advance curves checked and then tailored to the engine and the fuel used. (Note: MSD distributors come with a very conservative advance curve; included in the box are the bushings and springs to get the desired mechanical advance curve.) An incorrect advance curve may cause an engine to lack power, ping, use too much fuel or cause the engine to overheat. The advance curve used most often on a non fast-burn cylinder head 9-1 compression engine is an initial timing of 12 degrees with 24 degrees of mechanical advance at 3500 rpm plus a maximum of 10 degrees of advance from the vacuum advance. Most distributors equipped with a vacuum advance give too much advance (18-30 deg instead of 10 deg.); too much advance from the vacuum advance can cause a ping at light throttles and/or an engine to run hot. There are two methods we use to check the distributor advance curves; one is to put the distributor on a test stand, and the second is by using an advance timing light such as the Black Light series from OTC/SPX Tool Company.


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:

•  The first method is to use an illuminated magnifying glass to “read” the spark plugs. This method involves looking at the base of the spark plug insulator (white part of the plug) for a slight coloring on the insulator just above where the insulator comes through the steel case. If the mixture is too lean, it will leave no color, while a rich mixture will cause the fuel ring to become more prominent. Over-rich mixtures will give the plug a sooty appearance. In the days of leaded fuel and point ignition, this method worked quite well. Today the use of unleaded fuels and high-energy ignition systems has made this method much harder because very little color is seen on the spark plug; this method is thus a job for an expert.

•  The second method is to use 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, and then using trial and error to get the best engine feel. When setting the power and cruise mixtures, it is 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 then go leaner to get a 20-rpm drop in speed; this is known as the lean drop method.


•  The easiest and most accurate method that we have found is to use an infrared exhaust gas analyzer like the unit we use from OTC/SPX Tool Company called a MicroGas or the new Performance Gas Module that attaches on the back of the OTC Genisys Scan Tool to “read” the exhaust gases. By using the MicroGas or Performance Gas Module, 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.

•  An optional method of checking air fuel mixtures is to use a Lambda meter such as a unit available from

MoTeC called a Professional Lambda Meter (PLM) which uses an extended range oxygen sensor installed into the exhaust header. This method looks at the oxygen content in the engine’s exhaust and then the PLM supplies an air/fuel mixture reading; the readings are very accurate but false readings can be created by exhaust leaks, engine misfire or a high overlap camshaft at low engine rpm.   


The most accurate and easiest way to check the jetting (air/fuel mixture) of an engine is to observe the CO reading from any infrared gas analyzer, such as the portable unit that we are using, the OTC/SPX MicroGas or Performance Gas Module portable exhaust gas analyzer. First, the sample probe is placed into the tailpipe and then the unit “reads” the exhaust in order to supply the readings necessary to determine the air/fuel mixture.

The infrared gas analyzer or the lambda meter method allows part throttle fuel mixtures to be checked which otherwise is almost impossible. It is important to note that any changes other than jet changes and basic adjustments should be done by a proficient “carburetor expert”.

On Car Testing

After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, and a check has been done 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 the jets are changed in order to get the air/fuel mixture correct at the 2500-3000 cruise rpm range. Then the idle mixture is checked and set. 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 a check of 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.

Fuel mixture targets

A starting point for air/fuel mixtures for a stock engine:

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.

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 at 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 to perform up to its potential and give you a better running, more reliable ride! No matter whether you purchase a new or used infrared analyzer or take your vehicle to a shop that has one, this method of checking and setting the air/fuel mixtures can help you to get the most out of your engine. To obtain a manual on how to tune a specific type of carburetor, contact a company such as S/A Designs.


Ole’s Carburetor & Electric Inc.

120 El Camino Real

San Bruno, CA 94066      



John Bishop

Hot Rod Tuning

808 Burlway #2

Burlingame, CA 94010         



OTC/SPX Corporation

655 Eisenhower Dr.

Owatonna, MN 55060                        



Barry Grant Inc.

1450 McDonald Road

Dahlonega, GA  30533                                            




MoTeC Systems USA

West Coast:

5355 Industrial Drive

Huntington Beach, CA 92649


East Coast:

169-2 gasoline Alley

Moorsville, NC 28117



S-A Designs     

CarTech, Inc

39966 Grand Avenue