By: Henry P. Olsen

Continued from Part 1: Fine Tuning the Fuel System.

In part one of this tuning a carbureted engine technical article we showed why and how we tuned the ignition system of an engine to perform its best with the unleaded reformulated gasoline of today. The street rod we are using to demonstrate our tuning techniques is a Roy Brizio built 1936 Ford with a Roush Performance 427 R engine that is rated at 550hp, but these same tuning techniques will work on any carbureted engine. The first step in the tuning process, after we determined that the engine was mechanically sound, was the fine-tuning of the ignition timing (the initial timing as well as the mechanical advance curve and vacuum advance if used). Now with the ignition spark timing optimized for today’s unleaded reformulated gasoline, we will now proceed to the next step, which is tuning the fuel system. We will now show you the modern tools we use to “read” the air/fuel mixture that the carburetor is supplying the engine and how we use these modern tools to tune the fuel system so the engine can perform at its best. We will first explain the tuning process we use to tune the air/fuel and then show how we apply this process to the engine in the Roy Brizio built 36 Ford street rod we are working on.

The Air/Fuel Mixture

The leaded gasolines that most carbureted engines were designed to use allowed the engine to perform very good even if the ignition spark timing or the air/fuel mixture was not properly tuned. The same cannot be said for the unleaded reformulated gasoline of today, unless the ignition spark timing and the air/fuel mixture are properly tuned, the engine will not perform like it should. Most people think their engine is running too rich because the exhaust fumes burn their eyes; this is not true. An engine with a rich air/fuel mixture will have excessive carbon monoxide (CO) in the exhaust; carbon monoxide is an odorless gas that is quite deadly if you breathe too much of it. A engine that has a misfire from things such as: a rich air/fuel mixture, a lean air/fuel mixture or incorrect ignition spark timing will have a lot of unburned hydrocarbons (HC) in the exhaust. Exhaust gas with a high unburned hydrocarbon (HC) content will burn your eyes and make it difficult to breathe.

A lean fuel mixture (too little fuel for the amount of air in the cylinder) can cause an engine to have a surge or miss at idle and part throttle, stumble on acceleration, lead to engine overheating and cause a lack of power. A rich air/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 lack power or run sluggish.

Methods for Reading the Air/Fuel Mixture

A carburetor equipped street rod engine can be tuned to run as well or even better than it would have run with leaded gasoline by using modern tools such an exhaust gas analyzer to tune the engine. In the past, tuning a fuel system to supply an engine with the correct air/fuel mixture has always been a task that almost impossible for most tuners. A tuner had to attempt to tune the engine by looking at the spark plug, the exhaust port and the first 6 inches of the header for proper “color” and then make a guess at what air/fuel mixture change was needed. Tuners can now use the new, more scientific - modern method for checking the air/fuel mixture with tools such as an infrared exhaust gas analyzer and/or an extended range oxygen sensor based air/fuel ratio meter. This new technology analyzes the engines exhaust content to allow you can “read” what air/fuel mixture the engine is getting from the carburetor(s) at any rpm and load condition.

The new advances in exhaust gas analysis technology and wideband oxygen sensor technology have made it possible to “read” what the air/fuel mixture actually is under almost any driving condition. In the past exhaust gas analyzers have tended to be large and expensive, but the computer age has allowed the production of many new units that are not only compact and portable, but also affordable. Modern technology has also made it possible to use a wideband oxygen senor to “read” the air/fuel mixture with a digital air/fuel meter at an affordable price.

The easiest and most accurate method that we have found is using an infrared exhaust gas analyzer. An exhaust gas analyzer “looks” at the content of exhaust gases and then supplies readings that can indicate what the air/fuel mixture is and the efficiency of the engine. The infrared exhaust gas analyzer allows a tuner to “read” and record for future reference what air/fuel mixture is at any operating condition such as idle, cruise speeds or wide open throttle, the air/fuel mixture can be tailored to what your engine needs to perform at its best. The CO2 and NOx readings from the exhaust gas analyzer can be used as a method to determine if the ignition timing is correct for what the engine needs for maximum power and efficiency.

The air/fuel mixture can also be checked by the use of a wideband oxygen sensor installed into the exhaust header, the sensor is “read” by a digital air/fuel meter. This method determines the air/fuel mixture by looking at the oxygen/unburned combustibles in the engines exhaust; the readings can be very accurate, but false readings can be created by a exhaust leak, engine misfire, or a high overlap cam at lower engine speeds (these false readings are caused by the oxygen sensor misreading the extra oxygen in the exhaust that is there because of a misfire, an exhaust leak or the high overlap cam).

The original equipment carburetor(s) that are on many street rod engines were not designed or tuned for today's unleaded reformulated gasoline, so in most cases the engine will tend to run lean. The high performance and replacement carburetors sold today are built with a generic air/fuel mixture curve. These aftermarket carburetors should be designed with an air/fuel mixture that is rich enough for a wide variety of engines packages with different exhaust systems, but this is not often the case. Most of the aftermarket carburetors we see need a lot of tuning work to get the air/fuel mixture correct for the engines needs. The performance replacement carburetors we prefer from our engine tuners point of view are the part #1806 650 cfm Thunder carburetor from Edelbrock or any of the Mighty Demon carburetors from Barry Grant Inc. The #1806 Thunder carburetor works very well on a mild engine package with a driver that uses his or her street rod as a cruiser. When the driver is more aggressive or has a high performance engine with the goal of getting the maximum power from their Street Rod we prefer to work with the Mighty Demon series of carburetors from Barry Grant. When you are using a race-designed carburetor such as a Mighty Demon we always suggest that the customer select a smaller cfm most carburetor selection guides will recommend. A larger cfm (air-flow) carburetor will give you a little more high rpm power at the expense of low and mid range power and drivability, so unless you are always driving at very high speeds at maximum rpm your engine will perform better with a smaller carburetor!

The Barry Grant Mighty Demon carburetors have changeable air bleeds, which make it easy for a tuner to tailor the air/fuel mixture for the engines needs. The new Demon six-shooter 3x2 bbl carburetor & manifold packages that Barry Grant has for both small and big block Chevrolet engines (Pontiac and Ford packages are the next to come) promise to be great package that will combine good looks with lots of power. A modern 3x2 bbl carburetor & manifold package such as the new Demon six-shooter can allow an engine to get good fuel mileage if you are light on the throttle and keep your foot out of the two outer carburetors.

Jetting With a Infrared Gas Analyzer or Wideband Oxygen Sensor

The most accurate and easiest way to check the jetting (air/fuel mixture) of an engine is by observing the CO reading from an infrared gas analyzer and/or a wideband oxygen sensor. First place the sample probe into the tailpipe and then the unit will “read” the exhaust and supply the readings necessary to determine the air/fuel mixture. The infrared exhaust gas analyzer and/or the wideband oxygen sensor method allows the part throttle air/fuel mixture to be checked which otherwise is almost impossible, the readings from either method can be read in real time or recorded and later played back. It is important to note that any changes other than jet changes and other basic adjustments should be done by a proficient “carburetor expert” to avoid damaging a #’s matching original carburetor.

A starting point for air/fuel mixtures for most mild performance engines is:

Idle: 1.0% to 3.0 % CO or a 14.1-13.4 to 1 air/fuel mixture

Cruise rpm: 1.0% CO or a 14.1 to 1 air/fuel mixture with a mild performance engine: 1.0% - 3.0% CO or a 14.1 – 13.4 to 1 air/fuel mixture for a engine with a high performance cam. Note: a 0% CO or 14.7 to 1 cruise air/fuel mixture is chemically ideal but most high performance carburetor equipped engines will suffer misfire and drivability problems when operated that lean.

Power mixture and acceleration: 6.0% CO or a 12.5 to 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, in some cases may be able to use a slightly leaner power mixture of 4% CO or a 13.0 to 1 air/fuel ratio.

Tuning the air/fuel mixture with a infrared exhaust gas analyzer

CO (carbon monoxide): The 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 air/fuel mixture gives you the highest CO2 reading. The ideal ignition-timing advance will also create the highest CO2 reading

O2 (oxygen): A high O2 reading indicates a lean mixture; an exhaust leak or the engine has a “hot” cam. Note: if O2 is above 2 to 3%, the CO readings may not be accurate.

NOx (oxides of nitrogen): A gas created by excessive combustion chamber heat. In many cases a high NOx reading may be related to excessive ignition spark advance. The excessive spark advance can cause engine ping or detonation that can result in engine damage.

Fuel Pressure and Volume

The fuel supply system must supply enough fuel volume with the correct fuel pressure at every operating condition from idle to wide-open throttle! The fuel pressure that we recommend is 5-½ to 6-½ psi, if you increase engine power you may need to install a high volume fuel pump and increase the fuel line diameter to avoid starving the engine. The fuel pressure most not drop below 5 ½ psi at all engine operating speeds or the carburetor float level will drop and cause the carburetors air/fuel mixture to lean out. For every gallon of fuel the fuel pump pulls from the gas tank and supplies to the carburetor, one gallon of air must go into the gas tank. If the fuel tank is not properly vented, the fuel pump will be fighting against a vacuum build up in the fuel tank, this will cause the fuel pump not be able to supply the carburetor with enough fuel pressure and volume to feed the engines demands. The fuel system that would supply the fuel to an engine that could power a car to a 10 second run at a ¼ mile drag strip would need to be able to fill a 1 gallon gas can in under 25 seconds. If you run the engine to long without enough fuel pressure and/or fuel volume the air/fuel mixture will go lean and then the results could be a burnt piston or other engine damage.

Tuning For Fuel Economy

The fuel economy an engine can supply you with will be its best when the air/fuel mixture is at or near 14.7 to 1 a/f mixture, but a carbureted engine may not perform very well when the a/f mixture is that lean. Many carbureted engine will run lack power, misfire and/or run hot when you use a 14.7 to 1 a/f mixture so the engine will need to be tuned for the a/f mixture that works best for that engine package. The way we tune a carbureted engine to find what the ideal air/fuel mixture (read as CO) is for gas mileage is the use of an exhaust gas analyzer. When the an engine has the ideal a/f mixture, it will burn all the oxygen (O2) in the cylinder, create the lowest amount of engine misfire (HC) and the ideal air/fuel mixture will also cause the engine efficiency (CO2) to be at it’s highest. The best a/f mixture for gas mileage is slightly on the rich side of where engine misfire rate starts to rise. When you tune the engine for maximum fuel economy the power and drivability that an engine makes will suffer. When you tune the engine for maximum power and drivability the engine will burn more fuel, but a good tuner can supply you with both good fuel mileage and good power/drivability.

Tuning the power system opening point can also be used to increase gas mileage. As you tune the power system to open at a lower vacuum it will take more throttle opening to get the richer power mixture to kick in. If you have a carburetor with a power valve, you can change to a power valve that opens at 4.5 inches of vacuum instead of 8.5 inches of vacuum. A carburetor that uses a power piston and metering rods power system opening point can be modified to open later by changing the power piston spring to a lighter spring rate.

Proper ignition timing and advance curves must be used if you expect the engine to burn the a/f mixture at the correct time, making the power that pushes the piston down in the cylinder. A high NOx reading from an exhaust gas analyzer indicates that the ignition spark advance may be excessive for the fuel you are using, so the NOx reading from an exhaust gas analyzer can also be used as an ignition spark timing tuning aid.

Tuning with a Wideband Oxygen Sensor

The wideband oxygen sensor looks at the unburned combustible content in the exhaust and the air/fuel meter converts its signal into an air/fuel mixture reading. This method requires you to know what air/fuel mixture your engine needs for each driving condition, this data should be available from your engine builder or you can use a infrared exhaust gas analyzer to help you determine what air/fuel mixture your engine needs to run its best. An extended range oxygen sensor can “read” air/fuel mixtures as rich as 9 to 1 and it can read air/fuel mixtures as lean as 19 to 1 (a standard oxygen sensor is only accurate at air/fuel mixtures of around 14.7 to 1). This method has the advantage of extremely fast reaction times for the readings, but false lean readings can be created under certain operating conditions. The false readings are created when the oxygen sensor “sees” the excessive oxygen in the exhaust system that can come from a engine misfire, a supercharged engines “blow-thru” effect (at low engine speeds), a high performance camshaft’s valve overlap or an exhaust leak.

The wideband oxygen sensor based air/fuel meter that we use is the LM-1 Digital Air/Fuel Meter from Innovate Motorsports, this unit can allow you to sample and record the air/fuel mixture data at a rate of 12 samples per second over a period of up to 44 minutes, this data can allow you to “read” the fuel mixture the engine is getting and then confirm the tuning changes you make give you the fuel mixture you are targeting.

When we are tuning a fuel system we use both the infrared exhaust gas analyzer and the wideband oxygen sensor methods, this way we can take advantage of both tuning methods. The use of an infrared exhaust gas analyzer, while slower in reaction time, has the advantage that it not only reads the oxygen/unburned combustibles content of the exhaust like a wideband oxygen sensor, but also allows you to determine the air/fuel mixture by observing the CO reading (carbon monoxide); the engine’s rate of misfire can be determined by observing the HC reading (hydrocarbon); the engine’s efficiency can be determined by observing the CO2 reading (carbon dioxide) reading. Spark knock, detonation or engine ping that is in most cases is caused by overly advanced ignition timing can be seen by observing the NOx reading (oxides of nitrogen).

Caption: The above chart shows the gases in the exhaust that an infrared exhaust gas analyzer reads and how the gases change as the air/fuel mixture changes.

Reading the Air/Fuel Mixture

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 to determine what the air/fuel mixture is at idle through 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 through 1800 rpm, the idle channel restriction on a Quadrajet or the economizer restriction on a Carter AFB (also the Edelbrock Performer or Thunder series) 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 on light acceleration and 15 – 30 mph light cruise speeds; this lean off idle problem is also very common on many of the performance replacement carburetors sold today.

If the air/fuel mixture is too rich at idle and/or part throttle, the idle jet or part throttle idle restriction may be too big and may need to be replaced with a smaller one. Once you have the idle, part throttle and cruise air/fuel mixture curves correct, the next step is a road test. A road test using a portable infrared exhaust gas analyzer and/or a wideband oxygen sensor will allow you to check the cruise speed air/fuel mixture, followed by a check of the power air/fuel mixture under load. This type of test allows you to see what the air/fuel mixture is at the real world driving conditions you will drive your street rod at, during this road test you will be able to read and then correct the air/fuel mixture. If you “see” an air/fuel mixture reading that goes too lean at high engine loads, the first thing to do before you change jet size is to check that the fuel pressure. The fuel pressure must stay above 5 psi at wide-open throttle, if not the carburetor will starve for fuel. The low fuel pressure could be caused by the fuel pump not having enough volume or the fuel tank vent may not be letting enough air into the tank to replace the fuel being removed from it.

Tuning The Carburetor

A carburetor has an accelerator pump, idle, main jets, and in most cases a power system that is designed to supply the correct air/fuel mixture for the engine’s demands. An idle system will have an idle jet/restriction that must be changed to supply the desired fuel mixture for idle and off idle engine demands. If your engine is equipped with a Holley style carburetor (sold today by both Holley and Barry Grant as the Demon Series), the main jet size is what determines what air/fuel mixture is delivered to the engine at light load/cruise speeds (1500rpm and up) and the power valve restriction (under the power valve) is the determining factor in what the air/fuel mixture the carburetor will supply when the power valve is open; a 6.5 inch power valve will be open and supplying the richer air/fuel mixture needed under high power demands anytime the vacuum is below its 6.5 opening point. A power valve has a reputation for being a weak spot in this design of carburetors, but the carburetor can be retrofitted with backfire protection, which will help in reliability. A carburetor that uses metering rods in the primary jets such as a Quadrajet or Carter AFB (sold today by Edelbrock today as the Performer and Thunder Series) will use the metering rods to change the air/fuel ratio for both the power and cruise mixture demands of the engine; the larger the metering rod diameter the leaner the air/fuel mixture will be.

The accelerator pump system adds fuel as the throttle valves are opened, tuning the accelerator pump squirter volume and duration it’s tuning is mainly done by trial and error in-order to obtain the best throttle response, but a 12.5 to 1 air/fuel mixture is a good place to start. The problem we most often see on a engine with a Holley carburetor is a hesitation on quick acceleration, the cure we use for this is to install an accelerator pump upgrade kit part #1330 that we get from Ole’s Auto Parts, the pump upgrade kit makes the accelerator squirt more active. We also use an improved accelerator pump on the Edelbrock Performer (also know as a Carter AFB) & Thunder (Carter AVS) carburetors to make the pump more active and help avoid hesitation problems on acceleration. We buy the fuel system tuning parts we need from Ole’s Auto Parts, they have the accelerator pump upgrade kit we use for Holley carburetors (Ole’s part #1330) and an improved accelerator pump (Ole’s part #1010) for Edelbrock Thunder & Performer carburetors and the 9000 series AFB carburetors that Carter sold.

Tuning the fuel system in the 36 Ford

Now it is time to check out the air/fuel mixture the engine is getting from the carburetor, but first we checked the fuel pressure and confirmed it was at 6 ½ lbs. The next step was to check the air/fuel mixture the engine was getting by the use of our 5-gas exhaust gas analyzer. The readings showed that the air/fuel mixture was slightly lean at idle, as we increased the engine speed the air/fuel mixture the engine was getting from the off-idle system was very lean and then at about 3,000 rpm the air/fuel mixture the engine was getting thru the main circuit was much closer to the we would want to see for a high performance engine such as this. The exhaust gas analyzer also showed us that the accelerator pump was very lazy by the very lean readings we were getting as we snapped the throttle for quick acceleration, we could also hear the engine bog and try to backfire thru the carburetor when we snapped the throttle. The carburetor needed the idle system modified to supply the engine with a richer idle & off-idle air/fuel mixture and the accelerator pump would need a lot of help. The Holley carburetor also was a vacuum secondary unit with a two corner idle system, a engine with a big camshaft and/or a air-gap intake manifold will perform best with a carburetor with a four corner idle circuit. We would also in most cases prefer a carburetor with mechanical secondarys so the driver can control when the secondarys will open.

The Roush 427R engine we are working on has both a big camshaft and a single plane air/gap style intake manifold. In general unless you are building a race engine an air-gap style intake manifold will not provide enough extra power to make up for the tuning headaches that come with this type of intake manifold. A air-gap intake manifold can make more power because the air-gap design cools the incoming air/fuel charge, but when used on the street the air charge can become so cool that that the fuel and air do not stay properly mixed together. When the air & fuel are not mixed together properly you will lose power and waste a lot of very expensive gasoline. The intended driving style for this very powerful engine is maximum power, but since the main use of this 36 Ford street rod is on the street and not at a racetrack we recommended the intake manifold be changed to a non air-gap dual plane Edelbrock Performer RPM.

Rather than put a lot of work into a carburetor that is not very tuner friendly we decided to install a 650 cfm Mighty Demon carburetor with mechanical secondarys. The Mighty Demon is designed for a high performance engine such as this.It has a very responsive accelerator pump system and can easily be tuned to supply the richer air/fuel mixture that this high performance engine will need because of its air-gap style single plane intake manifold, large intake ports and aggressive camshaft. We installed the 650 cfm Mighty demon and then adjusted the idle speed & air/fuel mixture while observing the readings from the 5-gas exhaust gas analyzer. The readings we saw on the read out showed us the air/fuel mixture was right on target as it came out of the box and when we snapped the throttle the engine responded instantly. The idle air/fuel mixture the exhaust gas analyzer showed us was at our target idle air/fuel mixture of 3.0 % CO or 13.4 to 1 air/fuel mixture that this engine idled best at, the exhaust gas analyzer readings also showed us that the air/fuel mixture was right on the target air/fuel mixture of 3.0% CO or a 13.4 to 1 air/fuel mixture at all engine speeds thru 3000 rpm. When the throttle was snapped the exhaust gas analyzer now showed us that the Mighty Demon’s very responsive accelerator pump system was supplying the engine with the proper 6.6% CO or a 12 to 1 air/fuel mixture that is needed form the accelerator pump system and now the throttle response was much better and the engine no longer had any hint of a backfire.

Now it is time for the most important part of the tuning process, the test drive. We mounted the Innovate LM-1’s wideband oxygen sensor in their unique tailpipe probe in the exhaust pipe and now it is time for the test drive. The LM-1 readings allows us to we observe what the air/fuel mixture is in real time, while the car is driven on the road under real world driving conditions. The first thing we both noticed was when the Roy hit the throttle it no longer hesitated, it just pinned us in the back in the seats. The power was very impressive but as the engine rpm got above 4000 rpm the power started to fade. The readings from the Innovate Motorsports LM-1 showed us that the air/fuel mixture was going very lean.

Back at the shop we installed a fuel pressure gauge and again went for a test drive. As we again accelerated to 4000 rpm we noted the fuel pressure was dropping from the 6-½ lbs we saw at the shop to below 2 lbs at 4000 rpm. When we again got back to the shop we found that the electric fuel pump that was in the car was not a high enough volume unit to supply the fuel need to produce 550 horsepower. We next installed a high volume electric fuel pump along with larger fuel lines to solve this problem. This time on the test drive the engine accelerated to 6000 rpm without a trace of problem, the 36 Ford street rod was now scary fast. The air/fuel mixture readings on the Innovate LM-1 showed us the air/fuel mixture was right on target at every condition that Roy drove us at, the cruise mixture was reading right at the 13.4 to 1 air/fuel mixture that we use for a high performance engine like this 427/550hp power plant. The readings the LM-1 gave us also showed us that the power system from the Mighty Demon carburetor was supplying the correct air/fuel mixture of 12.5 to 1 that an engine needs for maximum power. As I looked at the air/fuel mixture readings when the driver hit the throttle, I noted the air/fuel mixture the accelerator pump was supplying the engine went right the 12.5 to 1 air/fuel it should have and stayed there as long as driver stayed in the throttle.

The problems that were present when we started this tuning story are no longer there, the ping problem is gone, the hesitation when the throttle is no longer present, the engine now has almost unbelievable power thru 6000 rpm and beyond and the exhaust fumes that were causing everybody’s eyes to burn is much less noticeable. The ignition spark advance curve and the air/fuel just had to be tuned correctly for the engine and the unleaded reformulated gasoline it will be burning.

Taking the time to properly tune your street rod’s engine air/fuel mixture and ignition spark advance will not only allow you to unlock all the power that was built into the engine while avoiding doing any damage to your engine due to a incorrect fuel system tune up or over advanced ignition timing. A properly tune engine will also get the best fuel mileage, with gasoline selling in the $2.00 to $3.00 a gallon range how can you afford to drive any car or truck with a un-tuned engine. For a more detailed explanation of the use of a 5-gas infrared exhaust gas analyzer in fuel system diagnosis on the web go to: www.automotiveu.com. Special thanks go out to the technical training & engineering staff at OTC/SPX Corporation as well the fine staffs at Bridge Analyzers, Barry Grant Inc. and Innovate Motorsports for their help, patience and input on the methods we use to tune an engines fuel mixtures and ignition advance systems with their products. Their help and advice has made it possible for us to explain to you how we tune an engine to perform it’s best with today’s unleaded reformulated gasoline!

Sources:

Ole’s Carburetor & Electric, Inc.

Ole’s Carburetor & Electric Inc.

120 El Camino Real

San Bruno , CA 94066      

650.589.7377

olescarb@sanbrunocable.com

John Bishop/Hot Rod Tuning

Burlingame, CA

Ph#650 343 4860

jfb396@sbcglobal.net

Demon Carburetion

Barry Grant, Inc.

706-864-8544

www.barrygrant.com

OTC/SPX Corporation

1-800-533-6127

www.otctools.com