Metering Rod Selection Hints

Before you start
Working with carburetors and gasoline presents a very real risk of fire. Engine components, especially turbochargers, can get very hot. If you are not confident of your own abilities to work safely and return the car to a safe condition, you should seek professional assistance.

On the other hand, carburetors are combinations of reasonably simple devices. The operation of each component is relatively simple. Once you understand the individual devices, you should be able to develop an understanding on the interaction between the systems. Once you do, you will have a better understanding of carburetors than most mechanics in this time of fuel injection and replacement of assemblies rather than troubleshooting individual components.

Work slowly. Change one thing at a time. Keep a record of the changes and test results. Make sure that you understand the effects of the change before changing something else. This method may seem slow, but it is the quickest way to get real repeatable answers.

The cars that use these carburetors are 35 to 50 years old. Before you start changing anything, you should determine the current condition of the systems that impact engine performance.

Things to check

Carburetor

Identity - the carburetor originally came with a thin triangular metal tag attached by one of the float bowl cover screws. The tag number is the most exact identification of the carburetor. If the tag is missing, look for the body number cast into the carburetor body on the top front near the throttle plate (see photo). This number will be reversed as you see it on the car. The number will include 0- then a four digit number. This number is unique to the year except for 0-1580 which could be late 1963 or 1964 or 0-1769 which could be late 1965 or 1966. See the table on the Replacement Metering Rod page.

Size - Body numbers 0-1392, 0-1507 and 0-1580 denote 1 5/16" venturi carburetors used on 150 HP engines. Body numbers 0-1750 and 0-1769 denote 1 3/8" venturi carburetors used on 180 HP engines.
Loose screws - the throttle body screws are often loose. This leads to vacuum leaks that will confuse your tuning efforts and could lead to engine damage.
Squashed throttle body gasket from long stud - the throttle body gasket is often squashed unevenly do to the way the carburetor is mounted to the turbocharger. The 2 lower studs pass through the base gasket and bosses on the throttle body only. The upper stud passes through the base gasket, throttle body, throttle body gasket and a flange on the carburetor body. Tightening the nut on the upper stud can crush the thick gasket between the throttle body and the carburetor body. Leaving the nut too loose can lead to air leaks. The only sure solution seems to be to bore out the upper stud hole in the carburetor body and throttle body gasket and inserting a sleeve to transfer force from the nut to the throttle body.

Warped throttle bodies - throttle bodies are often warped causing air leaks. Check with a straight edge. Light damage can be corrected by filing. Heavy damage may require machining or replacement.

Warped float bowls and covers - if the float bowl cover or the float bowl portion of the carburetor body are warped, fuel leakage, air leakage or internal leakage of the accelerator pump shot may result. Check with a straight edge. Correct with filing or machining if possible.

Throttle body and linkage - check that the throttle plate is fully open when the accelerator pedal is pressed to the floor.

Fuel supply

Fresh gasoline - gasoline evaporates starting with the more volatile components. Eventually, all that is left is varnish. Replace old gasoline before tuning.
Fuel pressure - insufficient fuel pressure can lead to fuel starvation during boost. This will cause leaning that can cause engine damage.
Fuel pressure - excessive fuel pressure can lead to flooding and high fuel consumption. A blocked or restricted fuel return line can cause high fuel pressure. A fuel pressure regulator can reduce this problem. Start with 2.5 PSI.
Filter, tubing or hose restrictions - can cause fuel pressure reductions as above.
Hose deterioration (MTBE) - the addition of MTBE to gasoline seems to cause rapid deterioration of synthetic rubber hoses and diaphragms. Check frequently.

Ignition

Turbocharged engines place great demands on the whole ignition system. A high output aftermarket ignition system is highly recommended. If you plan to run higher than stock boost levels, a better ignition system is mandatory.

Spark plugs - turbocharged engines place greater loads on spark plugs than conventional engines and require more from them.
Spark plug wires - Corvairs are notorious for rapid failure of spark plug wires. Turbocharged engines run hotter and cause even faster deterioration. Use premium aftermarket spark plug wires.

Proper voltage - make sure that you have full voltage to the ignition system.
Proper ground - it is equally important that you have good grounds.
Proper initial and centrifugal advance and pressure retard - make sure the pressure retard actually works. The distributor should be checked on a Distributor Machine to verify correct function of each system.

Exhaust

Exhaust leaks - this will cause a large reduction in boost pressure and horsepower.

Exhaust restrictions or blockage - will cause loss of performance also

Engine

Compression - the compression readings should be uniform for each cylinder. A low reading for one or more cylinders probably means that something is worn or broken.

Leak-down test - this is similar to a compression test except that an outside pressure source is used to maintain pressure in the cylinder and the location of the leakage can often be determined by listening for air flow at the exhaust, intake and breather. The readings should be low and uniform.

Oil smoke in the exhaust - oil smoke in the exhaust indicates that oil is getting burned in the cylinders. This lowers the effective octane of the fuel mixture and will confuse your attempts to tune the engine.


Get baseline readings

Although you can detect lean surge or gross richness without test equipment, in order to make precision adjustments, you should measure the mixture using an Oxygen sensor or Exhaust Gas Analyzer. We recommend using an Oxygen sensor since tests can be run in the car under load during a variety of weather and traffic conditions. An Oxygen sensor and Air/fuel ratio gauge can be purchased for less than $100. Wide band oxygen sensors are accurate or a much wider range and are available for about $300. We have had good success using Innovate Motorsports products. You should analyze 4 conditions: idle, cruise, power and the transition from cruise to power.

Idle mixture is adjusted by turning the idle mixture screw. All the other conditions are set by a combination of the main jet and the metering rod. The metering rod blocks part of the opening of the main jet reducing the fuel flow. The metering rod has 3 areas that control fuel flow. The larger diameter step limits fuel flow at lower airflow rates for better fuel economy. The smaller diameter step causes less restriction of the jet for greater fuel flow at high airflow rates for increased power. The taper between the steps effects the transition between cruise and power. The accelerator pump also affects the transition phase. The position of the rod is determined by a combination of throttle position and vacuum.

Air/fuel ratios are a comparison of the weight of fuel consumed compared to the weight of air consumed by an engine. The ratio by volumes is, of course, much greater. Carbureted engines produce best power at an Air/fuel ratio of between 11.5 to 1 and 13 to 1. Best fuel economy occurs with leaner ratios, typically between 14 to 1 and 17 to 1. The ideal or stoichoimetric ratio is 14.7, but engines will typically run leaner under light loads for better economy. Usually a ratio between 14 to 1 and 12 to 1 is required for a smooth idle since the engine is not very efficient at very low speeds.


Proceed cautiously

Make small changes and check the effects of each change with the engine at full operating temperature on a warm day. Ignition and carburetor settings that work on a cool engine on a cool day may cause damage to a hot engine on a hot day. It is easier to damage a turbocharged engine than a normally aspirated engine since producing the additional horsepower creates additional heat.

The most important engine protection device you have is your throttle foot. If you hear pinging or feel rough running, lift your foot and solve the problem before continuing.

Turbocharged and supercharged engines are limited by a combination of boost pressure, compression ratio, gasoline octane rating, cam timing, ring and valve sealing, ignition advance, fuel mixture and atmospheric conditions. Changing any one of these may require other changes to compensate.


Adjusting for seasonal changes

After you are satisfied with a jet & metering rod combination, make note of the weather conditions. If you drive in a variety of weather conditions or altitudes, you should track the effect of these changes to find out if you should adjust for seasonal variations.

As rules of thumb, the jetting should be 2.5% leaner for each 1000 foot increase in elevation and 1% leaner for each 10F increase in temperature. Increased humidity reduces the percentage of oxygen in air and thus makes engines run richer. You may need to adjust accordingly.

 

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