Metering Rods
Frequently asked questions
Why does Black Hawk Engineering make metering rods for Carter® YH carburetors?

There is a very limited range of metering rods and jets available for these carburetors making precision tuning difficult. The carburetors were only used on a small number of relatively low production volume vehicles like turbocharged Corvairs® and 6 cylinder Corvettes®, so there is little aftermarket support. I wanted to change the mixture on our own cars and found that it is very difficult to fabricate metering rods without expensive manufacturing tooling. Since I have now successfully developed techniques for fabricating individual metering rods, I decided to offer the metering rods for sale to help others improve carburetion on their cars.

What makes our metering rods unique?

The metering rods are machined using CNC (Computer Numerically Controlled) machine tools. This means that I can control the size and location of each feature precisely. Each metering rod is individually measured and the actual size is engraved on each metering rod. The metering rods are then tumble deburred to remove any surface roughness. This process results in a pleasing shiny surface that encourages uniform flow.

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Why not use the original metering rods and jets?

The formulation of gasoline has changed significantly since these carburetors were new. Federal and local smog laws have required extensive changes, particularly in the volatile components of gasoline. The current unleaded fuels are quite different than the leaded fuels for which the cars were designed 30 to 40 years ago. The current fuels are also formulated for fuel injected engines which are much less sensitive to reductions in vapor pressure than carbureted engines. Even if your carburetor and engine are like new, the current gasoline may require mixture changes. If parts of your carburetor are worn, air leakage will also change your mixture. In addition, modern oxygen sensor based fuel to air ratio gauges provide a much more accurate means of measuring the actual mixture supplied to the engine than was available at the time the cars were manufactured. This means that the mixture can be set more precisely for better fuel economy.

Can I change jets instead?

This is not the best method. Changing the metering jet changes the mixture at all airflow levels above idle. 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. Changing the metering rod enables changing the mixture in any of these areas without adversely effecting the others. Changing jets is an easy way to assess the effect of relatively large changes.

Can I use other stock metering rods and jets?

Yes, but these are available in a very limited range of sizes. Where two similar sizes of metering rod are available, the change in flow area is generally more than 10%. Note that the difference between a typical cruise air/fuel ratio of 14.5 to 1 and a typical power air/fuel ratio of 12 to 1 is just 21%.

Changing rod diameters by .001" results in a flow area change of only 2 to 4%. Carter jets are available in .003" increments. This results in a change in flow area of 10 to 13% between sizes. For comparison, the jets for the Holley® carburetors commonly used for drag racing are available in .001" increments resulting in a 2 to 3% change in flow area between sizes.

Will the same jet and metering rod work for all engines?

No. Each engine has differences in vacuum, valve timing, ring seal and many other factors which lead to changes in tuning requirements. Even similar engines may require different jetting.

What are the risks of changing the air/fuel mixture by changing jets or metering rods?

You can very quickly damage or destroy your engine by running too lean, too little octane and/or too much ignition advance. Major damage can occur in just a few seconds under full boost.

What is the best method to optimize mixture?
The best method is to use a chassis dynamometer to load the engine while analyzing the exhaust with an exhaust gas analyzer. These machines are fairly rare and quite expensive to operate.

The best method for the hobbyist is to use an oxygen sensor mounted on the exhaust pipe and Air/fuel Ratio gauge to determine mixture under load in use. Oxygen (also called Lambda) sensors are used on most current fuel injected engines and are available at reasonable prices. Suitable electronic gauges are available from a number of aftermarket vendors. We have had good success using Innovate Motorsports products.

A threaded bung for the sensor must be welded to the exhaust pipe. It can be blocked with a plug when not in use. The gauge can also be mounted temporarily and removed after use.

The engine should be warmed to full operating temperature for the readings to be meaningful. This may take 30 minutes of running at highway speeds.

Can I tell the difference between different jets and rods without test equipment?

This is possible. You should be able to detect pinging and lean surge caused by too lean a mixture. Too rich a mixture will result in soggy performance, fouled sparkplugs, a black tailpipe and poor mileage. You can probably identify the differences caused by .002" changes in metering rod diameter without test equipment (5 or 6%). With an Air/Fuel ratio meter, you can differentiate between changes of .001" in metering rod diameter.

I am getting 10 MPG. What should I do?

This seems very low unless it is all stop and go driving. First check that the float level is not too high and the fuel return line is not clogged. These conditions will drastically reduce mileage. Then check that the tires are properly inflated, the brakes are not dragging and the suspension alignment (especially toe-in) is correct. Then check the normal tune up items: the air filter is clean, there are no exhaust leaks, the muffler is not clogged, the compression is good on all cylinders, the float level is set correctly, the sparkplugs are fresh and properly gapped and the ignition timing is correct.

When all of these are correct, you can start tuning. Since the majority of driving is done without boost, concentrate on getting the mixture correct off boost. A smaller jet or larger metering rod cruise step will lean the mixture. Make small changes until you achieve Air/Fuel ratios above 14.5 to 1. If you are testing without instruments, back off if you encounter pinging or lean surge. Once you have optimized mixture for cruise conditions, you should analyze performance under boost. The power Air/Fuel ratio should be about 12 to 1.

What MPG can I expect with correct jetting?

This depends a lot on driving style, but here are some representative figures for a modified 95HP engine with the turbocharger & carburetor from a 1966 180 HP engine
(Otto cam, vacuum advance, MSD ignition with boost retard, K&N air filter, etc.) in an early model coupe in mostly highway commuting:

Worst mileage recorded (30% rich cruise step) 13 MPG
Stock 180 carburetor 16 MPG
Optimized jet & metering rod combination 18 MPG
The same engine without the turbo (95 HP setup) 22 MPG
The same engine with electronic fuel injection (and more power) 21 MPG

Will one metering rod & jet work for all seasons?

This is of course what the manufacturers did. Since these conservative settings are only correct for the worst conditions, the settings with be too rich the rest of the time. If you would rather be closer to the optimum more of the time, you can adjust for seasonal weather variations by changing the metering rods slightly. I offer Experimenter's kits with a new jet and a set of 5 metering rods with varying diameters for this purpose.

Can I adjust for high altitude operation?

Yes. As a rule of thumb, the jetting should be 2.5% leaner for each 1000 foot increase in altitude. You should also adjust1% leaner for each 10°F increase in temperature. I offer High Altitude Experimenter's Kits if you live at or visit higher elevations. These consist of a new jet and 5 metering rods with progressively leaner sizes for both the cruise and power steps.

Can you tell me exactly what jet and metering rod to use in my engine?

No. I can provide a standard replacement jet and metering rod set that will match the original parts in a stock carburetor. This will provide a good starting point for adjusting for current conditions.

First return your carburetor to the original specs if the engine is near stock. Once you have it running correctly with the original jet & metering rods sizes, you can be reasonably sure that the rest of the engine and fuel system is operating normally. Then you can systematically change metering rods to correct for wear, modern gasoline formulations, altitude, etc.

Does Black Hawk Engineering rebuild carburetors?

No, we just manufacture the metering rods. We recommend contacting Steve Goodman of Rear Engine Specialists in Golden, Colorado, phone (303)-278-4889. His e-mail address is rearengine dot steve at worldnet dot att dot net. Steve has also developed a technique for bushing worn out throttle shaft bores to stop air leakage. This is a highly recommended upgrade.

Still have questions? drop us a line at sales at blackhawkengr dot com

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