Choosing a Custom
Metering Rod
If you do not have experience tuning carburetors, start with the
Hints
page.
If you have questions, try the FAQ page.
Decide what you want to change
Remember that metering rods are the opposite of jets. Increasing the diameter of the metering rod blocks more of the jet and reduces the fuel flow. To increase fuel flow, you must reduce the diameter of the metering rod.
Decide which areas you want to change: leaner cruise, richer under boost, earlier transition, etc. Work on the cruise step first since most driving occurs in this area. Once cruise is optimized, work on the power step. If you still have pinging or lean sags when you make the transition from cruise to power, then increase the taper length.
Changing the cruise step .001" results in a 2 to 4% change for the original sizes. Changing the power step .001" results in a 1 to 2% change.
If your mixture is very far off, you can test large changes across the whole mixture range by changing jets to get in the ballpark for fine tuning. Since the jets are only available in .003" increments, expect about a 10% change in flow area for each jet size.
For a Turbocharged Corvair® and 6 cylinder Corvettes®, you can look up the original jet and metering rod combination in the Original Jet Metering Rod Tables.
As a rule of thumb, the jetting should be 2.5% leaner for each 1500 foot increase in elevation and 1% leaner for each 20°F increase in temperature.
You may compare individual jet and metering rod step combinations on the large General Jet & Rod Table and then calculate the change percentages.
If you are still unsure what to do, email me for suggestions sales@blackhawkengr.com. You will need to include the following information: year of the car, turbo size (150 or 180), carburetor size (150 or 180) and tag number if known, your current jet number or size, the metering rod number or sizes and a description of the problem you want to solve.
Choose dimensions for a new rod
If you have measured air/fuel ratio numbers for cruise and power, you can adjust the flow areas proportionately. For example, if you have a power A/F ratio of 11 to 1 and you would like to have a ratio of 12 to 1, the ratio is 11 divided by 12 or .92. If your current power flow area is 300, multiply .92 times 300 equals 276. Look in the General tables for the jet size you are using and find the flow area closest to 276. Then read the corresponding metering rod diameter. Do the same type of calculations for the cruise step.
Our current manufacturing process is limited to diameters of .054 to .094". If the metering rod you want falls outside of this range, you will have to choose another jet size to find a combination that works. The available jet sizes are .077, .080, .083, .086, .089, .092, .095, .098, .101, .104, .107, .110, .113, .116 and .119".
The effect of taper length
The standard taper from the small to the large step is about .040 to .050" long depending on the size. We have reduced pinging on transition by extending the taper to as much as .150". This is particularly important if you have leaned out the low flow (cruise) area looking for better fuel economy. If you want a longer taper, include the dimension in the taper column. If no taper dimension is listed, the standard taper length will be used.