3. As with Carter-style carburetors, each primary metering rod and spring is accessible from the top by loosening one screw. The metering rods are not interchangeable with earlier Carter carburetors. We know because we tried it. The stepped metering rods (arrow) maintain lean air-fuel ratios in high manifold-vacuum cruise applications. Separating the springs from the metering rods allows finite tuning, which can deliver excellent results.