There's a lot of myth and folklore out there for those seeking additional power from classic Mustangs. Stroker kits, superchargers, bigger carburetors, hot ignitions, high-flow exhaust systems, less restrictive air cleaners, fuel additives, and more will theoretically achieve gains in power. However, did you know your engine can make more power with simple analysis and performance tuning?
We visited Jon Enyeart of Pony Carburetors in Las Cruces, New Mexico, to discuss engine tuning and carburetors and how both directly affect performance. We also consulted with Marvin McAfee at MCE Engines in Los Angeles to get his take on how to unleash hidden power.
Pony Carburetors has been building concours-restored Mustang carburetors for 17 years. The company's nice-looking, factory-original Autolite, Holley, Carter, and Rochester carburetors look good on any concours restoration, not to mention a daily or weekend driver.
More than good-looking, Pony Carburetor's atomizers are high on function and improved performance because Jon wouldn't have it any other way. His approach to engine tuning is methodical, taking place one system at a time. He begins with the carburetor, then moves to the ignition system to ascertain a proper baseline tune. Most of the time, rough idle and lackluster performance are rooted in fuel- and ignition-system tuning. However, fuel and ignition systems cannot do their jobs effectively unless all other areas of the engine are healthy.
Any tune-up should include a compression check and cylinder leakdown test. Compression should be uniform within 5-10 psi across all cylinders. Leakdown differs from compression in that it shows whether or not the cylinder can maintain a proper seal. Compression is mechanical proof that each cylinder is making compression as the piston rises to the top of the bore with both valves closed. Leakdown is a test of cylinder pressure once compression happens and the fuel/air mixture ignites.
When compression and/or leakdown are poor, there's no point in tuning further until the fault is determined and repaired. Low compression and cylinder leakdown issues can be caused by burned exhaust valves, bad rings, piston damage, or a cracked casting.
Compression should be checked with the engine hot and again after it cools. Always check compression with the throttle wide open for maximum airflow into the cylinder. Compression should be uniform, within 5-10 psi across all bores. Usually low compression on one cylinder means a burned exhaust valve, broken piston rings, a damaged piston, or a blown head gasket. Low readings on two adjacent bores indicate a blown head gasket between the two cylinders. | 
There are a couple ways to perform a cylinder-leakdown test. Jon's people do it the old-fashioned way, which works quite well. Compressed air is administered to each cylinder at compression stroke with both valves closed. If the piston moves to bottom dead center and pressure holds, cylinder sealing is satisfactory. If noise can be heard through the intake or at the exhaust, valve-to-seat sealing may be unacceptable. If noise is heard through the oil filler tube, piston-ringntoncylinder-wall sealing may be poor. |
Main metering jets control the amount of fuel that flows to the boosters. The larger the jet (inside diameter), the richer the mixture. This is a Barry Grant metering block, which looks just like the Holley block. | 
Jets that are countersunk like the one on the left are preferred because the countersink reduces fuel-flow turbulence through the jet. The jet size is stamped into the jet. For example, 54 or 54F means .054-inch inside diameter. |
This is an Autolite 4100 with the air horn removed. Note the two float bowls, one for the primary throttle bores (left) and one for secondaries (right). The primary bowl contains main metering jets. Instead of being screwed into the main metering block like Holley and Barry Grant carburetors, the jets are screwed into the bottom of the fuel bowl. You can swap jets on an Autolite 2100 or 4100 without spilling a drop of fuel. | 
Float height is critical to proper fuel distribution, and each carburetor manufacturer has specifications to follow. Carburetor tuning includes a check of float height. If it's too low, the engine can suffer a lean mixture and stall out. If it's too high, you will experience flooding or an overly rich mixture. |