The GM HEI (High Energy Ignition) electronic distributors have a reputation for hassle-free performance. Used in GM rigs from its inception in 1974, the famed HEI can be retrofitted to almost any GM engine for a dramatic improvement over a point-style distributor. However, their moving parts do wear over time, and additional power can literally be locked under the cap-even in fresh, factory-spec HEIs.
To explain this in simplistic terms, an engine's piston travels faster than the spark, so, the spark needs a head-start (or advance) to arrive at the proper time for optimal combustion. Inside the head, the rate that the flame moves across the combustion chamber is affected by compression ratio, turbulence of the air/fuel mixture, spark-plug indexing, design of the combustion chamber, and other factors. A killer spark is usually needed for optimum combustion and maximum power.
In engines with an HEI distributor, variable ignition timing is controlled by engine vacuum advance and centrifugal advance. Vacuum is usually consistent in vehicles that have safe-and-sane cam profiles, whereas lumpy cams transfer the bulk of the timing burden to the centrifugal weights. In other words, all of this means that optimizing spark advance is often trial and error.
Professional distributor curving is now almost a lost art. But four-wheelers who like to tinker can tune their HEIs at home-or even in the boonies as part of trail-testing.
Inside an HEI, replacement springs and weights are available to control the centrifugal advance, and adjustable vacuum-advance canisters allow part-throttle spark timing to be tuned. Proform, a company that introduced the HEI Tune-Up Kit (PN 66945) some years ago, includes various weights and springs in the aforementioned kit; these items are also available la carte in an HEI Advance Curve Kit (PN 66948C) for people who don't need a cap, pickup coil, module, rotor, condenser, capacitor, wiring harness, and adjustable vacuum advance (available separately as PN 66952C).
Before pulling out your distributor, first make a mark on the valve cover that points to the vacuum advance. Pull the vacuum hose, unclip the wiring harness from the cap and the vehicle, then release the cap's hold-down clamps and remove the cap. | 
Use a distributor wrench to unbolt the hold-down bracket, and save the bracket unless you're using a replacement distributor kit that includes a new one. Wiggle out the distributor, noting the position of the rotor as it rotates. Reinstalling the distributor with the same orientation keeps the ignition timing in the ballpark. | 
The new Proform billet distributor is custom-curved at the factory with medium-tension springs to maximize torque (PN 66997). After applying moly-based grease to its gear and installing/lubing the gasket, the distributor's rotor and vacuum advance are aligned as before. If done correctly, the vacuum advance unit and rotor should be in the same position as the original unit. |
Snug the distributor into place with the hold-down (the Proform kit includes a billet plate), and plug the wiring harness into the cap. The plug is a one-way deal so it can't be incorrectly installed, but both plugs must be installed to function properly. | 
Secure the cap to the distributor with the lock latches, and then transfer the spark-plug wires (which are usually numbered) one at a time from the old cap to the new one. This makes sure you don't install the wrong wires on the terminals. If you do, get out your shop manual and start at the beginning. | 
Referring to a shop manual is necessary for the initial timing specs. Check the timing with the vacuum advance disconnected and all vacuum lines connected or plugged to prevent leaks. If timing is beyond the range of the timing marks, then the distributor gear is likely at least one tooth off from its original location. |