Priming the oil system prior to initial start-up eliminates the threat of oil starvation. Johnson used a priming tool and drill motor to spin the oil pump clockwise until the oil pressure gauge showed 65 psi. During break-in, be sure to use mineral-based oil rated for off-road use, meaning it has zinc, an EPA-regulated ingredient absent from most highway-rated passenger-car oil formulations. Despite costing 40 percent more than most other premium motor oils, JMS uses Valvoline VR1 racing oil, and while thin viscosities yield more impressive dyno numbers, we went with 20W50 for maximum protection on this fresh build. A pint of GM Engine Oil Supplement is also good antifriction/antiwear insurance at every oil change, especially with solid flat-tappet cams like ours. Avoid synthetic oil until after break-in; it's too slippery and the rings may not seat properly.
The 15-minute cam break-in phase went without incident, but during the first test attempts in Test 1, the 383 had a nasty-sounding misfire that came on and wouldn't quit between 4,200 and 5,500 rpm. In vain, Jeff (foreground) and Don Johnson checked everything from ignition timing to the dyno's fuel pressure setting for the cause before focusing attention on the carburetor, an 800-cfm Edelbrock 1412 EPS 800. We thought maybe there was some isolated flaw with our specific unit, so another box-stock EPS 800 was inspected and installed. Even with the new carb, the 383 stumbled as bad as before! Next a box-stock 750-cfm Edelbrock Performer 1407 was installed, and viola, the misfire cleared up, and the best of several pulls delivered 379 hp at 5,400 rpm and 405 lb-ft at 3,700 rpm. A good result, but the earlier stumbling episode haunted us. Was it caused by a case of overcarburetion with the big 800? Not so fast...
Though the traditional carb-sizing formula-cubic-inch displacement (383) times maximum rpm (6,500), divided by 3,456-would dictate the use of a 720-cfm carburetor, everybody at JMS agreed the "too big" 800-cfm rating was not the culprit. Rather, the attention focused on the design of the primary discharge nozzle used on the 1412 EPS800 carb. The pen points out how the nozzle is capped at the end and opens downward facing the primary throttle blade, making it susceptible to the effects of fuel reversion. Fuel reversion is a natural occurrence in which the column of fast-moving intake air reverses direction and stagnates or actually exits the intake tract with positive force moving outward past the carburetor inlet opening. Every engine experiences reversion at some point in the rpm curve, and most run right through it with no perceived ill effects. But once reversion takes hold, the normal vacuum signal becomes a pressure front that can enter the discharge nozzles and interrupt the flow of liquid gasoline from the bowls, through the emulsion tubes, and into the booster venturi. Why did the misfire symptoms vanish once the 750-cfm 1407 carburetor was installed? Because, it seems, the Edelbrock 750 is fitted with discharge nozzles (as are its 500 and 600-cfm siblings) that are not as susceptible to the effect of reversion. These nozzles terminate horizontally (left), so pressurized reversion is not as likely to enter the primary circuit and interrupt the crucial vacuum signal. We pondered the situation at length and this is the only logical conclusion we drew. So with the EPS800 on the shelf, all tests were made with a 750-cfm Edelbrock Performer 1407 in place.