Engine Tuning Guide - Carburetor Track Tuning For Speed

Greg's '55 is no slouch on the starting line. It yanks the left front wheel a solid 4 inches on almost every launch, yet the car is very streetable. All we had to do was dial in the carb with some subtle changes to make it run even better.

Greg's '55 is no slouch on the starting line. It yanks the left front wheel a solid 4 inch

There's a misguided perception in car crafting that all you have to do is bolt a few parts on your engine to instantly catapult it to its pinnacle of power. That myth rates right up there with Vice President Richard Cheney and expert marksmanship. The brutal truth is that bolting on parts is merely the first step.

You might remember Greg Smith from the story, "Fuel Delivery" (Dec. '05) where we solved some pump-to-carb routing problems for him. Greg's goal was to push his all-steel '55 into the 10s without resorting to the easy cheat of squeeze. His previous combination was an incredibly ancient, iron-headed 350ci LT-1 motor fitted with a 150hp shot of nitrous. It was fun for a few years until he grew tired of hearing, "Anybody can run fast on nitrous."

So Greg stepped up for a Scott Shafiroff-built 434ci small-block. This still-streetable thumper sports the large 227cc, 23-degree AFR heads, a huge Comp Cams roller with 264/269 degrees of duration at 0.050, and 0.631 inch of valve lift. All this is combined with an Edelbrock Super Victor intake and an equally manly Holley 950 HP carburetor that dyno'd at 618 hp at 6,500 rpm. Now it was time to see what Greg's thundering small-block could do in his '55 with a Turbo 400, a 10-inch Continental converter, and a 4.10-geared 9-inch.

Part-Throttle Tuning
Things became troublesome after Greg nestled the 434 into the engine bay. The first pass down LACR's high desert quarter-mile was less than impressive with an observed 12.10 at 112 mph. That was slower than his previous best with the little 350 motor on nitrous. The first step was to upgrade the fuel-delivery system to an Aeromotive fuel pump and regulator, which included building a return line back to the tank.

We tried increasing the idle air-bleed size before messing with the idle-feed restrictors, mainly because they're easier to access, right on top of the carb. The outboard bleeds are for idle air. Try making changes of 0.003 to 0.004 inch at a time.

We tried increasing the idle air-bleed size before messing with the idle-feed restrictors,

Next Greg blew a hole in the driver-side exhaust pipe, installed one of Innovate Motorsports' O2 sensors, and began data-logging the big motor's air/fuel ratio. It quickly became apparent that the Holley 950 HP carburetor was certainly on the fat side. On a sunny day in the San Fernando Valley north of Los Angeles, the light throttle cruise air/fuel (A/F) ratios were between 10.4:1 and 12:1. Wide-open throttle (WOT) A/F hovered around 11.6:1. In theory, best WOT power is achieved from around 12.5 to 12.9:1, while part-throttle should be much closer to 13.5:1 or leaner. It was obvious the carb needed attention.

The first step was to readjust the idle-mixture screws. Gentle tweaks on all four idle-mixture screws improved the idle A/F taking it from 11.6:1 to a much more efficient 12.6:1 in Neutral and an even better 13.3:1 in gear with idle vacuum at 8 inches. We also discovered that even the tiniest change in the idle-mixture screws brought about huge changes in the A/F, which told us we needed to lean out the idle (fuel) feed restrictor. The huge idle-mixture screw swings in A/F indicated the idle-feed restrictors in the metering blocks for the 950 carb were too big for this engine. This left us with two options. The first was to reduce the size of the idle-feed restrictor from its original 0.036 inch to more like 0.034 inch. While it's not much of a diameter change, it's a significant decrease in area, thereby reducing the amount of fuel to the idle circuit, which creates a wider range of adjustment for the idle-mixture screw. This makes it easier to fine-tune the idle-mixture, which also has a huge effect on part-throttle mixture since at light-throttle opening, the engine pulls fuel from the idle circuit, not the main jet circuit.

If you need to lean the idle-mixture more than the air-bleed can help, this tiny orifice called the idle-feed restrictor in the metering block is where to look. It's shown here with a drill bit inserted to show the restriction. A quick way to lean out the idle feed is to place a length of very small wire-about 0.015-inch diameter-in the restrictor with a J-hook at the end to keep the wire in place. By using different thicknesses of wire in the restrictor, you can get dialed in without making permanent changes to the idle restrictor.
If you need to lean the idle-mixture more than the air-bleed can help, this tiny orifice c
Many Holley HP carbs move the idle-feed restrictor up to this location in the metering block (arrow). Changes to this bleed are the same, it just moved to a different spot. This is a Holley billet metering block used in the 950 HP Ultra carb, but the cast block used in Greg's HP has the idle-feed restrictor in the same location.
Many Holley HP carbs move the idle-feed restrictor up to this location in the metering blo
Float level also affects the air/fuel ratio: higher is richer and lower is leaner. Float level is easy to change by loosening the screwdriver lock and moving the 5/8-inch hex nut. Clockwise lowers the float level while counterclockwise will raise the level. A minor adjustment will make a noticeable change, so work carefully. Keep the fuel level near the bottom of the sight plug.
Float level also affects the air/fuel ratio: higher is richer and lower is leaner. Float l