New-car makers haven't rolled a carbureted car off the line since the '80s, but it's plainly obvious that carbs have quite a lead on fuel injection as hot rodders' induction of choice. However, we'll bet that more often than not, parked out in the driveway behind your garaged, pampered, carbureted musclecar, is a late-model, fuel-injected daily driver--the reliable, practical transportation that gets groceries and shuffles the younguns to Little League.
Living with a carbureted daily driver isn't difficult--heck, 20 years ago, we didn't have a choice. As daily drivers, though, maintenance isn't "fun" and hence, we expect they'll run forever on gas and oil changes. Such was the situation with Greg Smith, art director of our sister publication Four Wheeler, and his commuter '67 Mustang. Its mild, Holley-carb'd 302 has cruised him back and forth to work for the past three years, but lately its neglected fuel mixer had been acting up. Hesitation, stumbling, and poor throttle response had become annoying reminders of long-needed maintenance, so we decided to help out a fellow car guy and enlisted the expert aid of TMP Carbs' Troy Patterson. TMP deals mainly in the science of high-performance carburetion. We're not throwing the term "science" around lightly, either. He's developed some seriously trick carburetors for a variety of applications. Patterson's experience helped identify and remedy the issues within Smith's ailing Holley.
We focused on easy mods for better driveability, but a good-driving carb also rewards you with better performance and fuel economy. So follow along; your carb deserves it.
Know Your Enemy: The test drive
No amount of explanation will substitute for a test drive to diagnose driveability problems. Patterson took the 'Stang for a spin and reported off-idle stumbling, backfiring through the carb after making right-hand turns, and generally poor throttle response (that's probably more than Smith would have admitted). Now we had a baseline "feel" for the tune.
Follow the Lead: Setting Ignition Timing
Get the timing set before tweaking the carb. Most distributors have combined vacuum-advance and mechanical-advance systems, but setting the base timing requires disconnecting the vacuum advance and plugging off its vacuum supply. The rate of increasing mechanical advance is as important on a street car as the total number of degrees when it's maxed out. Depending on the weights and springs used in the mechanical advance built into the distributor, you could see total advance at 1,000 rpm (ultra-weak springs) or as high as 5,000 rpm (ultra-stiff springs).
If you don't have a dial-back timing light, applying degreed timing tape to a stock balancer lets you read degrees of advance well into the 50s. We had all 31 degrees of advance in by 2,200 rpm, which gave a surge of low-end power but killed the midrange. Patterson replaced the weak springs with heavier ones from a recurve kit, which delayed the full 31 degrees of advance 'til 3,200 rpm. Another test drive showed more consistent power through the entire rpm range.
Mixing It Up: Setting Idle Mixture
The Mustang's single-feed 600-cfm Holley has two mixture screws for adjusting idle richness. Patterson turns one screw to lean out the mixture (clockwise) until engine rpm drops. Then he backs out the screw (counter-clockwise) to richen the mixture until the maximum idle speed is reached. A handheld tach or vacuum gauge helps for this (adjust for the highest rpm or vacuum then reset the idle speed). Repeat on the other screw, of course. Both screws should be adjusted nearly the same when you're done.
Patterson follows up with a lesser-known procedure--checking the off-idle circuit. This is what "bridges the gap" between the idle and main fuel circuits for a smooth power transition, also known as tip-in. Crank up the idle speed screw and verify that the engine rpm increases proportionally. When you see fuel dribbling from the boosters, back the idle speed down until they stop dripping fuel, and you're smack-dab into the off-idle circuit. Turning the mixture screws clockwise or leaning the mixture should reduce engine rpm, and richening the screws should increase rpm. Our engine rpm didn't increase when we tried richening the mixture because our idle-feed restrictions were drilled too small. Enlarging them, in our opinion, is carb guru territory.
Ever try adjusting the idle mixture screws, but find they don't seem to be doing a damn thing? Usually it's because your throttle blades are open too far--and here's why. As the throttle blades are cracked open, they expose the small transfer slots to a vacuum signal. The air/fuel mixture is drawn from the slots and helps smooth the transition off idle to the main fuel circuit. Having the throttle blades open too far at idle exposes more of the transfer slots' area to vacuum, and the added fuel that's drawn out literally drowns out the idle circuit. Patterson likes to see just 0.040-0.050 inch of the transfer slot showing under the throttle plates at idle.
Tuning for Power
An earlier test drive revealed a disproportionate surge of power at low rpm, so Patterson figured the power valve was opening too soon and dumping excess fuel in before the mild-cammed motor needed it. He hooked up a vacuum gauge to full manifold vacuum, gunned the motor a couple of times in neutral, and found a hesitation right around 6-inches of mercury (in Hg). Removing the primary bowl and metering block revealed a 6.5–in Hg power valve, which Patterson promptly swapped for one that opened at a 3.5–in Hg vacuum signal. He noted that correct power-valve sizing should help throttle response at tip-in.
But one thing often leads to another, and installing the new, later-opening power valve meant we'd lost low-speed enrichment and developed an off-idle hesitation. Off came the metering block once again, and the stock 65 jets were swapped for larger 66s. Problem solved.
The next test drive was the best yet. We had better tip-in, the hesitation was gone, and the car felt more powerful all around. Patterson thought that speeding up the slow-acting vacuum secondaries would yield even more improvement, so off came the vacuum actuator assembly. The standard stiff black spring was replaced with a lower-rate yellow coil to open the secondaries a little quicker. Patterson also drilled the vacuum bleed hole in the actuator housing to 0.065 inch for faster reaction when the pedal is floored.
Before the next test drive, we remembered the car's tendency to lean-backfire around right turns. The likely culprit was a low float level, starving the engine when the fuel sloshed around the bowl when turning hard. With the engine idling, Patterson unscrewed the sight plug, raised the float level slightly until fuel just touched the bottom of the sight plug threads, and called it good. The last test drive was the best, as we'd hoped. The 302 felt as strong as it did when it was new--well, as strong as a stock, low-compression small-block ever felt.
