The Too Fat Follies

Street Tuning A Big-Cammed Small-Block

Jeff Smith Jul 1, 2002 0 Comment(s)

Step By Step

We started by pulling Swanson’s immaculate ’64 El Camino into Moore’s shop and connecting the HC/CO “sniffer” to read the exhaust emissions. This would give us an accurate baseline. The carb is a Holley 750-cfm vacuum-secondary piece with a Percy’s adjustable primary jet plate installed.

First we replaced the existing suppression spark plug wires with a set of Crane Firewire 8.5mm spark plug wires. The original suppression wires checked between 8,000 and 10,000 ohms of resistance. The Crane wires came in between 200 and 300 ohms. With lower resistance, more spark energy is delivered to the plugs, which makes the engine run more efficiently.

Next we tried turning the idle- mixture screws but discovered an extremely small range of adjustment. This indicated an overly large idle feed restrictor so it was time to pull the carb apart.

Before we repaired the carb, we also checked the timing with MSD’s new self-powered timing light, which has an extremely bright strobe that is very easy to see. We found 14 degrees initial timing the first time, but after a couple of revs, this jumped to over 34 degrees. Clearly there was something wrong inside the distributor.

We removed the cap and rotor and discovered that the tiny plastic bushings that fit between the mechanical-advance weights and the pins were wasted. Moore replaced the bushings, and once the distributor was back together, we reset the initial timing to 18 degrees with 36 degrees of total timing.

We found a piece of 0.017-inch electrical wire and cut two short lengths that we placed in the idle feed restrictor to reduce the overall area from 0.55 inch to 0.038 inch. We hoped this would lean out the idle circuit and reduce the HC emissions.

The smaller idle feed restrictors dropped the HC level, but a quick testdrive revealed a now-worse off-idle stumble. We pulled the carb and found much of the transfer slot uncovered.

A cam with a ton of overlap will require a larger throttle-blade opening where a large portion of the idle transfer slot (arrow) is uncovered. This creates an overly rich idle mixture and an off-idle stumble.

After testing with another carburetor, we drilled the two primary throttle blades with a 3/32-inch drill bit. This bleeds more air into the manifold, allowing us to close the throttle blades so that only a small portion of the idle transfer slot is uncovered (arrow 1). Arrow 2 points to the curb idle discharge hole.

After resetting the idle mixture and idle speed, the next test drive revealed improved throttle response, but the engine still hesitated on light acceleration off-idle. We readjusted the accelerator pump arm and the El Camino instantly responded with its best throttle response to date.

1A clear indicator that the motor suffers from massive overlap is the black carbon that has coated the venturis of the carburetor. The carbon is the exhaust soot reversion present in the intake manifold at low engine speeds that contaminates the incoming charge. This is why the engine requires so much fuel at idle.

Where did the notion come from that adding more fuel to an engine makes more horsepower? It’s one of those automotive myths that clings to life even in the new millennium. It seems just about every hot street car on the planet runs too rich and could use a sharp tune-up. The latest victim in our quest to rid the world of pig-rich hot rods was our pal Don Swanson’s ’64 small-block El Camino.

A few months ago, CHP contributor Tim Moore put the tune-up on Kevin Doyle’s mild 350-powered Camaro (“Attention to Detail,” Dec. ’01). After we doubled the Camaro’s fuel mileage while sharpening both the throttle response and tailpipe emissions, it didn’t take long for his pal Don Swanson to ask us to do the same with his car. Swanson’s El Camino presented a much greater challenge since the small-block sported a lumpy Isky 280/292 flat-tappet hydraulic cam that idled in neutral with barely 8 inches of manifold vacuum. Compounding the problem was a tight converter that pulled the engine down to under 6 inches of vacuum in gear. The rest of the engine included ported iron heads, headers, an Edelbrock Torker single-plane intake manifold, and a modified Holley PN 80508 750-cfm vacuum-secondary carburetor.

Swanson complained that the small-block ran rich, fouled plugs, and had a serious off-idle stumble that would sometimes cause the engine to stumble under light acceleration from a stop light. Swanson brought the El Camino over to Moore’s shop, and we went to work. The first thing that Moore did was check the idle emissions with his Sun HC/CO meter to establish a baseline. The Sun machine tests four gases (HC, CO, CO2, and O2) but we were most concerned with hydrocarbons (HC) and carbon monoxide (CO). With the engine at operating temperature, we measured a ridiculously rich HC idle of 1,500 parts per million (ppm) and a CO of 2.4 percent. CO percentage can be used to indicate air/fuel ratio. The 2.4 percent figure meant the engine was running at a reasonable 13.5:1 air/fuel ratio. However, the HC level was especially high. Our goal was to reduce the HC and CO to help the small-block run leaner and crisper. We decided to shoot for reducing the HC down to around 500-600 ppm, which would be a decent idle for a cam this large.

First off, Moore noticed that Swanson’s motor was equipped with suppression-type plug wires. While almost brand-new, the wires measured 10,000 ohms of resistance. This is a little high, so we replaced the suppression wires with a set of Crane spiral-core FireWires that measured less than 500 ohms total. Moore then fired the engine back up and checked the timing. Using a new MSD self-powered timing light, the engine showed 14 degrees initial.

Moore also checked the idle mixture screws and found the passenger-side screw was turned out only 1/8 turn while the driver-side screw was over 1 turn out. Moore balanced the idle-mixture screws at ½ turn each. If the idle-mixture screws were turned out (richer) any more, the idle HC emissions went dead rich. This indicated that the idle feed restrictors in the carburetor were too large, allowing too little idle mixture screw adjustment.

But before he could tackle that problem, we noticed that after revving the engine a couple of times, the initial timing was stuck at 34 degrees instead of the original 14. We pulled the HEI distributor cap and discovered the small plastic bushings that fit over the mechanical weight pivot pins had failed, allowing the mechanical advance weights to stick. Moore offered two new replacement bushings, and he cleaned and lubricated the weights and reassembled the distributor.

With the timing stabilized, he could now address the rich idle mixture. After pulling the carburetor and removing the primary metering block, we discovered that the idle feed restrictor for the carburetor had been drilled larger. The standard idle feed restrictor size for most 750 cfm Holley carburetors is around 0.034-0.035 inch. The restrictors in this carb had been drilled to a larger 0.055 inch!

While this sounded bad, a typical big camshaft often requires larger idle feed restrictors because the increased overlap requires more fuel to keep the engine running. But we also knew that the idle-mixture screws offered almost no adjustment range. We used some 0.017-inch diameter electrical wire strands and placed one inside each idle feed restrictor in the metering block. This reduced the area of the restrictor, which leaned the idle circuit.

We reinstalled the carb and allowed the motor to warm up. Moore’s first step was to readjust the idle-mixture screws, which now had a much greater range of adjustment than before. However, a test drive revealed an off-idle hesitation that, according to Swanson, had always been there but was now much worse. He again removed the carburetor and discovered the next common problem with big cams. Because of the long overlap and reduced idle vacuum, the throttle blades had to be opened more to allow enough air in for the engine to idle properly.

This larger throttle-blade opening uncovers a large portion of the idle-transfer slot (see photo 9), which draws fuel from the slot at idle. When the throttle is opened more, there is insufficient fuel to compensate for the additional air entering the engine. All of this occurs before the main circuit tips in fuel from the boosters. The result is an aggravating off-idle stumble. The cure required pulling off the carburetor again.

We removed the baseplate from the carburetor and drilled two 3/32-inch holes in the leading edge side of the primary throttle blades. After cleaning all the metal chips from the baseplate, we reinstalled the carburetor for another testdrive. Drilling the holes allowed us to slightly close the throttle blades, covering most of the idle transfer slot. A quick test drive revealed that the off-idle stumble had changed characteristics, but was still there! We were frustrated.

By paying close attention to how the engine reacted, we figured out what was happening. If Swanson eased the throttle open, the carb worked fine. But if he hit part-throttle quickly, the engine would hesitate and sometimes die completely. This wasn’t a full-throttle stab, but more like a 1/8-throttle blip. After a few trips around the block, we returned to the shop and discovered clearance between the accelerator pump linkage and the pump arm at idle. This allowed the throttle to move roughly 10 percent before the accelerator pump squirted fuel into the primaries. The clue was that Swanson’s engine would accelerate just fine when the throttle was opened gently. We adjusted the accelerator-pump arm to move in unison with the linkage and solved the problem. Our next testdrive was much more rewarding—the engine responded to throttle input crisply and without hesitation. Swanson was thrilled.

Finally, we again plugged the El Camino into Moore’s HC/CO machine and recorded the numbers. We didn’t gain a tremendous reduction in HC/CO like we planned because the big cam just wouldn’t allow the engine to idle with a leaner air/fuel ratio. The best numbers we were able to achieve without hurting driveability was a rather high 1,008 ppm HC and a CO of 1.08 percent. What we did achieve was increased efficiency in part-throttle operation and a crisper throttle that has made Swanson’s El Camino a joy to drive. And that made all the effort worthwhile.

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