What's the old wheeze? "You can tune a Camaro, but you can't tuna fish." We're not much into deep-sea fishing, but we did manage to catch this little Camaro and put the tweak to it to make it run better.
This story is as timely and timeless as it gets. It's played out almost every day, with every kind of street machine in any garage in the country. In our case, the motor-vation is a 0.060-inch bored 283, with 292ci sitting in a little '67 Camaro in the garage. The engine had just been rebuilt with 9:1 compression, a Comp Cams Xtreme 250 hydraulic flat tappet cam, and mildly ported ancient iron heads. The engine ran OK, but that's about all you could say for it.
The players in this little game were a point-type distributor that had been long ago converted to a Stinger electronic distributor using a magnetic trigger pickup. We had new spark plugs, an Edelbrock Performer intake, and a 600-cfm Edelbrock Performer carburetor. The carburetor had been sitting around for a few years, and we thought it might need a rebuild since the engine ran a little richer than it should have.
Our plan was to apply some simple tune-up ideas to this engine in order to improve its part-throttle performance, sharpen its throttle response, increase fuel mileage, and reduce its emissions-ideas applicable to any engine from an AMC to a Lincoln Zephyr.
Our initial testdrive resulted in serious detonation at anything more than half throttle, and the engine fell right on its face. The car felt sluggish and smelled rich at part throttle. We had our work cut out for us.
The first thing we did was put a timing light on the engine to check the ignition curve. We discovered the engine had 23 degrees of initial timing at idle along with an astounding amount of vacuum advance that pushed the total timing at 3,000 rpm to almost 60 degrees! Clearly, that wouldn't work, so the quick fix involved disconnecting the vacuum advance and setting the initial timing at 15 degrees before top dead center (BTDC) with a total of 36 degrees.
We also pulled the distributor cap and rotor to inspect the advance weights and springs to make sure the ignition curve was working properly. The ignition curve was quick with mechanical advance all in by 2,800 rpm, so we left that alone. Before we went any further, we decided to establish an idle emissions baseline.
Most enthusiasts don't consider a smog test machine a tuning tool. But when we ran across an old Sun emissions test machine at our local swap meet for $100, we had to have it. After having it refurbished, the machine now can read out four different exhaust gases at idle, but the two we're most concerned with are hydrocarbons (HC) and carbon monoxide (CO). These two exhaust components can help the educated carb-tuner to get the most out of the existing carburetor. HC is reported in parts per million (PPM) and represents the level of unburned fuel in the exhaust. CO is the dangerous gas that is the result of mixing carbon and oxygen together during combustion. In emission testing, CO is expressed as a percentage. The idea is to adjust the idle mixture until we have the lowest combination of HC and CO. What makes the CO percentage more interesting is that we can estimate idle air/fuel ratio based on CO percentage. The accompanying chart can be helpful in estimating air/fuel ratio at idle.
Our initial test with the little-inch Mouse motor showed that the HC was slightly rich at around 800 parts per million (PPM) while the carbon monoxide was hovering around 2.5 percent, which represents an air/fuel ratio of 13.5:1-that's not too bad. These were somewhat rich numbers and indicated that simply adjusting the idle mixture screws would help our cause. One other point worth mentioning is that these numbers were generated with the engine temperature at just above 170 degrees F. As engine temperature kept rising, the HC readout did diminish slightly.
The idle mixture screws on the Edelbrock Performer carburetor are easy to access right in the front of the carburetor. The first thing we did was balance the two adjusting screws so they were turned out the same number of turns from fully seated. Then, as we adjusted the idle mixture, we made sure to keep the two adjustments consistent. After a few adjustments, we had cut the idle emissions down to 630 PPM and around 1 percent CO.
We felt that this engine could idle much cleaner, so we added a Pertronix Second Strike unit and a distributor to create a hotter fire under this small-block. After wiring the box in place, we fired the Camaro up, and after the engine warmed up to 190 degrees F, we noticed that the HC and CO emissions jumped radically. In fact, for a while, the HC skyrocketed up to over 1,000 ppm! This didn't seem right, so we decided to take the Camaro for a quick testdrive. After a 20-minute boogie where we took the Camaro out on a nearby freeway, a retest of the idle emissions now revealed much lower HC and CO readings.
We talked to our pal Norm Brandis of Westech Automotive in Silver Lake, Wisconsin. He has extensive experience with emissions testing and performance engines, and he told us that it's common to see an initial spike in the HC emissions after installing a strong ignition system. Brandis says the level will stay high until the ignition has completely burned away the top layer of carbon on the pistons. It's this carbon layer that spikes the HC levels. We now had reduced the HC down to between 410 and 500 PPM and a CO reading of 0.19 percent. Using our CO conversion table, this placed the little small-block at roughly 14.5:1 air/fuel ratio.
Next, we welded in an oxygen sensor bung in the collector of the driver-side header so we could use the Innovate Motorsports air/fuel ratio meter. We plugged the meter into the cigarette lighter for power and went for a part-throttle cruise to measure the air/fuel ratio. The carburetor we were using is the leanest of the Edelbrock carbs, and a quick check of the calibration made it appear that very few changes will have to be made.
One of the advantages of the Edelbrock carburetor is that this is a metering rod style carburetor, much like the system used in a Q-jet. In addition to the primary jets, there are also two primary metering rods that fit inside the flow orifice of the jets. At light throttle, these rods are buried deeply into the jets, reducing the flow area created by the jet, in essence making the jet flow area very small. As load increases, intake manifold vacuum drops off and the spring under each metering rod pushes up on a piston connected to the metering rods. The power tips of the metering rods are smaller, allowing more fuel to flow through the fixed orifice jet, richening the mixture. It's a simple system that works well, especially for part throttle tuning.
This 600-cfm Edelbrock carb is the leanest of the carburetors and is set up with a number 98 primary and a 95 secondary jet. The primary jet also has a metering rod with a 0.075-inch major diameter with a power tip that measures 0.047 inch. We experimented with leaning the stock 98 primary jet down to a 95 jet with the same primary metering rod.
With all these changes, we were able to improve the part-throttle cruise and fuel mileage, reduce the exhaust emissions, all while improving the throttle response and driveability. Not bad for a couple of afternoons' worth of tuning.
|Parts List |
|Component ||Manufacturer ||PN |
|600-cfm Performer carb, |
|elec. choke ||Edelbrock ||1406 |
|Electric choke kit ||Edelbrock ||1478 |
|Calibration kit for 1406 ||Edelbrock ||1479 |
|Rebuild kit for 1406 ||Edelbrock ||1477 |
|Air cleaner, 14-inch to clear carb ||Edelbrock ||1221 |
|Accelerator pump kit ||Edelbrock ||1475 |
|Billet distributor ||Pertronix ||D100710 |
|Second Strike box ||Pertronix ||500 |
|Air/fuel ratio meter ||Innovate ||LM-1 |
|Shroud, '67 Camaro, no A/C ||Year One ||1132N |
Air/Fuel RatioThe following chart converts carbon monoxide (CO) exhaust gas percentages into equivalent air/fuel ratios.
|CO ||Air/Fuel |
|(Percent) ||Ratio |
|0.1 ||14.7 |
|0.5 ||14.3 |
|1.0 ||14.1 |
|1.5 ||13.9 |
|2.0 ||13.7 |
|2.5 ||13.5 |
|3.0 ||13.4 |
|3.5 ||13.2 |
|4.0 ||13.0 |
|4.5 ||12.8 |
|5.0 ||12.6 |
|5.5 ||12.4 |
|6.0 ||12.2 |
|6.5 ||12.0 |
|7.0 ||11.9 |
5 Jenner, Ste. 100
2700 California St.
440 E. Arrow Hwy.
PO Box 129