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.