Just as you would tune a musical instrument, old cars also need to be tuned. This is a bit of an art and not everyone is good at it. Tuning an old car is a delicate balancing act. You are dealing with systems which are entirely mechanical and not capable of adjusting and correcting themselves like modern fuel injection.
An engine runs best when it’s air/fuel mixture is within certain limits. Stray outside these limits and the engine may misfire, hesitate, or be low on power. The goal of carburetor tuning is to keep the fuel mixture within these limits under all driving conditions. This results in maximum power and best drivability.
This is easier said than done. Anyone can make a car run well on the day they tune it, but what happens when atmospheric conditions change? Tomorrow may be cooler than today – which results in denser air. The barometric pressure may be different. Humidity may be higher or lower. Any one of these things could throw the air/fuel mixture out of limits.
Therefore a carburetor must be tuned for ‘average’ conditions. Say you tune an engine and it is running on the rich side, but it runs well. What happens on a day where it is hotter and the air is less dense? The fuel mixture could be much richer. Now the car that ran well stalls at stop lights, is down on power, and chuffs black smoke.
So how do you tune for average conditions? You can’t do it by using your senses that’s for sure. You may be able to tell an engine is running rich – but not how rich. Many old timers (and young whipper snappers alike) believe they can ‘tune by ear’. I say this is a poor excuse for not having, or not knowing how to use, the proper equipment.
You need the proper equipment and you need to know how to use it. For tuning fuel mixture, your best friend will be the gas analyzer. A gas analyzer is the same thing the use when they stick a sniffer in your tailpipe for a smog inspection. Its premise is simple – what goes in, must come out.
Your engine ingests a mixture of gasoline and air. The gasoline is made of hydrocarbons and the air is made up of mostly oxygen and nitrogen. The ratio of these is what comprises an air/fuel ratio. As the ratio of hydrocarbon and air being drawn in changes, the stuff coming out the tailpipe changes too. We can read these changes with the gas analyzer and have a better understanding of what is going on in the cylinder.
In my next post, I will discuss what these gases are and how you can use them to tune your engine to run well under all conditions.