Why do we tune classic cars?
One major difference between the modern cars and classics is how they mix the fuel and air before setting fire to it! Most older cars have distributors and carburettors instead of engine management and fuel injection.
By enlarge the latter are tuned from the factory and stay ‘in spec’ for the rest of their lives unless a component breaks down. The mechanical system on a classic has parts that ware out regularly such as points and rotor arms or components that react to heat and use, such as the inner workings of a carburettor.
It is also common for these parts to seize or gum up when left for long periods. Basically ‘a tune up’ is part of the regular maintenance your classic car needs to perform at its best.
What do you do then?
Before we start to tune an engine there are some basic checks:
- Timing – essentially when the distributor send the spark to ignite the fuel and air
- The ignition components are in good order
- That the carb is set at ‘base settings’ and working correctly.
- There is sufficient fuel pressure
Each car has a setting, generally read off the front pulley with the use of a strobe light. Noted in degrees ‘before top dead centre’ it is the point in the stroke of the piston that the spark occurs. Get it too early and the fuel is not compressed enough and too late and the bang comes too late as the piston is already on its way back down. We adjust the timing by slackening off the distributor and twisting it clockwise or anti clockwise to get the desired number.
All the fiddling in the world won’t help if the spark plugs are furred up or the points are burnt out, so making sure these are in good condition is very important.
Stale fuel leaves behind a sticky residue which attracts partials and gums up the inner workings. The insides of carbs are precision engineered. Imagine the workings of a mechanical clock. Everything needs to work at its best otherwise the clock will run slow. We use a special cleaning spray and check that the needles and floats inside are moving correctly giving us the ability to choose what the fuel and air mixture is at different throttle openings.
Fuel pumps degrade over the years and lines get clogged up or damaged causing a constriction. If the car is not getting enough fuel up to the carburettor it will run out of power as the revs rise. This is one of the most common complaints we see at Project Shop.
Tune me up baby!
Once all of these are checked it is time to actually tune the engine. What we are doing is a balancing act to get the engine running as best as possible.
If the car has more than one carburettor it important to make sure they are both doing the same thing. We check that the mixture screws are set at the same amount by screwing them all the way down and then coming out a couple of turns before using a balancing gauge to ‘fine tune’ each one. They work by showing the amount of air sucked in as a value on the gauge.
We use a CO meter to check the exhaust content. Too rich and the engine will not burn all the fuel, making it inefficient and too lean and the engine will run hot and be down on power. Both can cause premature engine ware.
We fine tune the timing, advancing or retarding it with in a few degrees to give the best performance on the road. Too far retarded and the engine will be slow to pick up and feel lethargic, too far advanced and the engine will ‘pink’ or ‘detonate’. This sounds like someone is knocking on the engine with a hammer. Neither are good for the engine or performance.
Its all well and good doing this in the workshop, but the real test is on the road. You would not believe the difference a yearly ‘tune up’ can make to the performance and economy of your car. Once you have experienced it, you will know when it is time to pop back in!
Cost and time
If the components are all in good condition a tune up usually takes a little over an hour, so for the cost of a full tank of fuel you will be getting the very best from your engine.