I want to “TUNE UP” my 1961 Corvette, and I want to do it myself. The only problem is; I don’t know exactly what a “TUNE UP “is. I have asked several people, and I always get a different answer. Some say that a tune up is just the replacement of the spark plugs and ignition points, others say it includes a carburetor rebuild, still others say it includes a valve grind. What is a TUNE UP???


The term ''tune up" has been used to mean a lot of different things over the years. For our purposes, we will include the following: Checking engine mechanical condition (compression or leak down testing)

Adjusting valves if required.

Installing new ignition parts as needed.

Checking and adjusting ignition and fuel system to factory specifications.

Inspecting and servicing the manifold heat control valve.

Checking and adjusting charging voltage and checking battery condition.

Checking drive belt or belts and adjusting or replacing as required.

Inspecting the cooling system, and servicing as required.

Visual inspection for leaks, or for hazards. Eg: belt rubbing against a hose or wire.

Checking all liquid levels, and changing fluids and filters wherever necessary.

Always remember that an engine that is not in good mechanical condition cannot be tuned. If the combustion chambers don't seal properly; that is, if the valves and rings and gaskets do not seal properly, the engine will not perform correctly. That is true for all engines, new and old. All too often an engine with a leaking head gasket, or a burned valve is tuned up to try to restore good performance, only to disappoint the owner with the same old problem after the effort has been made, and new parts installed. Any time a tune up is done; the compression should be checked to determine if the work will be satisfactory. After the engine is found to be in good shape, it is advised that a thorough tuning be performed to bring the engine back to its original power and economy levels. If the compression test reveals that there is a deficiency in the engines internal sealing, it is often necessary to do a cylinder leakage test. This test will tell where the leakage is located. That is, it will tell if there is a bad valve, a bad head gasket or worn piston rings, and which ones are bad. Basically, the way it works is that a fitting is installed in the spark plug hole which will allow for the shop air compressor to apply air pressure into the suspected cylinder while the piston is at top dead center with both valves closed.

The air pressure, being applied through a pressure regulator will leak out wherever the failure exists. The instrument will measure the percent of leakage. Normal leakage should be uniform, and it should be under approximately fifteen percent. If there is a significant leakage in one cylinder, it is possible to determine where the failure is located by listening as the air is being leaked out of the engine. If you hear a hissing noise over the carburetor or at the air inlet to the fuel injection unit, it will indicate a leaking intake valve. If you hear it at the exhaust pipe, it is likely an exhaust valve, and if you hear bubbling in the cooling system, it could be a leaking head gasket or a cracked cylinder head or engine block. If you hear air leaking out of the oil filler tube, or blow by tube, it may be defective piston rings, or a crack in the piston itself. The cylinder leak down test is a very useful way to determine what the problem is, where it is, and, therefore how to go about fixing it.

Another valuable test to know about is the CYLINDER BALANCE test. This test is used to find the failing cylinder when an engine starts "missing". (Running on less than all cylinders) Expensive shop testing equipment uses cylinder balance testing to determine which cylinder is failing. It is a very simple process: The engine speed is set at one thousand RPM. (This is done by simply turning the idle speed screw) As the engine is running, each cylinder is shorted out, one by one, causing the engine to loose RPM. The theory is that each cylinder is contributing to the set speed of the engine, and therefore, as you disable each cylinder, it will cause the speed to fall. When you come to the troubled cylinder, the set speed stays about the same when that cylinder is shorted, indicating that it isn't doing its part.

This is a very old but valuable method of determining which cylinder is bad, and it can be done using simple shorting probes that anyone can make up in his or her own shop. I use paper clips. I bend open one leg of the paper clip, so that the bent open leg is simply a straight wire extending away from the rest of the paper clip about one and one quarter inch. I make eight of these clips. I then insert the straight wire part of the paper clips down into the distributor cap towers, one next to each spark plug wire, to make an electrical contact with each spark plug wire terminal. I arrange these wires to point straight up in the air, and not contact one another or any thing else. I then make up a grounding probe which is simply a short length of primary wire with an alligator clip on each end. (Primary wire is any small gauge wire like the wire used to serve the lights, horns, or any accessories on the car) I attach one end of the wire to a good ground location anywhere in the engine compartment, and attach the other end to the blade of a screw driver with a plastic handle. The plastic handle is a good insulator, and will allow me to hold on to the screw driver without getting a shock. With the engine running at about 1,000 RPM, I ground the high tension spark at each terminal of the distributor cap, one by one, by touching the screw driver to the paper clips.

By going around the distributor cap in order, I observe the drop in rpm caused by each cylinder as it is shorted out. When I come to the one that doesn't cause a drop in RPM, I know which cylinder is the culprit. You must know which wire serves which cylinder in order to know where the affected cylinder is located, but that is a simple task once you know how to trace them down. By using the specifications listed in the Corvette Servicing Guide, or any other good "Motor" book, or "Chilton" book, you can become an expert in the art of knowing your way around the engine compartment. Learn the firing order, the direction of rotation of the distributor shaft, the direction of rotation of the crankshaft, the numbering sequence of the cylinders in the engine block, etc. so you can accurately find the offending cylinder. This kind of home work is what is necessary in order to get passed the mystery of it, and get down to the art of expert diagnosis and repair. The engineers designed the engine, ignition system, fuel system etc. to provide years of good service, but it must be kept in good condition and in proper configuration in order to accomplish this.

I have only covered a basic overview of the engine as per its mechanical condition. There is a lot more to the tune up. As an example, you will have to make sure that the fuel system is in good condition from the tank to the air filter. The ignition system needs to be operating according to specifications, including the spark advance systems, in order to get good performance and economy. Other systems like the cooling system, the charging system, the cranking system and the exhaust system all need to be working properly for the best performance. Those of us who work on cars professionally find an amazing array of broken or poorly operating items every day. Often it is just a matter of a lack of attention to detail by the owner or a technician that caused the problem, but, when all the various systems are operating together it is just like an orchestra. It makes beautiful music; maybe that’s why they call it a “TUNE UP” .