Engine tuning is based on many factors. Can you call the last owner to see if he or she can fill you in on any internal modifications that may have been made? I've seen many engines that have been built or loaded with high performance add-ons that only made things worse. If you have a base 289 for example, was it a C or A Code Engine? Are the Heads correct for the Engine? Have the Heads been modified in any way? Has the Cam been changed and if so, what are its performance characteristics? Do you have Hydraulic or Solid Lifters? Is the Intake original or has it been replaced with an aftermarket performance Intake. I can go on-and-on.
First you will need to determine which Engine Block you have and whether the Pistons are correct per the original specs. The attached photo is typical of a stock Piston. If your Pistons are flat and have four notches on the top or are domed in some manner, they likely are aftermarket replacements and may raise you compression ratio. The latter will require higher octane fuel.
Next you will need to know if the Heads are correct for that Engine and if not, what are they from? If they are correct or original, have larger Intake Valves been installed, does it have heavy duty Valve Springs or Springs inside of Springs? No matter what you have be sure that somewhere along the line Hardened Exhaust Valve Seats have been installed. If so, you can run on Unleaded Gas, if not, you'll need to add a Lead Substitute each time you fill up to prevent the Exhaust Valves and Seats from burning/eroding away.
Your Camshaft Design has to be known if it's not stock. The Camshaft determines when the Valves begin to open, how long they will be open, how far it opens the Valves and the RPM range for which is was designed. Are the Lifters Solid, Hydraulic or Roller? Does your Engine use a Timing Chain or is it Gear Driven?
The Intake Manifold, for street applications has to be Dual Plane and is likely what you have. But like all the other Engine components, is it original and designed for the Engine you have?
The Exhaust, original Cast Iron or Headers? Single or Dual Exhaust? Exhaust Pipe Diameter and Mufflers all enter into Tuning.
The Carburetor, more than any other component, is often incorrect; usually with a CFM rating beyond what the Engine can handle. Find out what the CFM rating of the Carburetor is and it's intended or original use. Whatever it is, it should have Two Primaries and Two Secondaries; the Secondaries can either be Vacuum or Mechanical in design. You do not want a Carburetor that has all 4 Barrels/Venturis operating at all times.
The Dual Point Distributor looks cool but for the street serves no real purpose; they were designed for high revving applications and the Advance Weights probably have stiffer Springs which means they won't fully advance under street or highway driving conditions. This effectively retards the timing and robs you of power. You can install lighter Springs so that the Timing will advance more quickly but will be a trial and error type of thing to find the correct combination. It would be better to pull the Distributor and take it to a Distributor Shop to have it tuned for your specific application. Does yours also have a Vacuum Advance? Personally, I would reinstall the correct Single Point Distributor and if you want, convert it with Electronic components such as those available from Pertronix.
In a nutshell, to be able to properly tune an Engine everything needs to be balanced. This means that the Air and Fuel capacity of the Carburetor cannot exceed the Intake Manifolds ability to carry it, the Valves ability to flow it, the Camshaft ability to keep the Valves Open long enough to move it and the Exhausts ability to expel it. If just one of these is incorrect, the Engine will never run properly and no amount of tuning will fix it.
Find out everything you can about the Engine, it's the only way to determine how to properly tune it. If you have a C-Code Engine and someone slapped on a 4 Barrel Intake with a big Carburetor and Headers but left the original Cam in place, that just won't work. Or if they did all that including the correct Cam but left it with Single Exhaust; again no good.
I'm sorry to be so long winded... but in actuality this is the short version. Without more and specific details, I can only be broad in the possibilities.
Let me know what you find.
Take care.
Ray
