Old Iron: Tuning A Carbureted Engine - Engine Builder Magazine

Old Iron: Tuning A Carbureted Engine

Even if you build the perfect engine all your hard work means very little if the customer is not satisfied with how their engine performs. So like it or not, you will have to become involved in engine tuning or at the least provide your customer with tuning guidelines if you want to have a satisfied customer.

Here is the method we use to tune a vintage carburetorated engine that was originally “tuned” for leaded gasoline so it can perform its best with the gasoline of today.

The mechanical and vacuum advance curves built into the original distributor of a vintage or muscle car from the 1950s, 1960s and 1970s were designed to allow the engine to perform its best with the leaded gasoline of the day. Most of the commercially rebuilt distributors we have checked the advance curves on have mechanical and vacuum advance curves that vary widely from those original advance curves. Even if the advance curves matched the OEM specifications, they will most likely still be wrong for an engine running on today’s reformulated gasoline.

The new high performance aftermarket distributors on the market will probably not have the correct ignition spark advance curve that your customer’s engine needs as it comes out of the box.

Today’s gasoline burns at a different rate than the leaded gasoline of years past plus the formulation of these modern blends of gasoline can cause the air/fuel mixture from a non-computer controlled carburetor to be leaner than it was with the gasoline of years past.

Last I checked, there were about 14 different blends of conventional, reformulated and oxygenated gasolines available at local gas stations. The formulation of gasoline available at your local gas station will also vary by season. Additionally, a multitude of unleaded high-octane blends are available from high performance gasoline suppliers. Federal and state regulations, environmental concerns, plus the drive to lower our reliance on foreign oil have brought us reformulated gasoline that may contain up to 10 percent ethanol in certian parts of the country.

The reformulated and oxygenated gasoline blends used today are less volatile and have at least 2 to 5 percent less energy than the leaded gasoline of years past. If your customer uses oxygenated gasoline in their part of the country, the oxygenate will result in an air/fuel mixture that is at least 3 to 5 percent leaner than it would be with leaded gasoline.

A modern electronic fuel injected engine performs very well with this today’s gasoline blends because it has a computer that is continually adjusting the ignition timing and air/fuel mixture. A vintage muscle car doesn’t have a computer to make minute timing and air/fuel mixture adjustments that allow it to run on modern pump gas.

The theoretical ideal ignition spark timing for power is just short of the point where detonation or pinging begins in any one cylinder (providing the octane requirement of the engine and the octane number of the gasoline are very close to each other). The correct ignition timing (initial timing plus the advance from the mechanical and vacuum advance mechanisms) enables pressure created by the burned fuel in the combustion chamber that is pushing the piston down to be at its peak when the piston is at about 12-15 degrees after TDC.

When an engine has too little advance it will lack power since the energy that should be pushing the piston down is lost as wasted heat through the exhaust system. When the engine has too much advance the engine will also lose power because the piston is being pushed down before the connecting rod angle from the piston to the crankshaft is correct. This means the energy from the fuel pushes the piston against the cylinder wall instead of pushing the piston down, as it should for maximum efficiency. Excessive ignition spark advance can also cause detonation and lead to piston or ring failure.

Ignition spark advance needs change as the air/fuel mixture, gasoline formulation, octane, engine temperature, air density, air temperature and humidity change, so we always recommend tuning the ignition on the safe side to avoid any detonation or preignition issues. The first rule of performance tuning is always tune the ignition curve(s) first, then tune the air/fuel mixture as needed. Today’s reformulated gasoline causes an engine that was designed for leaded gas to need more initial timing with a slightly faster mechanical advance curve, but the total advance (mechanical + initial) is about the same as it with leaded gasoline.                                                            

Mechanical Advance

The mechanical spark advance consists of three components: the initial advance, the rpm-based advance rate and the degrees of advance from the mechanical advance weights. The advance rate of most OEM and performance replacement distributors can be modified by the use of different advance springs, but changing the amount of mechanical advance from the distributor is not always easy. Each distributor manufacturer limits the mechanical advance differently, some of which are easy to tune while others can be more challenging.

A high performance distributor will almost always have a generic advance curve built into it that must be tuned to match your gasoline blend. Before you purchase any aftermarket distributor you should check to be sure that it’s “tuner friendly.” The ideal distributor will allow you to easily adjust the mechanical advance rate, amount of advance from the mechanical advance system and vacuum advance unit.

The best way to check both the vacuum and mechanical advance curves is on a distributor test stand. A test stand was standard equipment at one time but today they’re not as common. A test stand allows you to check the mechanical advance at any rpm without fear of over-revving the engine. Then you can check the vacuum advance curve with the use of a vacuum pump.

If you do not have access to a distributor test stand, you can check the vacuum and mechanical advance curves with the use of a hand vacuum pump and a dial-back/advance reading timing light (if the engine has a degreed harmonic balancer you can read the amount advance with a standard timing light).

First, observe the amount of mechanical advance in 250-rpm increments from idle until it quits advancing. Take care not to over-rev the unloaded engine. In many cases the mechanical advance could still be advancing the timing at engine speeds of 6,000 rpm or higher, which is why I prefer to first check the mechanical advance on a distributor test stand.

To check the vacuum advance curve, use the hand vacuum pump to vary the vacuum supplied to the vacuum advance and use the timing light to read the amount of advance given at different amounts of vacuum from one to 23? of vacuum.

One of the best guidelines for determining initial timing can be found in the Barry Grant catalog or Web site (in the Demon carburetor selection guide). The guideline recommends 10-12 degrees of initial timing when the camshaft duration is less than 220 degrees @ .050? of lift; 14-16 degrees with less than 240 deg @ .050?; And, 18-20 degrees with less than 260 degrees @ .050?.

When working with an engine that has an air-gap style intake manifold or high performance cam we often use the initial timing recommendation from the next step “hotter” cam. This is because the engine is less efficient at low engine speeds due to lower air speed in the larger intake tracks and the lack of heat in this type of high-rpm, race-inspired intake manifold. The heat from the exhaust gas crossover in stock or a non-air-gap intake manifold, plus the smaller intake runners on a “stock” intake manifold, help keep the fuel vaporized and properly mixed with the incoming air-charge as it travels from the carburetor to the cylinders at lower engine speeds.

The “performance” mechanical ignition spark advance curve we use most  allows full advance around 3,200-3,600 rpm. The mechanical ignition spark advance curve (as set by the advance springs) should not start advancing until the engine rpm is raised above the base idle speed. The advance curve must also not advance so quick that the engine experiences detonation. Most of the carburetorated engines we work on respond well to a “performance” advance curve that allows the total advance at 3,200-3,600 rpm, but this will vary by how well the octane matches the engine’s requirements.

A typical 9.5:1 compression engine such as a small block Chevrolet, Ford or Chrysler usually responds well to 36 degrees of total ignition advance (initial timing plus the mechanical timing). Engines with cylinder heads that have “fast burn” combustion chambers or high compression muscle car engines from the 1960s seem to produce more power when the mechanical advance is limited to 30 degrees of total advance (initial + mechanical advance).

Whenever you increase the initial timing you must check, and most likely reduce, the advance from the mechanical advance system so the engine has the same total advance. Only use the amount of advance the engine needs for the best power or fuel mileage. When you use as much timing advance as you can, you expose the engine to possible damage from detonation.

An engine’s ignition timing needs change with both engine load and air/fuel mixture. Leaner and less dense air/fuel mixtures at part throttle/cruise speeds take longer to burn than richer air/fuel mixtures at wide-open-throttle or heavy acceleration/high loads. This means that these leaner air/fuel mixtures need the ignition spark earlier in the compression cycle to allow more burn time so peak cylinder pressure is reached at the correct time for maximum fuel efficiency. A vacuum based ignition system will allow the ignition spark to be advanced when the intake manifold vacuum pressure is high so there’s more time for the leaner mixture to be burned.

When using a “performance” mechanical advance curve we tend to limit the amount of additional advance from the vacuum advance to about 10-12 degrees. Most original replacement vacuum advance units have 16-24 degrees of advance. And when you exceed 15 degrees of vacuum based spark advance with reformulated gasoline the engine may begin to misfire at light-load/cruise conditions. The advance from the vacuum advance should not begin until 8-10 inches of vacuum is reached to avoid detonation on light to moderate acceleration.

Most adjustable vacuum advance units only allow you to change the vacuum rate  with no way to limit the amount of vacuum based advance. If you can’t find a vacuum advance that matches your advance needs, you may need to add or build a stop to limit the advance/travel of the vacuum advance unit.

The vacuum advance needed, once the mechanical advance has been optimized for the engine and fuel, is lower than an engine used in the days of leaded gasoline. The 52-54 degree total advance limit (initial + mechanical + vacuum) most tuners used for a typical V8 engine back in the leaded gasoline days is  too much advance for the today’s reformulated gasoline burn characteristics.

When total ignition-timing advance exceeds 48-50 degrees, we often see the engine misfire rate increase and/or the combustion chamber temperatures increase with the reformulated gasoline available in California, but this may differ with the blend of gasoline you have in your area.


Henry Olsen, Jr., also known as “Professor Drivability,” is part owner and operator of Ole’s Carburetor and Electric, Inc., located in San Bruno, CA. Ole’s Carburetors is a complete auto parts store. They also offer “off the car” carburetor, fuel injector, starter, alternator, and generator rebuilding services. Additionally, Ole’s Carbs offer “on the car” fine tuning of fuel and ignition systems. today
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