Steve Zack Article:
Henry Olsen Articles:

Tuning a Carbureted Street Rod Engine - Part 1 of 2

Tim Wusz Articles:
spacer Solving Overheating

By: Henry P. Olsen


You have a hot-rod that is running too hot. You have replaced the radiator, the water pump and put in a high flow cooling fan but it is still running too hot. What do you do next? The engine burns gasoline to create power; about 25% of the energy from the gasoline is converted into power and the rest is converted into waste heat that goes into exhaust or the cooling system. When a vehicle has an overheating problem and everything in the cooling system is new, perhaps the problem is not the cooling system; maybe the engine is creating too much heat due to an incorrect air/fuel mixture and/or an incorrect amount of ignition advance.


Depending on the temperature and speed of the air flowing through the radiator, the radiator should remove about 25 to 50 degrees of heat from the coolant. One way of checking to be sure the radiator is doing its job is to use an infrared heat gun to measure both the inlet and outlet radiator water temperature. If the radiator is not cooling enough, first be sure that the airflow through the radiator is adequate; try driving the car without a hood. If this helps, maybe the air can�t get out of the engine compartment once it is blown through the radiator. Some cars we have seen have a lot of air getting around the radiator through air gaps that may allow the air to bypass it. These air gaps should be sealed so the cooling fan can do its job and all the air being forced through the radiator at highway speeds will go through it.


The speed of the coolant flow must also be correct. If the coolant flow is too fast, it may not cool as well as it should. A thermostat or flow restrictor is always recommended to properly regulate coolant speed through the cooling system. Always use distilled or deionized water; tap water contains minerals that can cause corrosion in the cooling system. Do not mix coolant types; the green and orange coolants may not mix properly, and mixing them may cause the coolant to lose all of its anti-corrosion properties. The water wetters can help cool an engine because of their ability to prevent air bubbles in the coolant. We often recommend that a customer put a piece of a zinc tab (available at any boat shop) in the radiator to be used as a sacrificial anode; they give any minerals in the coolant something to attack rather than corroding the radiator or engine block.


It is very important to have the air/fuel mixture correct as well as the correct vacuum and mechanical advance curves for all the engine load conditions. Correct ignition timing will cause the engine to have maximum cylinder pressure at about 12 degrees after top dead center; the optimum ignition spark timing will vary by engine speed, load and air/fuel mixture. If the spark occurs too soon, the engine will fight against the pressure in the cylinder creating extra heat and possibly cause a pinging problem that may do engine damage. If the spark occurs too late, the maximum cylinder pressure will occur too late, thus not converting the gasoline energy into power, but wasting it as heat that the cooling system must get rid of. If the ignition timing is incorrect, not only will the engine run too hot but it won�t have the power it should. An incorrect air/fuel mixture can also cause an overheating problem; a lean air/fuel mixture will cause an engine to create more heat than a rich air/fuel mixture.


An example where the reason for the overheating was not the cooling system but the �tune-up� is a 1947 Cadillac with a 500 cubic inch Cadillac engine that belongs to a local car collector; after everything in the cooling system had been changed, the car was brought to us to see if we could help. The first problem we found was the amount of mechanical advance; it was less than half of what it should have been. Also the vacuum advance was bad, thus not supplying the correct amount of advance. After repairing the distributor, we then checked the jetting; the idle mixture was very lean and could not be adjusted any richer. Next we checked the cruise mixture, which was also too lean. After modifying the idle and off idle systems to provide more fuel and increasing the main jets by two sizes, we had the correct air/fuel mixture; after a test drive, the customer said not only had the overheating problem gone away but the car had never run so good.


Ignition Timing & Advance


Correct ignition timing involves more than just setting the initial timing; the amount and rate of mechanical advance, as well as the amount and rate of vacuum advance, is very important in order to avoid overheating. Any distributor, original or performance replacement should have the mechanical and vacuum advance checked to insure proper operation. The advance curves in many cases may not be optimum for today�s fuels and the engine combo being used in your vehicle. Many performance replacement distributors come with a very slow advance curve installed and an assortment of bushings and springs to allow you to obtain the advance curve you desire. If you do not set the curve in the distributor, the engine will not run up to its potential. If you are running an OEM (original equipment) distributor that has been rebuilt by a �factory�, there is no knowing what vacuum and mechanical advance curves have been used. The mechanical advance could be too slow or too fast and have too little or too much advance, while the vacuum advance may have too much advance at too low of a vacuum.


Checking the Distributor Timing Advance


The best way to check both the vacuum and mechanical advance curves of a distributor is on a distributor test stand, but not everyone has a distributor test stand in his or her garage. Please note: any rpm or advance readings given by a distributor test stand are doubled since the distributor turns at � of engine speed; a reading of 12 degrees advance @ 1750 distributor rpm = 24 engine degrees @ 3500 engine rpm. The next best way we have found to check the vacuum and mechanical advance curves with the distributor in the engine is by using an OTC/SPX hand vacuum pump and an OTC/SPX advance reading timing light. To check the vacuum advance curve, use the hand vacuum pump to vary the vacuum supplied to the vacuum advance and then use a timing light to read the amount of advance given at different amounts of vacuum from 1 to 23 inches. If the engine does not have a degreed balancer, a timing light such as the OTC/SPX advance timing light we use can allow you to �read� the amount of advance. By observing the amount of advance at 250-rpm steps, you can now check the mechanical advance; again if the balancer is not degreed, the advance timing light will allow you to get advance readings. A distributor test stand is much easier because you can run the distributor at 6000 engine rpm or more without the fear of hurting an engine. In many cases a distributor may still be advancing the timing well above 6000 RPM; therefore the advance curve must be checked at all the rpm ranges the engine will be used in! The hot rod advance curve we see used most on a 9-1 compression engine is 10 to 12 degrees initial timing plus 22 to 24 engine degrees of additional advance from the mechanical advance mechanism; the advance in most cases is in the 3500 rpm range. The mechanical advance should not start advancing before about 1000 engine rpm. Too much advance at too low rpm may cause an overheating and/or a ping problem, which could lead to engine damage. If the advance is at too high or low of an rpm, the engine may overheat or not run up to its potential; it may feel lazy on acceleration.


The Vacuum Advance Effect on Cooling


A vacuum advance can help cool a hot running engine by giving the engine a little extra ignition timing advance under light load conditions that goes away under higher loads. In most cases when you are using a �hot-rod� mechanical advance curve along with a vacuum advance, the amount of additional advance from the vacuum advance should not exceed 10 degrees and not be in before 10 inches of vacuum. When you get too much advance at too low an engine vacuum from the vacuum advance you may get a vacuum advance ping at light throttle. The vacuum source we like for the vacuum advance is a ported source (no vacuum at idle). The reason for this is to avoid the timing change that happens on a vehicle with an automatic transmission when it goes into gear; the vacuum will go lower and cause the timing to change. This can affect the idle speed and quality when the car is in gear, especially if the engine has a hot cam (when the engine is in gear the vacuum is lower and therefore it has have less timing advance, thus effecting the idle speed).


The Air/Fuel Mixture Effect on Overheating


Now that the advance curves are correct, it is time to check the jetting or air/fuel mixture. It is also very important to be sure that the fuel pump can supply enough fuel volume and pressure, even at high loads. The fuel pressure we use most is 5 � to 6 lbs. at all speeds, and this pressure must be maintained even at high loads. If the fuel pressure drops at high loads, the fuel mixture will go lean and create a heating problem. An engine that is jetted too lean will create more heat than an engine that has the correct air/fuel ratio; if the jetting is too rich, the mixture may still be burning in the headers and exhaust system, again creating more heat. Many of the overheating problems that we have seen are due to a lean-at-part-throttle problem that is very common on many original equipment and aftermarket carburetors; this lean-at-part-throttle condition can create an overheating problem at light throttle. The newer, reformulated, gasoline that can shift the air/fuel mixture leaner by about 2.7% has brought many of these lean mixture problems on. If the air/fuel mixture is too lean at any driving condition, the engine will create even more heat for the cooling system to dissipate.


The two methods of checking the air/fuel mixtures that work best for us are: 1) using an exhaust gas analyzer, such as the unit we use from OTC/SPX Tool Company called a MicroGas , or 2) using a lambda meter such as the Professional Lambda Meter from MoTeC .


The most accurate and easiest method to check the air/fuel mixture (jetting) of an engine is by observing the CO reading from an exhaust gas analyzer, such as the unit that we use, the OTC/SPX MicroGas portable exhaust gas analyzer. All that you have to do is place the sample probe into the exhaust tailpipe and then the MicroGas will supply the readings that you can use to determine the air/fuel mixture. The second method involves using an extended range oxygen sensor that is installed into the exhaust manifold in order to read the amount of oxygen in the exhaust stream. We have been using the MoTeC professional lambda meter with very good results; it is able to supply us with the air/fuel mixture readings. This method using an extended range oxygen sensor can be very helpful, but if the engine has a �hot� cam, the readings may need to be confirmed at lower engine loads with an infrared analyzer because of the extra oxygen in the exhaust created by the overlap of the camshaft.


The air/fuel mixture that works best for a �stock� engine is: Idle mixture CO reading of 1 to 3% or 14.1-13.4 to 1 air/fuel mixture and at 3000 rpm cruise mixture CO reading of .75 to 1.25 or a 14.2-14.0 to 1 air/fuel mixture. A performance engine may need a richer cruise mixture of up to 3% CO or a 13.4 to 1 air/fuel mixture. If any readings taken are leaner than the target mixtures at any rpm range, that leaner air/fuel mixture may create an overheating problem. Power air/fuel mixtures also should be checked to be sure that the fuel system is supplying the richer mixture the engine needs when it is under higher loads; most engines use a power mixture of 6.6% CO or a 12 to 1 air/fuel mixture. In general if the air/fuel mixtures are richer than the target mixtures, the engine may not run too hot, but it may foul spark plugs and run poorly.


The Tuning Results


A properly tuned fuel and ignition system can help keep your engine running cool and also will help to allow the engine to run as well as it should. No matter which methods you chose to tune your fuel and ignition system, having the correct air/fuel mixtures and ignition-timing curves can help solve many overheating problems and make your hot rodding experience trouble free.



Ole�s Carburetor & Electric Inc.

120 El Camino Real 

San Bruno, CA 94066



John Bishop

Hot Rod Tuning 

808 Burlway #2 

Burlingame , CA 94010




OTC/SPX Corporation 

655 Eisenhower Drive   

Owatonna , MN 55060  



MoTeC Systems USA  

West Coast: 

5355 Industrial Drive  

Huntington Beach , CA 92649  



East Coast: 

169-2 Gasoline Alley 

Moorsville , NC 28117