Basic Tuning of Carbureted Performance Engines

CFS Carbs

The steps of tuning basically any carbureted internal combustion engine, particularly those used in competition, should be done in a specific order. This applies to the firing of a new engine, installation of a new component having to do with the ignition or fuel systems on an engine that has been running, periodic checking of normal tuning adjustments, or searching for the cause(s) of an engine performance problem. Checking and adjusting , done in this specific order, should help you get them all right without having to go in circles on your tuning adjustments. This helps determine if engine problems are due to improperly adjusted basic adjustments, or to some issue having to do with other variables.

Before making final adjustments to the carburetor, ignition timing must be properly set. Ignition timing affects how the carb works through the entire rpm range of the engine. This applies not only to when the engine is in the competition event, but also to its idle speed and part throttle running before and after the event, in this case , a pull. Determining the proper amount of initial timing(at idle), and the advance curve as the rpm's go up, is determined by many factors, including compression ratio, valve timing, and fuel octane, just to name a few. If you are uncertain about the ignition timing, consult with someone who has adequate knowledge of your engine combination. Getting the timing right is critical to the rest of the tuning adjustments. Next, are adjustments to the fuel delivery system and carb.

Before making carb adjustments, fuel pressure must be set to a pressure that is suitable for the size of the needle and seat size that your carb uses, and for the power that your engine makes. Assuming that you have an adjustable fuel pressure regulator(recommended), set the fuel pressure to the proper level, always with the engine running(idling). Next is setting the fuel level("float level") in the fuel bowl, if you have the type of fuel bowl with an externally adjustable needle and seat height. On the tractor carbs that I build , I use a Holley fuel bowl that is commonly used on their performance and racing downdraft carbs. This bowl type has an adjustable needle and seat height feature at the top of the bowl. Set the fuel level to the middle of the sight window, or the bottom of the sight hole, depending on which type of Holley bowl you are dealing with. This, as with fuel pressure, must always be done with the engine running, with the fuel pressure already at its proper setting. Fuel pressure regulators(conventional restriction type regulators), and needle and seat/float mechanisms used in carburetor fuel bowls, both require a small amount of fuel flow to function properly. The amount of fuel flow that an engine uses while it is running at idle speed, is enough to satisfy that need, assuming that the regulator, needle and seat, and float are all in proper working order. Your fuel pressure gauge should be mounted where the driver can easily see it during a pull, so he can monitor it closely. The next adjustments are to the idle speed and idle mixture.

Now that the fuel pressure and fuel level in the bowl are set, then the idle speed and idle mixture can be set. You may need to go back and forth between the two a couple of times, to get both where you want them, to achieve your best idle quality. These two adjustments should be done with the engine fully warmed up.

To recap, here's the proper order :

1)ignition timing, 2) fuel pressure, with engine running, 3) float level, with engine running, 4) idle mixture and idle speed, with engine fully warmed up.

Another system that can really help with fuel related tuning, is having a wide band O2 sensor system that tells you what the air fuel ratio is in real time, having the readout gauge located so the the driver can easily read it during a pull and during all other situations when the engine is running. The kind I like to use is called the AFR500, available from Ballenger Motorsports. This system, used with an upgraded NTK exhaust sensor, can be used with heavily leaded racing gas, without fouling the sensor. I have two on my engine dyno that I've been using for years, and have never fouled a sensor, and have run many engines with leaded racing gas. I suggest that you investigate the AFR500. Other similar systems that do not or cannot use the NTK exhaust sensor, typically have sensor fouling problems, giving you unreliable air fuel ratio readings. On pulling tractors, I usually look for air/fuel ratios around 12:1, both at idle and under power.

If you are dealing with an engine performance problem, and all your basic adjustments are correct, including your air/fuel ratios, then additional investigation will be necessary. Things like faulty spark plugs and plug wires, fuel contamination or stale fuel, too much fuel pressure drop from idle to full power, dirty air filters, and clogged fuel filters can cause problems. and this just to name a few. Determining engine performance problems can be a frustrating process, because frequently, they are caused by more than one factor. Eliminating one factor at a time, as a possible contributor to the problem, is the process needed to solve the problem. It can be frustrating and time consuming, but in most cases will solve the problem.

Norm Schenck/Competition Fuel Systems/Birch Run, MI/
520-241-2787/[email protected]


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