Tuning a Marine Carb

Excerpts from Family & Performance Boating, by Henry Olsen

Summer is here and gasoline prices are up and out of sight, this has many boat owners wondering how they tune their engine so they can get more hours out of a tank of fuel without losing power or doing any engine damage from going to lean on the air/fuel mixtures. Properly tuning a carburetor or programmable fuel injection to supply a boat engine with the correct air/fuel mixture for performance, lower fuel use and reliability has always been thought of as an art that those few people that can “read” a spark plug could do properly. Those top tuners looked at the spark plug, the exhaust port and the first 6 inches of the header for proper “color” and then made an educated guess at what jet size change was needed. One of the disadvantages of this method is that the header and spark plug can only indicate what the mixture was at the exact rpm and load condition the plug check was done at, so you were mainly just tuning by trial and error. The more modern day tuners are now taking advantage of the technology being used to make today’s new boats run so good by the use of a wideband oxygen sensor to read the content of the engine’s exhaust. A wide-band oxygen sensor reads the oxygen or unburnt combustionables content of the exhaust to determine the air/fuel mixture. This new wideband oxygen sensor technology is being used by the top race teams in Nascar and NHRA Pro-Stock as well as some of the top Super Cat off-shore race teams.

The proper tuning of any engine can make the difference between a great running engine and an engine that always sounds and runs like it needs a “tune-up”. For most marine tuners, one of the biggest mysteries is how do you “jet” the engine in order to obtain the correct air/fuel ratio necessary for a high performance marine engine, to not only supply drivable horsepower when you want to go fast, but also supply the engine with the correct air/fuel mixture for while just cruising or when idling thru a no wake zone.

Tuning with a Digital air/fuel meter

The air/fuel meter method uses a wideband oxygen sensor to determine the fuel mixture by analyzing the unburned combustibles in the exhaust gas. A wideband oxygen sensor can read air/fuel mixtures as rich as 9 to 1 or on the lean side it can read air/fuel mixtures lf 19 to 1 or leaner (a standard, narrow-band oxygen sensor is only accurate at air/fuel mixtures of around 14.7 to 1). This method has the advantage of extremely fast reaction times for the readings but the accuracy of the readings can be affected on an engine with a “race” cam or a supercharged application at light load/low rpm testing conditions because of the excessive oxygen in the exhaust created by the cam overlap or the superchargers blow thru effect at low engine speed and low load conditions. The digital air/fuel meter method requires you to know what air/fuel mixture your engine needs for each driving condition.

A starting point for air/fuel mixtures for most performance marine engines is:

Idle:   14.1-13.4 to 1 air/fuel mixture

Cruise/light loads rpm 14.2 –14.0 to 1 air/fuel mixture

Power mixture and acceleration: 12.5 to 1 is the ideal air/fuel mixture for a “normal” engine, but many marine engine builders prefer a 11 or 11.5 to 1 power mixture; a high performance engine with improved combustion chamber design such as a Pro-Stock or a Winston Cup engine, which are being used in some of the top offshore race boats, can use a slightly leaner power mixture of 13.0 to 1 air/fuel ratio. A supercharged engine can use a power mixture richer than 12 to 1 as a method to help control the detonation that can be caused by the higher cylinder pressures.

The performance and replacement marine carburetors sold today have a generic “tune-up” or jetting unless the carburetor is built for a specific engine package and fuel. A carburetor not built and tuned for a specific engine, exhaust system, and fuel should supply an air/fuel mixture rich enough for a variety of engines (but this is not always the case). If the carburetor is supplying too lean of an air/fuel mixture, the engine will lack power, run sluggish, overheat and the lean mixture could cause engine damage. If the carburetor is supplying an air/fuel mixture that is too rich, the engine may tend to load up, foul the spark plugs, run sluggish and lack power.  The carburetor must have 5 ½ to 6 ½ lbs. of fuel pressure at all rpm and load conditions, if the fuel pressure goes to high the carburetor will tend to” flood”, while if the fuel pressure goes to low the carburetor fuel level will drop causing the engine to starve for fuel at higher loads and engine speeds. If the fuel pressure drops below the proper pressure, the carburetors air/fuel mixture will go lean and engine damage may follow. A fuel pressure in excess of 6-½ lbs. can cause the carburetor to flood especially when operating in rough water conditions such as when you jump a wake or if you land in the trough after coming off a large swell, this flooding can be seen as black smoke coming from the exhaust or the engine begins to miss and run sluggish till it clears out.

One area often ignored is fuel tank venting, if air can not get into the tank to replace the fuel the engine is consuming fuel can not get out, the fuel tank must have a vented gas cap or some other type of vent system. Doug Schriefer from Barry Grant Inc. often points out that for every gallon of fuel you use a gallon of air must be allowed into the fuel tank so if you are adding power into your boat you may need to upgrade the fuel tank vent system with larger hoses or additional vents.  If the fuel tank vented by a hose routed to a external vent always be sure to route the hose so it has a loop in it that is above the external vent, this will keep any water spray from traveling from the vent thru the hose and into the tank because the water will not easily travel up hill. Also do not forget to upgrade the fuel filter/water separator, you are operating in a marine environment and water is everywhere you need to keep the water out of the carburetor or fuel injectors. Proper fuel pressure/volume and fuel tank venting are very important because a marine engine is expected to supply more power for a much longer time than any other form of Motorsports, the fuel pump must be able to supply enough fuel for the carburetor while the engine is supplying the boat with full power for sometimes hours at a time!

 The fuel you use (reformulated pump or race), the air density (i.e. altitude, barometric pressure, air temperature, humidity), compression ratio, camshaft, exhaust system, ignition timing curve, engine condition, fuel pressure, air flow thru the flame arrester, etc will all effect the carburetor “tune-up” needed to get the correct fuel mixture for your engine. The first order of business is to get the correct ignition advance curve for the engine and fuel being used, then the fuel pressure must be checked to be sure it has the proper system pressure at all engine load conditions. Too much or too little ignition timing can give you a false lean or rich air/fuel ratio reading, so first check and set the ignition advance and advance curve!

Ignition Timing and The Advance Curve

Before checking the air/fuel mixture, the ignition timing and advance curve must first be correct. No mater how what ignition system you use, if the ignition spark timing is not correct for the engine needs, the engine will not produce all the potential power built into it. Once the ignition advance curve has been confirmed to be correct, many of the problems that we see can be traced to the fuel mixture being incorrect for the engine’s needs.

Checking the air/fuel mixtures under load with a carbureted engine

After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, the next step is to determine what the air/fuel mixture is at idle through 3000rpm. If the cruise mixture is off, first change the jets in order to get the air/fuel mixture correct at the 2500-3000-cruise rpm range. Then check and set the idle mixture. If the air/fuel mixture is too lean at idle or part throttle and the idle mixture screws do not provide enough adjustment, the correction may involve enlarging the idle “jet”.  This lean condition at part throttle condition will cause the engine to miss or stumble, this is due to the lean air/fuel mixture, this problem is very common on many of the performance carburetors sold today. If the air/fuel mixture is too rich at idle and part throttle, the idle jet/restriction may be too big and may need to be replaced with a smaller one.

The next step is to read the air/fuel mixture while operating the boat under power using a wideband oxygen sensor to check the cruise speed air/fuel mixture-main jetting, followed by a check of the power air/fuel mixture under load. During a drive test you are able to read and then correct the air/fuel mixture so you can have them correct at idle, cruise/light throttle and full throttle. If you are buying a engine package that has been “dyno tuned” or developed and run on an engine dynamometer, get the engine builder supply you with what air/fuel mixture they ran the engine on the dyno for both maximum power and cruise rpm loads and then be sure they are the same with the engine in the vehicle. If you have a engine builder build a custom engine for you and they run it on a dyno, have them record the air/fuel mixture with a air/fuel meter such as the Innovate Motorsports unit and then you can use the recorded data to tune the fuel curve to supply the engine with the same air/fuel mixture that the engine builder used on the dyno.  Many of the engine packages we have checked the ignition spark timing and air/fuel mixture curves on have had the correct ignition spark timing and air/fuel mixtures for high rpm/wide open throttle operation but need a lot of tuning work at low rpm/part throttle/normal driving conditions. In most cases when an engine is run on a engine dynamometer, they only check for maximum power while using race style headers with open exhaust (non water jacketed marine headers) and they supply the engine with air that is not of the same density the air you will see when your boat is in the water and the carburetor probably did not have a flame arrestor or any other type of air cleaner on it.

The air/fuel mixture and ignition timing curves should be corrected for the real world operating conditions of your boats engine compartment with coming thru your flame arrestor, along the changes in exhaust backpressure created by the exhaust system you are using which may cause the air/fuel mixtures to change from their original baseline causing the engine to not perform with the same power that was seen on the dyno.

Tuning A Fuel Injected Engine

The Innovate Motorsports wide band oxygen sensor based air/fuel meter can be used to read the exhaust in order to be sure the air/fuel mixture that a fuel injection system supplying is correct for the engines demands. If the fuel injection system is programmable the correction may be as simple as a few simple computer program changes, but if it is not programmable there is still a few tricks that can be done. The easiest way to change the air/fuel mixture on a non-programmable fuel injection system is to change the fuel pressure or the flow rate of the fuel injectors. As the fuel pressure is increased the fuel flow will be increased, thus making the air/fuel mixture richer. Increasing the fuel pressure has a limited effect on fuel mixtures, if you increase the fuel pressure too much the fuel injectors windings may not have enough power to open the fuel injectors against the increased fuel pressure, this will cause the mixture to go lean as the fuel injectors start to lock closed. If you need to increase the fuel pressure from a base fuel pressure of 40 psi (pounds per square inch) more than 15 % to try to get a rich enough air/fuel mixture, it may be time to buy larger fuel injectors. Lowering the fuel pressure will cause the air/fuel mixture to go leaner but again limit you change to no more than 15%, if more correction is needed smaller fuel injectors may be needed.  The fuel supply system must be able to supply enough volume and pressure to maintain the proper fuel

The Results of Proper Tuning

A properly tuned fuel and ignition system will allow your marine engine to perform up to its potential and will supply you with a better performing, more reliable and efficient running marine engine that will be a whole lot more enjoyable to use. Taking the time to properly tune your boat’s fuel and ignition advance systems will not only allow you to have a more reliable marine engine with more drivable engine power, it can also help lower your engines fuel consumption. This properly tuned marine engine that has the correct air/fuel mixture and ignition advance curve will help you have a boat that runs great, has more power, uses less fuel and is free from the problems and this proper tuning may help avoid the engine damage that can result from incorrect ignition timing or overly lean air/fuel mixtures.