Frequently Asked Questions (FAQs)

Clarify Your Doubts In The FAQ Field Below

The alternator puts out 20 amps at idle and 50+ at operating speed. Our engines are also fitted with a rectifier that converts AC to DC and controls battery charging.

We recommend a group 24 automotive starting battery. Group 27 is okay. Deep cycle batteries can be used, but are not as good in the long run.

You can run two batteries, one deep cycle and one starter, but remember, use an isolator or battery selection switch if you are using the deep cycle battery to run a trolling motor. The isolator allows one battery to be discharged separately from the other and still allows both to charge from a single source.

Storage – Remove the battery and place indoor on a piece of wood or rubber for the winter. Fill with distilled (not tap water) if needed. I like to put a trickle charger (1.5 regulated amp) on the battery when in storage.

On a hot day after I run the engine for a while then shut down, the engine won’t restart or it starts and idles fine, but when the throttle is advanced the engine bogs down or quits running and will not restart. Why?

It sounds like your engine may be vapor locking. One factor that contributes to vapor lock is fuel. Fuels produced for use during the winter and early spring months around the United States are designed to vaporize at a lower temperature. This lower vaporization point can cause the fuel in the boat supply lines and engine fuel system to vaporize prior to being used.

Another contributing factor to vapor lock is over-heating. Make sure debris is not blocking the engine air intake and the air filter is clean.


  • Use synthetic oil. It reduces friction and allows the engine to run cooler.
  • Let the engine idle for 3-5 minutes before shutting it off.
  • Lighten the load.

Remove the valve cover . Remove the spark plug, put finger on plug hole and turn over the engine until you feel the compression stroke air pressure .

  1. Turn the flywheel to close both valves.
  3. Insert a narrow screwdriver in to the spark plug hole and touch the piston . Turn the flywheel clockwise past top dead center until the piston has moved down 1 /4″ . Use the screwdriver to gauge the piston’s range of motion . PLEASE NOTE: This procedure must be performed for each cylinder on V-Twin engines .
  5. Check the valve clearance by placing a feeler gauge between the valve head and the rocker arm.
  7. The settings for the valve lash are as follows: Boss 3500 In take 0.005 ” Exhaust 0.005 ” Boss 3700 EFI In take 0.005 ” Exhaust 0.008 ” Boss 4400 In take 0.006 ” Exhaust 0.006 ”
  9. Adjust the clearances as required by turning the rocker screw. Once adjustments are completed , tighten the rocker nut . 6. Install the valve cover , using new gaskets , as required , and make sure the cover is secure . NOTE: Please read and abide by any applicable Safety Information (PDF) before performing any engine work . This information is not meant to take the place o f work performed by a Briggs & Stratton Authorized Dealer. Terms and Conditions apply to all of our information provided on this website . Always be sure to read and understand your engine Operator’s Manual.

Our large block Vanguard engines come with a Champion RC12YC spark plug . Although the gap can vary f rom .030 to .040 on new motors, they should be at .035. We have found that the AP3923 spark plug is a good upgrade. It is an Auto lite Platinum plug and comes in AP and APP – the later being double platinum and lasts longer . It is a slightly cooler plug and although the details regarding why we use this are long , we like to use this plug on performance motors and/or when replacement time comes. Don’ t underestimate the value of having fresh and well performing spark plugs. For starting, acceleration and top speed – good plugs, produce good results . They are the heart of your engine .

What octane rating fuel should I use in my mud motor . United States and Canada Use a major brand of automotive unleaded gasoline with a minimum-posted octane rating of 87. Leaded gasoline is not recommended . Our BOSS 4400 performance engines require 91 or higher octane.

Ethanol? Engine manufacturers are working on new Flexible Fuel Engines.

Will the use of fuels containing ethanol void my engine warranty? Fuels contain ing up to 10 percent ethanol are considered acceptable for use in our engines. Fuels containing higher levels of ethanol are not considered acceptable for use, and the use of fuels containing ethanol higher than 10 percent can void the warranty.

Does ethanol affect horsepower or fuel-efficiency? Ethanol has a heating value o f 76 ,000 BTU per gallon, which is approximately 30 percent less than gasoline’s heating value (which is approximately 109,000 to 119,000 BTU/gal ). The result is E-10 gasoline which should yield slightly lower mileage – a decrease of approximately 3 percent. Fuels containing higher levels of ethanol will have a corresponding reduction in mileage. For example, E85 fuels produce mileage approximately 30 percent less than gasoline . The octane rating of pure ethanol (200 proof ) is about 100 and is therefore useful in elevating the octane value of gasoline. In E -10 blends the presence of ethanol provides about 2.5 to 3 percent of the overall octane rating. The effect on engine horsepower is determined by the octane result of the blended fuel. Care should be taken to select fuels having the octane rating recommended for the engine you have.

What are ethanol and ethanol-blended fuels? Ethanol for fuel is highly refined beverage (grain ) alcohol, approximately 200 proof, that can be produced from natural products such as corn , sugar cane and wheat. New technology will allow ethanol to be made from “cellulosic ” feedstocks including cornstalks, grain straw, paper, pulp, wood chips, municipal waste, switchgrass, and other sources. Ethanol used for fuel has been “denatured,” or rendered unsafe to drink by the addition of a hydrocarbon (usually gasoline). The ethanol -blended fuel E-10 refers to fuel that contains 10 percent ethanol and 90 percent gasoline. Similarly, E-85 refers to fuel that contains 85 percent ethanol and 15 percent gasoline. E-85 is intended only for engines specially designed to accept high-ethanol content fuel blends, such as the Flexible Fuel Vehicles (FFV) made by some car companies.

What are the characteristics of ethanol? Ethanol is an oxygenated hydrocarbon compound that has a high octane rat ing and therefore is useful in increasing the octane level of unleaded gasoline. The EPA, the agency responsible for setting some of the requirements for all gasoline used in the U.S., has allowed the use of ethanol in gasoline at levels up to 10 percent as an octane enhancer and to provide beneficial clean-burning combustion characteristics that help improve some emissions. Ethanol is hygroscopic (it has an attraction for water) and will more readily mix with water than with gasoline. It has different solvency behaviors than does gasoline, which allows it to loosen rust and debris that might lay und is turned in fuel systems. And it can more readily remove plasticizers and resins from certain plastic materials that might not be affected by gasoline alone. Loose debris will plug filters and can interfere with engine operation. Additionally, ethanol is corrosive to some metals, especially in combination with water. Although gasoline does not conduct electricity well, ethanol has an appreciable capability to conduct electricity and therefore can promote galvanic corrosion. Basically, the choice is yours at this point.

Several basic items should be checked.

First, is the fuel tank full of fuel and vent open? Water in fuel or excessive amounts in water separator. Stock motor engines should idle between 1250 and 1450 . Stock EFI motor engines should idle between 850 -900. Modded Mikuni Carburetor and EFI aftermarket engines should run between 1000-1200.

Second, is the fuel line pinched from a heavy object sitting on the line, or is a tight connection restricting flow? Are there cracks in the line or connections, allowing air to leak in to the fuel system? If you located your fuel tank near the front of the boat, did you use a recommended 5 /16 ” or 3 /8 ” fuel line? If you added a quick disconnect to the fuel tank, especially OMC type, you may be restricting fuel.

Do you have rope, weeds or wire wrapped around your propeller? I s your fuel filter dirty or plugged? Is your engine oil low? Tip: If you use an in-line primer bulb, do not over pump the Mikuni carburetor aftermarket engines. These carburetors have very small needle and seat valves and if you over-pump the bulb, you can push small particles of debris past the fuel filter element and into the needle and seat where it will plug the carburetor. This can stop the engine or restrict fuel during acceleration or at full throttle. Also, the Mikuni carburetors have a low speed idle jet on the side of the carburetor. (brass screw). If your motor experiences hesitation at throttle-up, turn the screw in. The idle screw is normally set at 1 and a half turns out, but it can be run as far as all the way in. Each engine and carburetor are slightly different. If you are in a state that is using 10% ethanol and you aren’t using our fuel tank and new fuel lines, the ethanol can deteriorate the older system and cause particles to enter the fuel system, plug up fuel filter, etc .

Break-In is important to ensure correct engine performance and life. Even though many of us have stated in the forum that you can run the engine at full throttle after five minutes, the official answer is to follow the procedure listed in the Engine Operation, Maintenance and Warranty Manual which usually states don’t run at full throttle for the first 2 hours, then only briefly until 5 hours and always vary the throttle level during operation the first ten hours. Generally, for the first two hours, we recommend avoiding extended idling, sustained periods of wide open throttle, or holding the engine at one speed for extended periods of time. You can use regular automotive, synthetic or synthetic blend oil during break in. 10W30 is best. I say use the best oil you can buy. It protects your engine, the engine runs faster and longer. Use 87 or higher octane, 91 or higher for the performance engines. Change your oil, lubricate all drive points and check the sight glass on the upper transmission on the drive at 20 hours. This is the single most important thing you can do for your engine and drive.

I accidentally reversed the battery connections when I reinstalled the battery. What potential damage could I have caused?

Trim motor and bilge will operate backwards. Engine may not start. On your Vanguard engine, even after you have corrected the problem, you may have burned out the ignition switch diodes located near the coils. A service center will be able to install new diodes. Check this by turning over the engine and observing the grounded spark plugs for spark.

You can damage the voltage regulator. Check the voltage regulator by connecting a voltage meter anywhere in the system, run the engine up to 2200 rpm or higher and the circuit should be 14 volts or more. If 12 or less, your system is not charging. Check your rev limiter by slowly increasing the engine speed until it is near the rev limiter cut off point. If the engine speed exceeds the 4300, 4600 or 5100 rpm rev limiter maximum, the rev limiter has burned out. Replace the rev limiter immediately.

Other: If your battery has low power anytime, stock motors with carburetor fuel shut off valves may not start. The low voltage will not allow the fuel shut off valve in the carburetor to operate. If your engine does not start with a good battery, pour a little fuel in the carburetor. If the engine starts the fuel shut off valve may be damaged.

What should I do to prepare my mud motor for out-of-season storage or prolonged storage?

Clean the engine and drive of all foreign matter and mud.

The major consideration in preparing your mud motor for storage is to protect it from rust and corrosion. We recommend you read the storage information in your engine owner’s manual. Damage from improper storage procedures is not covered under any of the engine manufacturers.

Change Oil: It is best to change the oil at the end of the season. Small amounts of acid from burning fuel builds up in the oil and when left during storage can cause corrosion that can lead to loss of engine power and engine failure.

What is the purpose of using Storage Seal in my engine?

Storage Seal Rust Inhibitor sold by many outboard marine stores is a blend of corrosion inhibiting additives designed to coat internal and external engine components during engine storage to reduce the formation of surface rust.

Use Storage Seal Rust Inhibitor as directed with the product. We basically, remove the spark plugs and spray the recommended amount in each cylinder, turn over the engine slowly and replace the plugs.

Fuel System care if very important. BOSS DRIVES recommends the use of a fuel stabilizer to prevent fresh fuel from undergoing degradation and oxidation during fuel storage. Additional benefits of the product includes: easier engine starting after periods of storage, elimination of the necessity to drain and dispose of fuel, prevention of the formation of deposits that could clog filters and tiny passages in the fuel system; and elimination of corrosion and rust throughout the fuel system. It also absorbs water and prevents fuel line freeze-ups. Fill the fuel tank to 90%, leaving room for hot weather expansion. Add stabilizer, and then run the engine up to five minutes to fill the fuel lines and carburetor with treated fuel. You can drain the fuel from all our Mikuni after market carburetors, but this isn’t needed with fuel stabilizer.

We now recommend changing the oil in the lower unit every 50 hours or at the end of the season. Remove the lower bleed screw from the drive and let the old gear oil drain. Your BOSS DRIVE takes approximately two and a half quarts of 75w-90 synthetic gear oil.

The best method to refill is to pump gear oil from the bottom bleed screw hole to the top of the drive unit. This prevents air pockets from forming, which prevents you from assuming the drive is full when it’s really not.

Inspect the seals in the seal cap for foreign matter and fishing line. Replace the seals if needed by returning to BOSS DRIVES or an authorized service center for a rebuild. The seals must be installed with special depth gauge tooling.

Paint chips should be sanded lightly and re-finished to avoid corrosion.

Yes, but be careful around others and pets, and do not engage the drive longer than five minutes. Guys, as smart as our hunting dogs and children are, they will walk right into a blurred, spinning propeller. The patented BOSS DRIVES seal system uses a special configuration of seals that prevents the outer most seal from being lubricated other than when running in water. If for some reason you need to run the drive longer than five minutes outside the water,  remove the propeller and spray oil or silicone on the outer seal.

What precautions should I take when operating my engine in Salt Water or Polluted Water? Mud motor engines are not designed to run in salt water, so we need to ensure these suggestions are followed if you run in this type environment. We recommend that you flush the outside of the motor and drive every time you exit salt water. Run the engine at idle and slowly inject a water flow into the flywheel so that the mist will wash off the coils and wires under the engine cowl. If you keep your boat moored in the water, always tilt the outboard so the gear case is completely out of the water (except in freezing temperature) when not in use. Add a corrosion control anode to the boat side of the skeg. Any small configuration works. Each month, spray Mercury Precision Lubricants or Quicksilver Marine Lubricants Corrosion Guard on external metal surfaces and on all exposed wire connections and harnesses. (DO NOT spray on corrosion control anodes as this will reduce the effectiveness of the anodes). If you power-wash the mud motor, do not spray the high pressure into wire harnesses or near oil seal areas.

Mooring and Hunting – When moored in freezing or near freezing temperature, keep the mud motor tilted down at all times so the lower outdrive is submerged. When you park your boat overnight and during hunting, park in a location where the waves are not splashing against the engine. This will cause severe ice buildup on the outdrive and trim assembly. This can prevent your motor from turning and trim can freeze in position. If this happens, don’t hammer the ice off the trim unit since this can break the wires and seal surfaces.

Throttle Cable – If any water enters the throttle cable it will freeze and prevent you from running your motor. If you thaw out the cable with your hands or some other type of heat, I guarantee it will freeze when you are driving and you will have an unpleasant surprise. The throttle will stick and you will need to turn the key off or use the safety kill switch to stop. On short transom boats, the sudden stop will cause a wave to come over the transom.This is unsafe.

Prevent water from entering the cable by elevating both ends all the time. We elevate the engine side cable and you need to elevate the throttle end when you receive and install the handle on new motors. Use a tight wire tie 6″ from handle to elevate the cable slightly.

Remove water in the throttle cable by removing the cable, hanging it vertically and insert anti-freeze. The best way is to slide a tight fitting rubber hose over the throttle cable and pour antifreeze into a container and use the hose to funnel the fluid into the throttle cable until it runs out the lower end. Now install the cable with both ends elevated.

Cover the throttle and handle controls with a plastic bag in freezing rain. The ice will build up and cause the switches to freeze in place. If you hammer the ice from the switches this can cause breakage.Carry a zip-lock, cover, or handle cover when in these freezing rain conditions . Protect your fuel by removing ice and snow from your fuel tank. Shine a flash light into the fuel tank at night and the little diamonds floating in the bottom of the fuel tank are water droplets. Don’t allow ice and snow to enter the tank when refueling. Use moisture dissipating fuel additives in cold weather. One small ice particle in the tank, fuel line, filter or carb will make for a very short or long day on the water.

Carb Freezing can occur when running on mist or foggy water when temperatures fall be low 35 degrees, not 32, 35 degrees. The accelerated damp air moving through the intake system will cause a sudden temperature drop and ice will build up in your carburetor. This will cause loss of power, exhaust smoking, accelerated fuel consumption and a headache. The Vanguard engines have cold weather icing problems; turn the air filter intake towards the exhaust so warm air enters the motor.

If your engine in take freezes, shut off the warm engine and cover with a motor cover, decoy bag, jacket or anything to keep the engine heat on the carburetor. It will thaw out in 10 minutes.

Parking a boat on steep snow covered shorelines can be dangerous. Approach a snow covered bank slowly. The boat will power up the snow covered shoreline easily and when it slides back into the water, the transom can submerge.

Snow and ice is the boat and motor’s worst enemy. Cover your boat and motor when not in use. When the boat and motor become full of snow and ice builds up, it is necessary to garage the boat and completely thaw out all the components. Ice builds rapidly in the boat, adds weight and affects the stability and safe operations of the boat. Ice covered wires easily breaks. We have lots of fun when the weather gets very cold, but preventative maintenance is imperative.

Proper positioning of the weight inside the boat (persons and gear) has a significant effect on the boat’s performance, for example:

– Shifting weight to the rear (stern)

– Generally increases top speed.

– If in excess, can cause the boat to porpoise.

– Can make the bow bounce excessively in choppy water.

– Will increase the danger of the following – wave splashing into the boat when coming off plane.

– Shifting weight to the front (bow) – Improves ease of planing off.

– Generally improves rough water ride.

– If excessive, can make the boat veer left and right (bow steer).

BOTTOM – For maximum speed, a boat bottom should be nearly a flat plane where it contacts the water and particularly straight and smooth in fore -and-aft direction.

– Hook: Exists when bottom is concave in fore-and-aft direction when viewed from the die. When boat is planing, ‘hook ‘ causes more lift on bottom near transom and allows bow to drop, thus greatly increasing wetted surface and reducing boat speed. ‘Hook ‘ frequently is caused by supporting boat too far ahead of transom while hauling on a trailer or during storage.

– Rocker: The reverse of hook and much less common. ‘Rocker ‘ exists if bottom is convex in fore -and-aft direction when viewed from the side, and boat has strong tendency to porpoise.

– Surface Roughness: Moss, barnacles, etc., on boat or corrosion of mud motor gear housing increase skin friction and cause speed loss. Clean surfaces when necessary.

WATER ABSORPTION – It is imperative that you not allow water to sit in your boat during the off season and in storage. Water intrusion in to the transom core and/or inner hull will result in additional boat weight (reduced boat performance), hull decay and eventual corrosion and structural failure.

CAVITATION – Cavitation is caused by water vapor bubbles forming either from a sharp edge or angle on the gear case or from an irregularity in the propeller blade itself. These vapor bubbles flow back and collapse when striking the sur face of the propeller blade resulting in the erosion of the propeller blade surface. If allowed to continue, eventual blade failure (breakage) will occur. Keep debris off the skeg. Hammer impact areas and breaks on the prop back into alignment. Do not grind these off which will reduce the prop diameter.

It is a known fact that weather conditions exert a profound effect on power output of internal combustion engines. Therefore, established horsepower ratings refer to the power that the engine will produce at its rated rpm under a specific combination of weather conditions.

Manufacturers internationally have settled on adoption of I.S.O. (International Standards Organization) engine test standards, as set forth in I.S.O. 3046 standardizing the computation of horsepower from data obtained on the dynamometer, correcting all values to the power that the engine will produce at sea level, at 30% relative humidity at 70 degum (20 Dodge) temperature and a barometric pressure o f 29.61 inches of mercury.

Summer Conditions of high temperature, low barometric pressure and high humidity all combine to reduce the engine power. This, in turn, is reflected in decreased boat speeds- -as much as 2 or 3 miles-per-hour (3 or 5 Km per-hour ) in some cases. Nothing will regain this speed for the boater, but the coming of cool, dry weather.

In pointing out the practical consequences of weather effects, an engine- -running on a hot, humid summer duma encounter a loss of as much as 14% of the horsepower it would produce on a dry, brisk spring or fall day. The same is realized when our motors run over 3000 feet elevation.

The horsepower, that any internal combustion engine produces, depends upon the density of the air that it consumes and, in turn, this density is dependent upon the temperature of the air, its barometric pressure and water vapor (or humidity) content, plus elevation.

Accompanying this weather – inspired and altitude loss of power is a second but more subtle loss. When designed, the engine was equipped with a propeller that allowed the engine to turn within its recommended rpm range at full throttle. With the coming of the summer weather and the consequent drop in available horsepower, this propeller will, in effect, become too large. Consequently, the engine operates at less than its recommended rpm.

Our motors and propellers run best in cool weather. Just so happens many of us are duck hunters and our loads increase during the fall and we need every bit of this power. Winter and spring generally produce the best weather for maximum power. Summer however, is different.

Most operators realize this and generally eliminate some of the load to regain lost power or live with it during warm weather. Others, strap on a slightly worn propeller with smaller diameter which in most cases increases RPM and boat speed.

Boss Dr ives have a neutral safety start switch located under an access cover on the top of the transmission on the Boss Drive. If the switch is out of adjustment it will start in gear or may not start at all. See the owner’s manual and adjust the safety switch. In an emergency, the owner’s manual has procedures to bypass the switch. There are also emergency trim and shift procedures in the same section.

Your propeller is affected by wear. You will see decreased acceleration, poor hole shots, inability to carry a large load and your speed will decrease. Check your propeller by measuring the outside distance, diameter of the propeller. If you are running a 12 X 10 Big Blade for example, the 12 is the diameter and the 10 is the pitch or angle of the blade. If you measure the propeller and it has 1 /4″ or more decrease in diameter, then replace the blade.

The one most single thing you can do to prevent your motor from shaking to pieces, loose bolts, cracks and gasket leaks is to ensure your propeller is not worn, bent or out of balance. If so, replace it immediately. Check by revving the engine to full throttle in a safe manner. Observe the skeg and front o f the engine. If it begins to vibrate excessively after 2500 rpm, and especially at 4300 rpm, change the propeller immediately.

Lubrication is important in a marine environment. Lack of it can cause premature wear and early machine failure. See your owner’s manual for specifics. If your unusual or harsh environment, lubricate more often and remove any signs of corrosion. Lubricate the throttle cable at both ends with grease, to include the slide mechanism on the carburetor. Lubricate all swivel points with marine grease which you can purchase at any auto parts store {marine wheel bearing grease is best}. Lubricate the propeller threads or hex slip shaft with marine grease or never seize. Your drive is filled with approximately 2 ½ quarts of 75w-90 synthetic gear oil. It is recommended to change the gear oil after 50 hours or at the end of the season, whichever comes first. Refer to your owner’s manual for re-fill specifics.

A rather common failure of the large block vanguard engine is the gasket area on the PTO cover (located on the back of the engine). This is caused from excessive vibration from a worn or out of balance propeller. Or even a bent drive shaft. Check the PTO cover bolts every 20 hours and retighten if necessary. Use 242 thread locker on loose bolts. Do not over tighten. If you have a major leak, change the gasket or see a Vanguard repair center for assistance. You have a three year on the 3500 and 3700 EFI and 1 year on BOSS 4400.

Yes, indeed. Performance mufflers like the BPS single crossover and Q mufflers have larger internal pipes and measured pipe lengths that increase exhaust flow.

Trim until you feel the tiller torque or boats low down. If you are near a rev limiter, then trim down a little.

For the best hole shot results, you should be close to the running position. This will be greatly affected by the boat you are driving. You need to let the surface drive prop “breathe” and develop RPM, then trim down if necessary.

Typically, a two blade prop will have a better hole shot and mud performance. In comparison, usually the 3 blades will have more speed. However, the Raptor propeller offered by BOSS DRIVES is the most advanced 3 blade prop. Our testing shows the hole shot and load carrying capability will compete right up there with the best two blades. It offers a higher top end and less vibration, yet still performs like a BOSS in the mud.