"Dreams can't come true without first dreaming...there is no harvest without first sowing seed"
James J Steele

Mercury 40 HP Four Stroke

40 ELPT Command Thrust Mercury® FourStroke


We choose the Mercury 40 HP Four-Stroke for several reasons, the biggest being the extra cost to jump to the next level $$$, the second being trying to minimize the weight that we were pulling with the 2007 Toyota Tacoma, and finally, many of the rivers and runs we will be visiting are ‘No Wake’ spring rivers. I continue to read the advice from everyone saying to get the biggest you can afford…that is what we did, both $$ wise and weight-wise. So, now it’s time to learn about the engine’s use, capabilities, break-in, maintenance, and troubleshooting…

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Engine Details ~ Break-In Procedures ~ Performance on the Bass Buggy 18 ~ Troubleshooting

Engine Details…

40 HP Mercury Four Stroke
40 HP Mercury Four Stroke

High-displacement, four-cylinder, single-overhead-cam design delivers optimum power and a smooth ride. Great for aluminum tiller, aluminum console, inflatable rigid hull (RIB) and soft hull, watersports, and fish & ski boats.

Specifications – 40 EFI 4-Cylinder…

  • HP / kW – 40 / 30
  • Engine type – Inline 4
  • Displacement (CID/CC) – 60.8 / 995
  • Full throttle RPM – 5500-6000
  • Fuel induction system – 2 valves per cylinder, single overhead cam (SOHC)
  • Alternator amp / Watt – 18 amp / 226 watt
  • Recommended fuel – 87 octane / up to 10% ethanol
  • Recommended oil – Mercury FourStroke 25W40 Marine Oil
  • Engine protection operator warning system – SmartCraft Engine Guardian
  • Compatible with SmartCraft digital technology – Yes
  • Starting – Electric (turn-key)
  • Steering – Big Tiller
  • Shaft length – 20″ / 508 mm
  • Gearcase ratio – 2.33:1
  • Dry weight *Lightest model available – 260 lbs / 118 kg
  • CARB star rating – 3
  • Bore and stroke – 2.56 x 2.95″ / 65 x 75 mm
  • Ignition – ECM 07 Digital Inductive
  • Fuel system – Electronic Fuel Injection (EFI)
  • Cooling system – Water-cooled with thermostat
  • Gear shift – F-N-R
  • Gearcase options – Command Thrust
  • Trim system
    • Gas assist
    • Power Tilt
    • Power Trim
  • Exhaust system- -Through prop
  • Shallow water trim range (degrees) – 20
  • Remote fuel tank (optional) – Yes
  • Color – Phantom Black

Engine Break‐in Procedure…

image-engine-breakdownIMPORTANT: Failure to follow the engine break‐in procedures can result in poor performance throughout the life of the engine and can cause engine damage. Always follow break‐in procedures.

  1. For the first hour of operation, run the engine at varied throttle settings up to 3500 RPM or at approximately half throttle. What we did, starting at 1500 RPMs, ran it for 6 minutes, then increased the RPMs by 500, ran it for six minutes and did this increase up to 3500 RPMs, then did the same procedure backing down from 3500 to 1500 RPMs.
  2. Do the same procedure as above for the second hour of operation, running the engine at varied throttle settings up to 4500 RPM or at the three‐quarter throttle, and during this period of time, run it at full throttle for approximately one minute every ten minutes.
  3. For the next eight hours of operation, avoid continuous operation at full throttle for more than five minutes at a time. Our WOT is 5700 RPMs.

Performance on the Bass Buggy 18 DLX…

The figures below are a guide based on two performance tests of different individuals and locations but can be used as a guide when planning trips…

Mercury 40 HP 4 Stroke Performance Results

WOT 5500 – 6000 (Wide Open Throttle )

Rpms    Mph    Knots    Gph    Mpg   Nmpg     St.Mi/gal    NM    dBa
700        1.8         1.5          0.2      8.75      7.61           276          240      65
1000      2.2        1.9          0.3      8.8       7.65           277           241      67
1500       3.3        2.9         0.4      9.43      8.2            297          258      67
2000      4.5        3.9         0.5     10          8.7            315           274      68
2500       5.3        4.6        0.7      8.15       7.09          257           223      68
3000      6.5        5.6          1       6.79        5.9            214          186       69
3500       7.4        6.4         1.3     5.88        5.11           185           161      75
4000      8.6        7.4         1.6     5.34        4.65          168          146      77
4500      10.8       9.3        2.2 5   4.35        4.35          158           137      80
5000      13.1       11.3        2.8     4.75        4.13           149          130      81
5500       15.13     13.3        3.3     4.62        4.01          145          126      81
5800       16.8      14.6       3.8    4.41        3.83           139          121       84



Engine Losing Power ~ Engine Overheating ~ Engine Won’t Start ~ Engine Suddenly Stops ~ Prop Vibration ~ Unable to Shift Gears ~ Fix Steering ~ Trim is Stuck

Boat Engine is Sputtering and Losing Power

  • Gas Tank is Empty
  • Filter Problem
  • Fouled Plugs
  • Gas Tank’s Air Vent is Not Open
  • Fuel Lines Are Obstructed or Bent
  • Fuel Connector Has Become Loose

Solution: Replace the in-line fuel filter. Carry an extra. If not, remove and clean the filter element of any debris, and drain any accumulated water.

Prevention: Leaving a tank near empty for long periods of time can cause condensation and water in the gas. For long-term storage, fill the tank, and for periods of more than three months, consider a fuel stabilizer. If so, run the boat long enough to get the treated gas into the engine as well.

If it isn’t the gas, it might be the spark plugs. Carry spares, along with the tools to change them.

Carry Onboard: Spare filter or filter element and a filter wrench.

What to Do if Your Engine Is Overheating

The needle on the temperature gauge is rising. This almost always means you have a lack of water flow in the cooling loop. If that water stops flowing, the engine heats up and can fail.

Solution: Trace the source. In a vast majority of cases, the problem is an obstruction in the raw water intake – like weeds, mud, or a plastic bag. Locate the intake and clean it out.

A loose hose clamp or a split or burst hose can also slow water flow, and it can spray damaging moisture around the engine.

Prevention: Regularly service and replace the impeller. Also, look at the condition of its housing. Scarring or pitting of the metal housing can cause even a good impeller to lose pumping power.

Every so often, have the exhaust risers and associated components opened up for inspection.

Engine Won’t Start

This is most likely an electrical issue — a low or dead battery, or a break somewhere in the ignition circuit.

Solution: Check the kill switch. Make sure the shifter is in neutral. Then pay special attention to the starter switch itself. Sometimes, a dash-mounted ignition switch will simply become loose in its fitting, allowing the entire switch mechanism to turn with the key. Fixing this can be as simple as getting behind the dash and tightening up a retaining nut or mounting screws. If the starter groans but won’t engage, it could be a low battery, but it also might be a loose or poor connection.

Prevention: Inspect, clean, and, if necessary, replace your wiring periodically. If your crew habitually drains the battery by cranking the tunes while at anchor, consider installing a secondary battery bank or one of those metering devices that monitors supply and saves enough reserve to ensure a restart.

Carry Onboard: Screwdrivers with insulated handles; wrench set or crescent wrench; Allen wrenches. A battery charger is also good to have on hand. Clean the connections on your marine battery with a wire brush.

Your Boat Motor Stopped Suddenly

Check the kill switch. Or you could be out of fuel. If neither of these checks out, this usually represents some type of electrical failure. It could be a blown fuse or tripped breaker, a loose connection, or corrosion.

Solution: Start with simple scenarios. On any boat equipped with a kill switch and lanyard, make sure the lanyard key hasn’t come loose. Sometimes, it might seem to be engaged but has actually slipped just enough to activate the switch.

Ignition switches can also fail or suffer loose connections, and though this will most likely show up at start-up, it’s worth fiddling with the switch a bit (and checking its attendant breaker or fuse) before moving on to the engine side of things.

Back at the business end, where the big wires live, corrosion is your most likely source of problems. Even boaters who contentiously maintain the battery terminals might forget that there’s another end to those wires, and they also require the occasional cleaning.

If it turns out to be something more complex — such as an ignition chip on an EFI engine — you might have to pull out the cell phone or put out a call on channel 16.

Prevention: Learn the various components of the ignition system, and periodically inspect, clean and coat each exposed connection with an anti-corrosion product.

Take a look at the main fuse on your outboard motor. This is usually held in a big red holder on the engine’s wiring harness. In most cases, it will be a 20-amp fuse that you can replace cheaply and easily.

Carry Onboard: Wire brush to clean terminals and Corrosion X spray.

How to Fix Vibration from the Engine Prop

The faster you try to go, the worse the vibration is. You might also notice the engine racing, while the boat loses speed.

Solution: Something’s likely gone wrong with the prop. A nick or gouged blade can create imbalance and vibration; a towrope or fishing line can snarl the shaft; a direct hit on an object could remove or misshape enough metal to make the prop ineffective.

Sometimes a seemingly good prop might have enough unseen distortion or damage to cause cavitation and vibration. Short of changing to a spare prop — which isn’t always possible or advisable when on the water — your best option is to slow down and concentrate on getting to shore.

If the line — especially monofilament — has worked its way into the prop hub, you might have to trim up the motor until you can remove the prop and clean it out. Most outboards and I/Os can stand a bit of mono, but if there’s enough to cause a noticeable decrease in performance, you shouldn’t ignore the problem, as it could lead to permanent damage.

With outboards, the rubber bushing inside the hub can begin to slip and fail, causing a loss of power. Again, you might need to idle home.

Prevention: Consider carrying a spare prop, along with the necessary tools to make the swap. Practice changing props so there are no surprises if you have to do it away from home.

Carry Onboard: Gloves to protect the hand from prop blades and a brand-specific prop wrench.

Your Engine Won’t Shift into Gear

You pull away from the dock and push the shifter. The boat never leaves idle speed. The shifter is not engaging the transmission.

Solution: If you have e-link electronic controls, it might be a fuse. But, since 90 percent of small boats still use mechanical cable shifts, it’s probably a stuck or broken linkage. Start at the gearbox to make sure the cable hasn’t become detached from the shift lever on the transmission housing. If internal corrosion has caused the cable to stick, try wiggling it free — or if need be, shift manually at the engine/transmission — just don’t try any fancy docking maneuvers. If the problem seems to be on the transmission side of the linkage rather than the cable side, it might be an actual transmission failure — there’s probably nothing you can do out on the water. Major boat transmission problems require work at an engine mechanic.

Prevention: The leading cause of transmission failure is lack of fluid or gear oil, so keep those levels topped off and changed as prescribed. Regularly maintain the end fittings and hardware, and periodically service the cable.
Check your boat’s fluids and gear oils regularly

Carry Onboard: Extra transmission fluid and wire, tie wraps, and J-B Weld for quick linkage repair.

Fixing Steering in Your Boat

When you turn the wheel, the engine/outdrive doesn’t turn. Or, it’s frozen in place.

Solution: Most likely, the steering system is low on hydraulic fluid, or it has sprung a leak. Add fluid as needed to get the system working. If you do notice fluid seeping out of the console or a fitting near the motor, see if you can tighten the fitting. If the drive is frozen in place, it could be a mechanical failure — possibly a loose connection on the steering arm. On boats with full mechanical steering, the glitch could involve any part of the cable system. Tracing the lines is the best way to find the problem.

Prevention: Check the steering fluid level periodically, and lubricate and service mechanical systems.

A bilge pump protects against rising water in your boat.

Carry Onboard: An extra jug of hydraulic fluid and a small funnel.

The Trim Is Stuck on Your Engine

You’re back at the ramp and the outdrive/outboard won’t raise so you can get the boat on its trailer and ready for the highway.

Solution: Assuming it’s not a bad fuse, it’s some sort of mechanical/hydraulic problem. The simple solution is to wade out back and raise it by hand. To do this, you’ll need to know the location of the trim release valve, which is usually a small screw near the base of the outdrive/outboard. Opening this valve will release pressure from the hydraulic loop, allowing the drive to tilt.

Prevention: Maintain adequate fluid levels and inspect periodically to ensure there are no leaks or water intrusions into the fluid reservoir.

Carry Onboard: Large slotted and Philips head screwdrivers to open the release valve.

The No. 1 reason people call for towing assistance is they run out of gas. Make sure your boat’s fuel gauge is accurate — or plan accordingly if it’s not. In addition, knowing a bit about your boat’s fuel burn and operating range could save you from guessing.

Additional Mercury Resources…

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