Petrol Engine for a Motor Pump or Generator: Choosing the Right Power and Maintaining It

On a farm, in an orchard or on a site far from the power grid, the petrol engine remains the most self-sufficient way to run a motor pump, a sprayer or a generator. But an undersized engine struggles and wears out quickly, and a poorly maintained engine ends up refusing to start at the worst possible moment. This guide helps you choose the right power for your use and adopt good maintenance habits.

Displacement, power, torque: the basics

Most engines used on motor pumps and small generators are single-cylinder 4-stroke petrol engines with a horizontal shaft and recoil (pull-cord) start. Three figures matter:

• Displacement (cm³): the volume of the cylinder. The larger it is, the more energy the engine can deliver.
• Power (hp or kW): 1 hp is roughly 0.74 kW. It determines the flow rate and pressure a pump can reach.
• The fuel tank: it sets the running time between refills.

A concrete example: the RB20-B petrol engine available at Brico MIM is listed, according to its product sheet, at 183 cm³, 5 hp, a 3.8 L tank, 0.6 L of oil and 15 kg. It is a common size for powering a water pump, a sprayer or small farm equipment.

What engine power should you choose?

For a motor pump

The power required depends on two things: the desired flow rate (m³/h) and the total head to overcome (suction + delivery + pipe losses). A rough formula:

Power (kW) ≈ flow (m³/h) × head (m) ÷ (367 × efficiency), with pump efficiency often close to 0.5 on small motor pumps.

Example: 20 m³/h over 20 m of total head → 20 × 20 ÷ (367 × 0.5) ≈ 2.2 kW, or about 3 hp. You then add a safety margin so the engine does not run at full throttle all the time. Also keep in mind that suction is physically limited: beyond a few metres below the pump, you need to move the pump closer to the water or switch to a submersible pump.

The connection diameter is also a good indicator: our 2-inch (50 mm) motor pumps suit medium-sized surface irrigation and emptying ponds, while the 3-inch motor pump is designed for large volumes (large areas, wells, site drainage).

For a generator

1. Add up the power (in watts) of all the appliances that will run at the same time.
2. For motor-driven appliances (refrigerator, pump, compressor, air conditioner), allow for the starting surge, which can reach 2 to 3 times their rated power.
3. Add a margin of about 20 to 30%.

Rough rule: allow about 2 hp of engine per continuous electrical kW, since the alternator also has losses.

For treating trees, for example, we offer a 22A petrol sprayer with RB20-B engine and 2 × 50 m of hose. All our engines are in the Petrol engines collection; prices are shown on each product page (RB20-B engine from 1 189 DH at the time of writing).

Petrol engine maintenance: the schedule

The manufacturer's manual always takes precedence. As a rough guide, here are the most common intervals for a small 4-stroke petrol engine:

• Before each use: oil level (engine on level ground), air filter condition, fuel, no leaks.
• After the first few hours (running-in): first oil change, often recommended at around 20 hours.
• After that: regular oil changes (often every 50 to 100 hours or every season), with the 4-stroke oil recommended by the manufacturer.
• Air filter: frequent cleaning, much more often in dusty conditions (fields, building sites). A clogged filter cuts power and increases fuel consumption.
• Spark plug: periodic checking and cleaning, replacement if the electrodes are worn.

Starting and storing properly

• Motor pump: always prime it by filling the pump body with water before starting. A pump that runs dry can damage its mechanical seal.
• Fit a strainer at the end of the suction hose to avoid drawing in sand and debris.
• Use clean, fresh petrol; petrol stored for a long time clogs the carburettor.
• For a break of more than a month, empty the tank or run the engine until it stops to empty the carburettor, then store it in a dry, sheltered place.

Safety: rules you must never forget

• Never indoors or in an enclosed space: exhaust fumes contain carbon monoxide, which is invisible, odourless and deadly.
• Refuel with the engine off and cold, without smoking, away from any flame.
• Do not touch the exhaust during and just after operation.
• Wear hearing protection during long sessions.
• Generator and home: never connect a generator directly to your home's electrical installation. The connection must go through a transfer switch installed by a qualified electrician, to prevent any dangerous back-feed into the grid.

Frequently asked questions about petrol engines

My engine won't start, what should I check first?

In this order: fuel tap open and fresh fuel, the choke, the oil level (some engines have a safety cut-out that prevents starting if the oil is too low), then the spark plug and the air filter.

Can I replace the engine on an old motor pump?

Often yes, if the new engine has equivalent power and a compatible shaft (diameter, length, mounting). Compare the dimensions before buying or send us a photo on WhatsApp.

Which pump should I choose with my engine?

See our guide which water pump to choose and our Pumps collection.

Do you deliver everywhere in Morocco?

Yes, Brico MIM delivers to all cities in Morocco, with cash on delivery.