Alternative Fuels For Your Two-Stroke Engine

Two-stroke engines have a reputation for being high-maintenance, but they deliver a powerful performance. They are simple machines, and anyone with basic mechanical knowledge should be able to rebuild them. Two-stroke engines require a mixture of oil and petrol to function, and modern two-stroke road bikes have a separate oil tank that automatically mixes the two fluids in the correct ratio. It is important to use good-quality oil and to always warm up the engine before use.

Oil and petrol ratios

Two-stroke engines are known for being temperamental. To keep them running smoothly, it's important to get the correct ratio of oil to petrol. Two-stroke engines don't have an internal oil reservoir, so lubrication is provided by the oil being mixed directly into the petrol.

The precise ratio of oil to petrol depends on the make and model of your equipment, so it's important to check the manual that came with your machine. If you can't find the specific ratio, 40:1 is a commonly accepted average ratio for two-stroke engines. This equates to 125mls of two-stroke oil to 5L of fuel, or 25mls of oil to 1L of petrol.

It's important to get the ratio right. Too much oil may stop the engine, generate carbon buildup, and blow smoke, which is bad for the environment. Too little oil will, over time, permanently damage your engine and cause it to overheat.

You can use a two-stroke fuel calculator to work out the exact amount of oil to add to your petrol. It's also important to use a good-quality, low-smoke oil to ensure a clean-burning engine.

Oil types

Two-stroke oil, also known as 2-cycle oil or 2T oil, is a specific type of motor oil designed for use in crankcase compression two-stroke engines, which are commonly found in small gasoline-powered engines. Unlike four-stroke engines, two-stroke engines use the crankcase as part of the induction tract, requiring oil to be mixed with gasoline to provide lubrication throughout the engine. This mixture is known as premix or petroil.

The base oil used in two-stroke oil can be petroleum, castor oil, semi-synthetic, or synthetic oil. The oil is mixed with petrol/gasoline at a volumetric fuel-to-oil ratio ranging from 16:1 to 100:1. Modern two-stroke engines, particularly those powering garden equipment and chainsaws, may require synthetic oil to avoid high emissions and oily deposits on spark plugs.

One key difference between regular lubricating oil and two-stroke oil is that two-stroke oil must have a lower ash content to minimise deposits that can form when the oil burns in the engine's combustion chamber. Additionally, non-2T-specific oil can turn to gum if mixed with gasoline and not immediately consumed.

Since the 1980s, different types of two-stroke oil have been developed for specialised applications, such as outboard motor two-strokes and motorcycle two-stroke oil. "Racing" oil or castor-based oil offers excellent lubrication but may require more frequent maintenance.

Some common additives for two-stroke oils include detergent/dispersants, antiwear agents, biodegradability components, and antioxidants (zinc compounds). Higher-quality oils may also include a fuel stabiliser.

Engine health

Two-stroke engines have a reputation for being temperamental. They can be running well and then suddenly break down. However, this is often not the case, as there are usually warning signs that something is wrong.

Observing Symptoms

One of the most obvious signs that something is wrong with a two-stroke engine is when it struggles to start. This could be due to carburetion or injection issues, but it could also indicate a problem with the cylinder. Worn piston rings or reed valves that are no longer sealing properly can cause the engine not to build up good compression, making it hard to start.

Inconsistent Performance

If the engine struggles to hold a tune or needs constant attention to the jetting, it could indicate a problem with the carburetor or spark plug. However, the more serious issue could be with the engine seals or gaskets. Stator side crank seals, leaking base gaskets, or intake manifold gaskets can all result in air leaks, leading to a lean air-fuel ratio. This can cause excessive combustion temperatures, which can melt a hole in the piston or seize it in the cylinder bore.

Gearbox Oil Consumption

Gearbox oil loss is abnormal and should always be investigated. It is usually due to leaking seals or gaskets. If the bike has tipped over or cartwheeled, gearbox oil can exit through the gearbox/crankcase breather. If the leak path is not external, the drive side crankshaft seal may be leaking, allowing gearbox oil to migrate into the crankcase and burn in the combustion chamber.

Excessive Smoke After Warm-Up

While some smoke is normal for a two-stroke engine, excessive smoke after the engine has warmed up could indicate a problem. Blue smoke may indicate that gearbox oil is burning in the combustion chamber, while white smoke may indicate that coolant is burning.

Excessive Coolant Loss

It is normal for coolant to exit the overflow tube when the bike has been tipped over or has overheated. However, if it is happening regularly, it could indicate a leaking head gasket. Dribbles of coolant around the coolant pump indicate a faulty water pump seal, which, if left unattended, can cause overheating and significant damage.

Excessive Top-End Noise

A "metallic slap" sound is common in two-stroke engines and is known as piston slap. It is caused by the piston rocking back and forth in the cylinder bore. While this is normal, if the intensity increases, it can indicate excessive piston slap, which can damage the piston and weaken the skirts.

Diagnostic Checks and Tests

Coolant contaminated with black specks can indicate a leaking head gasket or o-rings. Additionally, the composition of the gearbox oil can provide clues about the engine's health. Milky oil indicates moisture intrusion, while the presence of metallic particles suggests accelerated wear on internal parts.

To further diagnose potential issues, cylinder leak-down testing, compression testing, and crankcase leak-down testing can be performed. These tests help assess the health of the piston rings, cylinder bore, cylinder head seal, and various seals and gaskets.

Carburetor issues

Lean Air-Fuel Mixture:

• Symptoms: Mild backfiring when going downhill, unsteady or abrupt acceleration, excessive choke during starting, white or light grey spark plugs, excessive or white smoke.
• Causes: Low fuel level in the carburetor bowl, faulty inlet manifold sealing.
• Solution: Adjust the fuel level as per the user manual. If the problem persists, inspect the sealing and jet tubes, and consult a mechanic if necessary.

Rich Air-Gasoline Mixture:

• Symptoms: Fewer miles per gallon of gasoline, sluggish or inactive acceleration, starts in cold without a choke, strong smell of gasoline when idling, uneven running, black or smoke-colored spark plugs, black smoke from the exhaust.
• Causes: Dirty or clogged air filter, high fuel level in the float chamber, worn-out float valve needle, overflowing float chamber due to a stuck valve assembly.
• Solution: Clean or replace the air filter, adjust the fuel level, and inspect the float valve and valve assembly as per the user manual. Consult a mechanic if issues persist.

No Gasoline in the Carburetor:

• Check the fuel tank and its vent valve. Clean the vent valve if blocked.
• Inspect the fuel line from the tank to the carburetor. Clean or replace the fuel line if necessary.
• The float assembly and float valve may be stuck. Refer to the service manual for guidance, and consult a mechanic if needed.

Engine Flooded with Gasoline:

• This can be due to a high fuel level in the float chamber, a worn-out float valve needle, or an overflowing float chamber caused by a stuck valve assembly.
• Solution: Adjust the fuel level, inspect and replace the float valve needle if necessary, and ensure the valve assembly is functioning correctly.

Carburetor Out of Adjustment:

• Symptoms: Poor engine performance, improper or erratic acceleration.
• Causes: Clogged jet tubes, stuck jet needle, stuck float valve, faulty gaskets or diaphragms.
• Solution: Clean or replace clogged or stuck components, and replace gaskets or diaphragms if necessary.

Engine Knocking:

• Causes: Incorrect air-gasoline mixture, carbon build-up inside the engine, improper ignition timing.
• Solution: Check and adjust the air-gasoline mixture as per the service manual. If the problem continues, clean the carburetor and engine head.

It is important to consult the user manual and service manual for specific instructions and guidelines when troubleshooting and performing any maintenance on your two-stroke engine.

Worn piston rings

Another issue that can arise from worn piston rings is increased oil consumption. This is because the oil can migrate into the crankcase and be transferred up into the combustion chamber during the scavenging process, where it is then burned. This can also lead to excessive smoke after warm-up, as the gearbox oil burns in the combustion chamber. While some smoke is normal for two-stroke engines, excessive smoke can indicate a problem.

To diagnose issues with worn piston rings, several tests can be performed, including a cylinder leak-down test, a compression test, and a crankcase leak-down test. The cylinder leak-down test pressurizes the engine's combustion chamber and compares the pressure going in with the pressure retained, allowing you to identify any leaks. A compression test can also help identify low-pressure readings, which may be due to problematic piston rings. Finally, a crankcase leak-down test can assess the sealing integrity of the crankcase and cylinder, helping to isolate any problematic seals and gaskets.

Frequently asked questions