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Engine overheating

by Had Robinson

Overheating in paramotor engines is almost always caused by fuel starvation when the engine is run at high loads (1/2 throttle or above).  Rarely, it can be caused by the addition of too much oil to the fuel mixture or failing to mix oil with the gasoline.

When too much air and not enough fuel enters the cylinder, the temperature of combustion is much higher than normal.  The engine cooling system is unable to do its job and the engine temperature increases.  As it increases two critical things happen: the piston begins to expand and the lubricating oil mixed with the gasoline begins to burn in the wrong places.  As the temperature increases, the piston can expand enough so that it seizes in the cylinder.  This causes irreparable damage to both parts.  The pilot may notice a sudden drop in RPM.  While the engine may run after it cools down, the cylinder is damaged enough that power output is dramatically less.

In a normally operating engine, the cylinder wall and piston are cool enough so that the lubricating oil will not burn when it contacts either surface.  Because the piston has no way of conducting heat except by contact with the cylinder, it is always the hottest of the two parts and where we will first see evidence of overheating.  The oil must not get too hot in order to do its job: lubricating the piston as it moves up and down in the cylinder.  When combustion temperature exceeds a certain value, e.g. 300ºC, the oil burns and can no longer lubricate the moving parts.  In addition, the piston cannot cool fast enough and will begin to expand.  At this point, damage occurs.

Here are photos of some pistons, one normal and two that overheated.

normal Top 80 piston  overheated piston  melted overheated piston

When an engine is functioning normally, the fuel oil mixture burns inside the cylinder but not on the actual metal surfaces.  As the piston on the right heated up, the oil on its surface (and, to a lesser extent, on the cylinder wall) became scorched and started burning on the surface of the piston.  Instead of oil lubricating the surfaces, there was just brown gunk.  If the piston gets overheated quickly, the top may get a hole burned right through it.

In addition, the piston ring may stick in its lands (groove).  That is, the overheated, scorched/burnt oil will form a type of glue between the piston ring and the lands.  If the piston ring is no longer free to move about the lands, it can't do its job of sealing the combustion chamber, especially when the fuel air mixture is ignited by the spark plug.  Overheating is the #1 cause of piston ring sticking, regardless of the type of fuel or oil used, as Red Line (an oil manufacturer) notes in one of their technical documents,

The time indicated is the time required for the lubricant to decompose to a sticky mass capable of sticking a two-cycle piston ring.

Often, the top of the piston during normal operation will get hot enough to harmlessly burn the oil on the underside but this can only be observed if the piston is removed from the connecting rod.

If the piston and cylinder wall are not lubricated properly, they will quickly wear out and fail.  It is very important that pilots do not let their engines overheat.  In other words, how can fuel starvation be prevented? 

Here are the most common causes:

1. Fuel pump diaphragm failure -- Using ethanol fuel blends greatly accelerates the deterioration of the material used in the diaphragm.  The pump will still work but its output declines.  When the engine is run at full throttle, the pump cannot keep up and the engine leans out.  We recommend that the carburetor be rebuilt and the metering lever spring replaced at least once a year.  This (and changing the spark plug) is the most important routine maintenance task on a paramotor.  Rebuild kits and metering lever springs are available from Miniplane-USA.

2. Clogged fuel filters -- There are three filters on the Miniplane engines (others similar): one at the bottom of the pickup tube in the fuel tank, one inline filter near the carburetor, and a very fine screen filter between the fuel pump and metering diaphragm chamber in the carburetor.  The filter on the end of the fuel tank pickup tube is the most subject to clogging, especially if ethanol fuels are used.  Ethanol is a powerful solvent which also attracts water.  It can form gooey gels that will quickly clog this filter.

3. Clogging of the fuel pump port/tubing to the crankcase -- The fuel pump is operated by the pressure pulses created in the engine crankcase.  Some engines have a small piece of tubing that goes from the crankcase to the fuel pump.  It is rare that these engines experience failure of the tubing and fittings that transmit the pulses.  Other types of engines (like the Top 80) have a port through the carburetor and the reed valve that goes to the engine crankcase.  For the latter, incorrect installation of the gasket that goes between the carburetor and the reed valve will cover the hole that goes to the crankcase.  The engine will still run, even without a fuel pump -- but barely.  As soon as the pilot goes to full throttle, the carburetor is unable to deliver enough fuel to the engine and it leans out.  Another common cause is the overuse of sealants.  More is not better.  The mating surfaces of the carburetor do not need any sealant, anyway.  However, pilots will goop everything up and the small port going to the crankcase gets clogged and the pump fails.

4. Loose reed valve body (Top 80) -- On rare occasions, the reed valve body can be loose either from the factory or because of operation.  If the body is loose, the pulses from the crankcase to the fuel pump will vent to the outside and the fuel pump will fail to work properly.  For more detail, go to our page on the reed valve.

5. Incorrect jetting -- The WG-8 has a fixed main jet which works well at sea level but not at high altitudes (HA).  If a pilot changes out the jet for a HA jet to fix a rich mixture at above 4,000' MSL, then goes back to low altitude BUT forgets to put the original jet back in, he can quickly overheat his engine.  On the other hand, the WB-37 used in many engines has an adjustable main jet.  Pilots who tinker with this adjustable jet are asking for trouble.  Leave it alone unless you install a CHT and then ONLY make adjustments by 1/8 turn or according to the instruction manual that came with the engine.

6. Incorrect fuel/oil mixture -- The instruction manuals from the various manufacturers warn about changing the fuel/oil mixture.  They note that too much oil in the gasoline does nothing to prolong the life of the engine.  Instead, it causes the engine to run hotter because the fuel has a higher viscosity and does not move through the jets in the carburetor at the same rate as fuel with less oil.  The result is that the fuel/air mixture is leaner and the engine runs hotter.  If a lower ratio mix is used, the jetting needs to be adjusted accordingly.  On the Top 80, the high speed jet (the most important one) is not adjustable so the increased oil in the gasoline will lean out the mixture and possibly overheat the engine.  Generally, pilots must stick closely to the manufacturer's instructions on the type and amount of oil that should be mixed with the gasoline.  If you make changes, be aware of the consequences.

7. Use of winter blend gasoline -- Most are unaware that the gasoline refineries tweak the formulas for winter and summer gasoline in order to cut costs.  It is does not matter what grade of fuel is used.  The winter blends have a lower boiling point than the summer blends.  Using a winter blend of gasoline during the summer or hot weather can result in vapor lock -- and a leaning out of the air/fuel mixture.  It can be so severe that no fuel may reach the engine and it will stop.  Typically, pilots will see "fizz" where the fuel exits the inline fuel filter that is high up on the engine.  It looks like an air leak but it is gasoline that that has reached its boiling point.  Unfortunately, most paramotors have fuel pump at the carburetor and locate the fuel tank well below the engine so that there is a significant vacuum "head" on the fuel that does not occur in automobiles.  The latter have the fuel pump sitting in the bottom of the fuel tank so that the gasoline is always pressurized which ensures that it is unlikely to boil.  AVGAS, of course, is manufactured for all temperatures and will not have this problem -- another reason to spend the extra money for quality fuel.  Nonetheless, even AVGAS will fizz if it gets hot enough and the pressure is low enough.

Fuel starvation and ignition failure have common symptoms but must not be confused.  The page on performance issues will help solve many problems with the engine.