Engine overheating

by Had Robinson
updated January 10, 2020

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.  In some cases, the high temperatures will burn a hole right through the top of the piston.  The pilot may notice a sudden drop in RPM.  While the engine may run after it cools down, the cylinder is damaged enough so that power output is dramatically less.

Here is an overheated spark plug from a Top 80.  The engine was fuel starved.  The insulator is almost pure white rather than a medium brown color.  The engine was not damaged.

overheated spark plug

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 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.  Full synthetic oils, for example, must get much hotter than ordinary oils before they will begin to burn/cake – and why pilots would be wise to use full synthetic oils.

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

normal Top 80 piston

overheated piston

melted overheated piston

Here is an overheated piston.  Note the black (burned) oil in the lands.  This causes ring sticking and severe wear of the cylinder walls of the engine.

top 80 piston with burned oil in the lands

Same piston on the inside.  The charred oil underneath and the sunken crown (seen above) of the piston are indicators that this engine overheated badly.  The pilot had a cheap inline fuel filter which allowed contaminants to partially clog the inlet valve filter screen inside the carburetor just enough so that the engine could run at full throttle but not experience the cooling effect of a rich (and correct) fuel mixture.  When fuel starvation gets too severe, the engine will not run at or near full throttle and will not overheat.

burned top 80 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 in the middle 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 black/brown goo.  If the piston gets overheated quickly, the top may get a hole burned right through it, as in the last photo.

Overheating may cause the piston ring to 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 cannot 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.  Red Line (an oil manufacturer) notes this 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 a little of the oil on the underside but only in a small spot near the middle of the piston.  This problem can only be observed if the piston is removed from the connecting rod or the cylinder removed and a mirror and light used to look up inside the piston.  It is always better to remove the piston because the connecting rod and bearing may hide signs of overheating.

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, the usual cause, be prevented? 

Here are the most common causes of overheating:

1. Incorrectly adjusted metering lever – This is the number ONE problem that can cause engine overheating.  The Walbro carburetor Rebuilding and tune up part 2 page will give the directions on how to fix this problem.  The default settings of parts in rebuild kits are often not correct.

2. Fuel pump diaphragm failure – Using ethanol fuel blends greatly accelerates the deterioration of the material used in the diaphragm and stiffen it.  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.  It makes no difference whether the engine was run or not.  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.

3. 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.  I recommend that pilots make a 6mm (1/4") hole in the pickup tube filter.  Here are the instructions on how to do this. This will forever prevent this filter clogging.  If a quality inline fuel filter is used, the pickup tube filter is NOT needed.  The filters, especially, the inline filter, must be examined carefully if fuel starvation is suspected.  A quick system fuel test can also be performed.

4. Clogging of the fuel pump port/tubing to the crankcase – The fuel pump is operated by pressure pulses created in the engine crankcase as the piston goes up and down.  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 that use the WG carburetor 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.  MAKE CERTAIN THAT THE LENGTH OF THE ENTIRE PULSE PASSAGEWAYS UP TO THE CARBURETOR ARE CLEAR.  I advise increasing the size of these passageways to help.  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 often goop everything up and the small port going to the crankcase gets clogged and the pump fails or operates poorly.

5. Loose reed valve body (all owners of the Top 80) – On occasions, the reed valve body can be insufficiently torqued down against the engine crankcase either from an assembly error at the factory or because of operation.  If the reed valve body is not properly attached to the crankcase, the pulses from the crankcase to the fuel pump will vent to the outside and the fuel pump will fail to work properly.  I estimate that about one in twenty motors have this problem.  For more detail, go to our page on the reed valve.

6. Incorrect jetting – The WG 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 used in many engines has an adjustable main jet.  Pilots who tinker with this adjustable needle jet are asking for trouble.  Leave it alone unless you install a CHT and then ONLY make adjustments by 1/8 turn and carefully follow the instruction manual that came with the engine.

7. Incorrect fuel/oil mixture – The instruction manuals from the various manufacturers warn about increasing the amount of oil added to the fuel.  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 and passageways 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.

8. 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 reaches the engine and it stops.  Typically, pilots will see bubbles or fizz where the fuel exits the inline fuel filter.  It looks like an air leak but it is gasoline that changed from a liquid to a gas.  Unfortunately, most paramotors have a fuel pump at the carburetor and the fuel tank is located 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 t.

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