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Troubleshooting your engine

by Had Robinson

"HELP! My engine won't start!" -- "My engine runs poorly!"

If this is what you are thinking or experiencing, you have come to the right place.  Below are the most common issues I have found that cause problems in paramotors.  When looking through the information below, pilots should also reference the respective specifications page for additional data for their motor.  (Even though these pages are specific for the listed motors, the information is largely the same for other paramotors, as well.)  It is always a good idea to do all of the tests as, often enough, there can be more than one issue that is causing problems.  (Note: Pilots must be cautious when using gasoline containing ethanol.  Please follow these recommendations.)  Before performing these easy tests be sure that the low speed adjustments on the carburetor have been done correctly. Here are the links for the WG-8 and WB-37 carburetors.  The WG-8 is generally used on engines that are 130 cc's or smaller, like the Top 80 or Thor 100 or 130.  The WB-37 is used on many larger engines.

A. Fuel issues

It is assumed that you are using the correct fuel and oil at the correct ratio.  Check this first if there is any doubt.  The tests below will ONLY work for diaphragm type carburetors like the WG-8 and WB-37.  If you have a Bing carburetor, you will need to purchase their service manual and do the service yourself or send the engine to us.  The Bing is much more complicated to tune than the Walbro.  The Bing service manual is more a discussion than a technical document.  It needs to be completely rewritten by an actual service technician.

Fuel starvation is the #1 most common problem that I see in the motors coming in here.  It leans out the engine and can destroy it due to overheating.  Symptoms of fuel starvation can be rough idle, fast idle, no idle, and decreased power output (fading) at full throttle.  Damage can include stuck piston rings, scuffed cylinder walls, low compression, and actual holes in the top of the piston.

Quick fuel system tests

To see if fuel is able to pass through all of the tubes, filters, and valves of the fuel system:

  1. Remove the airbox/filter, if it has one.
  2. Tip the engine so that the throat of the carburetor is lower than where it attaches to the engine.  This must be done so that fuel will run OUT of the carburetor and not INTO the engine.  Some engines, like the Minari, cannot be tipped to prevent possible flooding.  In this case, hold the throttle wide open with a piece of tape or string wrapped around the throttle handle.  This will help fuel that runs into the engine evaporate quickly.
  3. Pressurize the fuel tank with the primer tube and hold the pressure with the tip of your tongue.  If you have a primer bulb, pressurize the fuel system by squeezing it several times.  You must maintain pressure on the bulb to do this test.
  4. Press down and hold the primer lever on the carburetor.  (With the Minari, only hold the primer lever just long enough to see if fuel begins to dribble out of the holes in the throat of the carburetor and down into the engine.)
  5. Fuel should dribble in a steady stream out of the carburetor and out of the airbox/filter, if the engine has one.  (For the Minari, leave the throttle wide open for another 10 or 20 minutes while you are doing other tasks so that the excess fuel can evaporate.)
If the fuel does not dribble out, remove the fuel line going into the carburetor and use the primer tube to pressurize the fuel system again.  If fuel comes spurting out of the fuel line, you know something in the carburetor is stopping the flow of fuel, probably a clogged fuel inlet screen.  A rebuild of the carburetor is necessary.  If little or no fuel comes out of the fuel line, you probably have a clogged tank filter or a crushed fuel line.  Ethanol fuels often create goo that can clog the pickup tube filter inside the fuel tank.  Take it out of the tank and examine it carefully. 

Perform this next test on the fuel pump if step #5 above passes.

To test the fuel pump:

  1. Remove the spark plug.  Reconnect the spark plug to the secondary wire and lay the plug down in the area of the spark plug hole so that the metal shell of the spark plug touches a metal part of the engine.  (This grounds the spark plug which MUST be done.)
  2. Remove the airbox/filter, if it is not already removed.
  3. Tilt the engine to the carburetor side about 40 degrees.  With the Minari, see step #2 above.
  4. Push down on the primer lever.
  5. Crank the engine.  Fuel should dribble out of the carburetor throat and out of the airbox/filter, if the engine has one.

For the Top 80: If the fuel pump test fails and the carburetor has been recently rebuilt, then it is possible that the reed valve body is loose and leaking air.  This often happens only when the engine heats up to normal operating temperature so this must be checked when the engine is hot.  The pump will not work if the passageways to the crankcase are not tight and clear of obstructions.  If the carburetor gasket is put on backwards or upside down, the fuel pump will not work even though the engine may run, though poorly.  This problem creates a lean-out condition and it is easy to burn up the engine!

For all other engines: If this test fails, remove the hose that goes from the crankcase to the carburetor at the carburetor end.  Crank the engine again.  You should hear and feel a "whoosh-whoosh" of air going in and out of the hose as the engine is cranked.  There will be a few pounds of pressure in the hose as the engine turns over.  If this test fails, something is blocking the flow of the air pulses from the crankcase that work the fuel pump.  It could be something in the hose or the fitting that screws into the crankcase that is clogged.

If this test passes, then something is wrong with the carburetor fuel pump.  It could be debris in the pump valves, a bad diaphragm, or a clogged inlet filter screen.  In any case, the carburetor will have to removed, disassembled, and thoroughly inspected.  If there are any doubts per the conditions of the metering lever and fuel pump diaphragms, rebuild the carburetor.  Pilots need to look carefully at the carburetor and see if they can spot the problem.  If this step is baffling, send the carburetor to us and we will take care of it.

If there is any doubt about the performance of the fuel pump, perform a thorough test of the fuel system.

If all of these tests pass, the fuel system is most likely OK and you can go to section "B" below and do the simple ignition test.

If there is a lot of dirt in the fuel system, the low speed needle valve may be clogged, a rare event.  Remove the needle and blow out the disassembled carburetor with compressed air.

Thorough fuel system test

If fuel does not dribble in a stream out of the carburetor in the tests above, perform this more thorough test of the fuel system and fix the problem.  If it has been a year or more since the last rebuild, it is time to rebuild the carburetor regardless if there are problems with performance.  The guts of the carburetor age in the presence of fuel whether the engine is run or not.

Fading near/at full load in the Top 80

Fading at full load can be caused by a loose reed valve body in the Top 80, an overlooked problem that must be checked in motors that have problems.  Most paramotor engines do not port the crankcase pressure pulses to the fuel pump through passageways in the carburetor and reed valve but do it directly through a fitting on the crankcase and a tube going directly to the fuel pump diaphragm.  Fading at full load may also be caused by mechanical and ignition issues -- see below.

Hard starting hot

This is usually due to vapor lock in the fuel system.  If any parts of the fuel system get near the exhaust system, the heat will cause vapor lock.  As I note the fuel/oil specifications page, cheap or cold weather gasoline will fizz if the engine is run during the hotter seasons.  Ethanol blends will fizz more easily and this is just one more reason to stay from these blended fuels.  Once vapor lock has occurred, the only remedy is to let things cool off.  Taking ice water and dumping it over the fuel tank can help get you in the air again.  Once airborne, air movement helps keep things cooler.

B. Ignition Failure

The #2 most common problem is a failure of the ignition system.  Thankfully, the only symptom of ignition failure is poor engine output, not overheating which can quickly destroy an engine.  If you have more than 10 or 15 hours on the spark plug, replace it!

Simple secondary ignition system test

With a digital multimeter (<$10 from Harbor Freight), measure the resistance from the metal tab inside the spark plug boot to one of the cooling fins on the engine (ground).  It should be 17K Ohms ±10% on the Top 80.  For other engines, if you have resistance between 7K and 20K, you are probably OK.  However, if it is open (no resistance), you have a bad secondary wire which must be fixed.  Your options are limited to replacing the wire (easy) or replacing the coil (a difficult and expensive operation).  An internal break in the secondary wire will cause missing and a drop in engine output at high engine loading and becomes more acute as the engine heats up and internal pressures increase.  When the break gets sufficiently wide, the spark cannot jump the gap in the wire, the engine will not start, and, as likely as not, the coil will be destroyed from internal overvoltage.  Here are more details on testing the secondary ignition wire.

If the preceding test is good, place the paramotor out of bright sunlight.  Remove the spark plug.  Reconnect the secondary wire to the plug and firmly push the metal base of the spark plug against any metal part of the engine, like the exhaust pipe.  Pilots who are nervous (me) about getting a shock can use masking tape to FIRMLY hold the plug in place.  THE SPARK PLUG MUST BE GROUNDED OR THE IGNITION COIL CAN BE DESTROYED!  Pull the starter as if starting the motor.  You should see a bright spark at the end of the plug every time the motor makes a complete revolution.

If both of the above tests are OK, continue with the next section.  If not, perform a more detailed ignition system test.  It is easy to do and requires minimal tools.  Engines will never reach full output if there are problems with the spark plug and secondary wire.  Too great a coil gap will make the engine hard to start.  Do not waste your time tinkering with the carburetor if the ignition system is not working properly.

Kill switch test

If you experience sudden engine failure, the kill switch circuit could be shorting out.  The green arrow points to the kill switch wire going to the throttle and light blue arrow points to the primary wire that goes to the coil.  These wires are just in front of the carburetor and next to the cooling box.  The engine will stop if the primary wire is grounded.  Sometimes, the wire going to the throttle will break or rub against the Bowden cable and short out.  The switch in the throttle can also be bad.  Separate the connection between the primary wire and the kill switch wire.  Set the multimeter to "Ohms" and its lowest range.  Connect the red lead of the  multimeter to the terminal inside the red boot (others similar) and the black lead of the multimeter to a fin on the engine.  Use some test leads to help unless you have three hands.  The kill switch lead should be "open" -- no resistance.  Now press the kill switch a few times.  Each time you press it, the resistance should be less than 3 or 4 Ohms.  If it is any other value, you will need to find out why....

Top 80 primary ignitiion wire

Spark plug

The #3 issue I see is problems with the spark plug.  It must be torqued properly and the gap set to the specifications for your motor.  An under-torqued plug will leak, maybe overheat the engine, and the high end performance of the engine will suffer.  On the other hand, over-torqueing can destroy the cylinder head.  If the gap is too great, the spark will be quenched at high loads, the engine will misfire, and power will drop.   Always be certain that the aluminum washer-spacer is installed with the spark plug.  If there is any doubt about the condition of the spark plug, replace it.  Plugs should be replaced after 25 hours.  Of course, how will you know how many hours the plug has unless you keep a log and have an hour meter/tachometer?

C. Mechanical issues

Mechanical issues are not as common as issues "A" and "B" above.

Note: few shops have the tools or knowhow to perform mechanical repairs on paramotors, especially adjusting the timing or replacing the coil on the Top 80.  Those who service racing two stroke engines, like dirt bikes and go-karts, are the best bet.  For example, it takes about $300 of special tools to service every part of the Top 80 correctly.  Thankfully, most pilots will only need a modest tool set that costs much less.

Poor idle

Poor idle can be caused by clutch drag.  You cannot adjust the idle correctly unless the clutch does not drag.

Loose cylinder head

Sometimes, the factory or pilots fail to properly torque down the cylinder head.  A more serious cause of a cylinder head loosening is the use of inexpensive gasoline which causes fuel mixture pre-ignition (knocking).  Using ONLY premium gasoline or AVGAS (preferred) will eliminate the cause of this problem.  However, those who purchase service station premium grade gasoline may not be getting high octane fuel unless it is purchased from major refiners e.g. Chevron, Shell, BP, etc.  AVGAS, on the other hand will not cause pre-ignition in any engine as it has the highest octane of any gasoline made other than racing fuels.

To tighten the head, remove the cooling air duct/shroud to access the cylinder head nuts and be sure they are torqued to the correct value (9 Nm for the Top 80).  Tighten the head nuts in a cross-pattern to 4 Nm and then to 9 Nm or the final value for your respective engine.


Check the space between the head and the cylinder for any leaks.  The presence of leaks indicates that the head gasket is faulty and should be replaced.  NEVER REUSE A HEAD GASKET!  I am still amazed at how some DIY's mangle simple things like a head gasket.  Don't risk harming the engine: if you are going to do your own service, get the necessary parts to do it right.  Engine leaks are common.  Note that a leak can originate at one point on the engine and air passing rapidly over the engine can move it around to somewhere else.

If you think you have a leak, FIRST thoroughly CLEAN the engine THEN run it an hour and see if you can pinpoint the leak.  If the leak is black and gummy, it is a leak AFTER the fuel/oil mixture is burned.  If the oil is clear in color, it is a leak BEFORE the mixture is burned.  For example, leaks between the cylinder and crankcase will have oil that is clear in color because it has not been burned yet.  Leaks between the cylinder and head will always be black in color.  Carburetors on 2 cycle engine always will leak somewhat -- it is impossible to prevent.


Verify engine compression.

Carburetor gasket in the Top 80

Sometimes, the gasket between the reed valve assembly and the carburetor will be installed incorrectly on the Top 80.  The small hole in the gasket which transmits the pulses from the crankcase must line up with the holes in the reed valve and the carburetor.  This problem will be evident during a thorough test of the fuel system.  The engine may run at low speeds but not under load.

Timing and coil gap

The ignition timing and the coil gap must be correct on all engines.  Pilots should send their engines to competent mechanics to have these values checked and adjusted if they cannot do it themselves.  These are rare conditions that, ordinarily, will not need to be checked.


The cooling system should be checked to be sure there is nothing clogging the system.  Some engines do not have forced air cooling, like the Simonini and Minari.  The engine below had a slight problem....

Top 80 engine with debris in the cooling system causing it to overheat

D. Midrange performance issues

This problem is caused by too much fuel entering the engine via the low speed/idle circuit in the carburetor.  It is relatively easy to fix but most do not bother and put up with roughness in the midrange.  The higher the altitude where the motor is run, the more annoying the problem. Here are the instructions on how to fix this.  Having your motor purr in the midrange is a very nice feature.  Fixing this problem in the Polini Thor engines makes them the smoothest and quietest engines in the world.


If you are completely frustrated and need help getting your engine properly working, you can send it to us.  For service, go to our main paramotor page and read the information in paragraph #2.