If your motor doesn’t start on the first few attempts, something is wrong. Continued pulling without corrective action will just wear the starting system and decrease your respect for the prop—making a life-changing prop injury more likely.

Troubleshooting is usually best started with the simple and cheap. Here are some that qualify.

  • Replace the spark plug if it’s not running right. I’m amazed how often that cures a problem.
  • Replace the Carb with a new one, a $100 quick-change that can eliminate many hours of tribulation.
  • Tighten everything down: carb, head bolts, motor and prop mounts, redrive mounts, exhaust mounts, etc.
  • Disconnect the kill switch if easily done. You’ll obviously need a safe alternative to kill the motor such as blocking intake air, starving fuel, choking (as equipped) or flooding with the primer bulb. Removing the kill circuit isolates what can cause a surprising array of problems, even ones that don’t seem like they’d be related.
  • Make sure there’s new gas in the tank and take primer bulbs our of the fuel circuit temporarily, if easily done.
  • Clean out the carb filter, if present, by spraying carb cleaner through it in reverse.
  • Pressurize fuel lines and see if fuel leaks. There could still be an air leak but this will find the most grievous holes.


Possible solution or link.

Start Problems

Pull starter

Go here for pull-starter (recoil starter) issues.

Electric starter:

Nothing happens when I press the starter.

There are a few mostly obvious causes here. Be extremely careful when troubleshooting these problems and ALWAYS take the spark plug off before working on it. Several pilots have suffered grievous injuries when they activated the start circuit unexpectedly and the motor roared to life.

1. The battery is completely dead.

2. A wire is broken/disconnected or the start switch is bad.

Electric starter:

I hear a motor spin but the prop doesn’t move.

See the caution above about the extreme danger of working on electric starter issues.

1. This probably means the solenoid that drives the starter gear onto the motor’s gear is not working. It may be able to be repaired through lubrication or other means. Otherwise, the starter must be replaced.

Prop won’t spin all the way around, even when turning it by hand.

added 2010 Mar 17

You can move the prop back and forth a quarter turn but it feels like something is stopping it hard.

1. The prop is on backwards. On props with a root section that’s thicker than the hub, this root section can hit parts of the reduction drive or paramotor.

2. There is something stuck in the cooling fan or staring bell (for pull starts).

Motor getting harder and harder to start. Pulls (or cranks) ok but is taking more and more pulls to start.

1. May be a fuel feed problem.

3. There’s a carburetor problem.

4. There’s a crankcase air leak.

5. On belt machines, the belt may have come loose.

6. 2010-09-16  The spark plug cap is loose (see Troubleshooting Ignition). Black sooty material in the cap or plug is evidence of this.

7. The coil/stator may be working loose or failing internally. Here’s more on troubleshooting ignition.

Motor starts then dies

1. There’s a fuel feed problem.

2. There’s a carburetor problem.

3. There’s a crankcase air leak.

4. There may be a short between the kill wire and frame triggered only by vibration.

Motor won’t even fire

1. The master switch (as installed) is off.

2. Spray starter fluid into the air intake. If it still doesn’t fire there may not be a spark (Ignition problem).

3. It may be flooded. Remove the spark plug. If it’s wet, pull the motor over a few times and retry without priming or choking.

4. The carburetor may need adjustment. Rarely does a carburetor just go out of adjustment on it’s own so, if it ran before, and you’re at the same basic elevation (within a 3000 feet) then it should start.

Motor usually won’t fire

1. There is a carburetor problem. If its a Walbro Carb, the pop-off pressure may be too high.

2. Could be ignition related. Change the plug, make sure the cap is on tight (no black soot).

Motor fired once but now won’t fire again

1. It may be flooded. Pull the spark plug, disconnect the fuel, pull the starter (or run the e-starter) through about 20 revs. Put the plug back on (but not the fuel line) and try to start. Once it starts and quits, put the fuel line back on.

2. The plug may have fouled.

3. The decompressor valve, if installed, is stuck open or has other problems.

Motor is difficult to prime when pressing needle release


Idle Problems

(updated 2010-June-24, thanks to Lance Marczak and Alex Varv)

Cuts out periodically like the kill switch is periodically activating. Mid to high throttle it’s ok.

This is indicative of something interfering with the spark current since it only appears during the lower intensity spark at idle.

1. Replace the spark plug.

2. Clean the spark plug cap.

3. Unscrew the spark plug cap and see if there’s black soot. Clean it and insure a good insertion.

Idles fine but dies when I throttle up

1. If it dies suddenly then it may be that the carburetor needs adjustment.

2. There’s a Reed Valve Problem.

3. There’s an air leak.

4. The fuel may be bad.

Idles rough but is ok at higher power settings

1. It may be flooded.

2. Low idle setting on carb needs adjustment.

3. If a Bing carb, it may be caused by a tear in the hose from the engine block. Too much air may prevent fuel from being sucked up from the bowl on the idle circuit.  Replacing the hose cures the problem. (2010-Mar-24 thanks to Phil Adkison)

(Top 80) Hear a ticking noise

1. The starter pawls rub against the engagement piece if engine RPM is too low. Make sure the carb is adjusted properly and that the idle is set high enough.

Idles rough

1. The carburetor may need to be adjusted

2. The idle is too low. If the motor is not mis-firing but shakes around a alot, increase the idle until it is smooth.

Idle has gradually decreased.

1. (WG8 & Others) Change the diaphragm on the fuel side (where you press something in the hole to prime).

2. Insure the pop off pressure has not changed.

3. Idle adjustment screws have changed–readjust. This is extremely unlikely. If you find that you’re having to mess with either the idle stop screw (adjusts where the throttle rests at idle) or adjust the mixture, there is probably an internal problem building.

Above Idle Problems

Motor dies abruptly when I throttle up from idle.

1. It could be a Reed Valve Problem.

2. There’s a problem related to the motor’s torquing (twisting against its rubber mounts). In one case, above about 50% power, the motor torqued enough to touch the kill wire on

3. The electrical connections may be inadequately crimped. It took Warren Smith two years to figure out why his machine consistently died when revving it up. When he re-crimped the electrical connections, the problem went away and didn’t come back.

Motor dies off quickly but smoothly when I throttle up from idle.

1. This suggests that the mixture may be getting too lean as you open the throttle. Clean the carburetor screen (if applicable) such as on the Walbro WG series.

2. There may be an internal plug or other problem in the carb. Save yourself the hassle of further troubleshooting and just put a new carb on.

3. Fuel filter is clogging although this problem usually takes longer to manifest.

Motor accelerates slowly when I throttle up from idle.

1. The idle mixture may be too lean. See carburetor adjustment.

Motor dies out after flying a while (Bing Carb with Fuel Pump).

Applies to machine with separate vacuum driven fuel pump (Fly Products) with Bing Carb.

Motor would die after 1 minute to 30 minutes of flying. Pilot could use primer bulb to prime motor then start it and it would run for 30 minutes then die again.

Carb rebuilds did not work. A replacement carb didn’t work. It wasn’t a vent problem. It wasn’t a clogged fuel feed. It turned out to be the vacuum line from the carb to the fuel pump.

 (Thanks to Scott Richie 2011-09-16 )

Runs rough in the mid-range

1. It could be a Reed Valve Problem.

2. Bing Carb: It is possible the Jet needle (the one you change for launching at different altitudes) needs replaced. The needle can look perfectly normal, but Bing suggests that replacing this needle can frequently solve this problem. Pilots have confirmed this issue.

3. Bing 84 Carb: The motor is running a bit too rich. Keep the Main Jet, which may be properly sized, but lean it out this way. Change the Holding Plate on the Jet Needle from position 2 (stock) to position 1 (up one notch). lean out the motor for much smoother operations through entire throttle range. (Thanks to Scott Richie 2010-10-03 )

Run rough at full power

1. It is likely to be a Reed Valve Problem.

2. There could be an ignition problem. If it dies suddenly then comes back to full then an ignition problem is more likely.

  a. If the spark plug has a screw-on tip, make sure it is tight.

  b. Make sure spark plug cap is screwed into the ignition wire tightly.

  c. Make sure all wire connections to the coil are solid.

  d. Change the spark plug to eliminate that as a possibility.

3. The carburetor may need adjustment although if it just started running rough then this is very unlikely. You will generally notice a power decrease before rough running. An excessively rich mixture, however, can cause rough running. Check

Quits randomly or

Gets to full power then dies

1. If using a Walbro carb, the internal fuel filter screen may be clogged.

2. If you changed the prop, it may be too big or has too much pitch (overpropped).

3. The fuel vent is blocked or other fuel delivery problems exist. If air cannot get into the tank to replace fuel sucked out by the engine, a vacuum will build. Eventually, it will reduce fuel flow when the fuel pump can no longer suck fuel up to the engine, causing it to run lean, rough and/or quit.

4. The mixture is too lean. As the motor heats up the mixture becomes leaner. If it started out somewhat lean, it may lose power plus, there is a high risk for melting the piston to the cylinder wall (and seizing).

5. A head bolt is loose. If any bolts won’t tighten, you’ll need to helicoil the cylinder to accept a new bolt.

6. The motor is getting excessively hot.

7. If there’s a fuel line bubble at a membrane carburetor replace the fuel line. If that doesn’t work, replace the carb. See more details on carburetors(thanks to Andrew Solano)

8.  Bing Carb: The float arm became bent while rejetting or the float arm pin is worn, restricting fuel from entering the carb. Check the float arm and/or use carb rebuild kit. Here is a parts replacement description (as of 2010-10-03).(Thanks to Scott Richie 2010-10-03 )
9. The kill circuit, that includes wire running out to the throttle-mounted kill switch, has bare wire exposed. It could be a part of the paramotor, pilot, or harness pushing the wire against the frame, essentially pressing kill switch.(Thanks to Beery Miller, 2017-06-23)

10. Fuel line is pinched somewhere along its run. It may not be observed on the ground but pressing occurs in flight once the pilot is seated. (Thanks to Beery Miller, 2017-06-23)


Motor cuts out at higher RPM.

(updated 2010 Sep 16)


1. Spark plug is bad. It may look fine but could have internal problems.

Not as much thrust as it used to have (has happened over weeks)
(updated 2017, Jun 19)

1. The prop is worn. Nicks and prop tape degrade performance.

2. The piston ring(s) are sticking.

3. Spark plug. Even thought the motor starts and runs great. Thanks to Larry Koral for this tip.

4. Exhaust problem. Could be an interior failure or packing (frequently fiberglass) material is clogging the interior. Check to see that internal baffle is in place. On some motors (Top 80) there is a rivet whose absence indicates this problem is, or will be, likely. On the Top 80 it manifested by a max RPM of 7600. Thanks to Lance Marczak.

Power Fades from high RPM and/or cuts out suddenly

I would NOT have thought this could be the cure if I didn’t witness the whole troubleshooting problem myself. I would have called it an ignition problem because one of the way the motor would get to RPM, fade, then just cut out like the ignition source shorted.

Replaced the needle valve and membranes, using the white plastic membrane option. Thanks to Mo Sheldon

Suddenly cannot achieve Max RPM.

1. Prop is on backwards. Thrust is in the same direction, it’s just a lot less with the prop on backwards.

2. If you’ve put on a different prop and the problem appeared, the prop is either too big or has too much pitch. Consider changing (increasing) the redrive ratio or getting a lesser pitched prop.

For example, on Top 80’s, the MAH carbon fiber prop works only on the 19/73 (19 teeth on the small gear, 73 teeth on the big gear) but NOT on the 20/72 redrive. Thanks to Lance Marczak for this tip.

3. Mixture too rich. If it is four stroking (running rough but in a way that’s regular, like it’s missing every other power stroke) then it needs leaning out.

Motor is getting louder with time

1. A loose or damaged exhaust system is the most likely problem.

2. Next to exhaust, a loose or damaged air intake silencer is the most likely cause..

3. Far less likely is that is off, causing the spark either too early ,or too late, timing


There is a lot of vibration at idle

RPM may be too low, adjust upwards.

Check the motor mounts by flexing the prop while it’s vertical and then horizontal. Most motor mounts have no metal that runs all the way through them so a broken mount leaves the possibility for the engine to leave the frame.

Low RPM Vibration that increases as the engine RPM increases.

The prop is out of static balance meaning that doesn’t balance from the exact center of the prop hole.

If the prop is statically balanced yet it still vibrates, the prop may not be aerodynamically balanced. Insure that the prop is flat on its mount, especially that there is no horizontal offset (as opposed to lengthwise offset) will make one blade have a greater pitch than the other. Having two bolts on one side excessively tightened could cause this. If it’s still aerodynamically unbalanced, such as one blade is sanded incorrectly, there is no practical cure and the prop should be replaced.

High RPM vibration that increases as the engine RPM increases

1. Flywheel related problem. Many engines don’t have flywheels but, if it does, it can be source of this vibration.

Motor runs rough in the mid range

The engine “misses” occasionally

1. It may have an Ignition problem.

2. There could be contaminants in the fuel although this is unlikely since the motor is more likely to just quit.

Shutdown Problems

The motor won’t shut off when I press the kill switch.

Go here for shutdown related problems.

Throttle Problems

Throttle is sticky or doesn’t return

There is dust in the throttle cable. Also consider using dry lubricant graphite powder. You can sometimes solve this problem by adding another spring on the throttle arm.  stronger spring to the carb ret

The spring on the carb is broken or disconnected.

Power not steady

1. There could be debris in the carburetor screen. See curburetor.

Miscellaneous Problems

Carbon Buildup

Some carbon buildup on 2-stroke engines is inevitable. If it’s excessive, here are some considerations.

1. Inferior Oil. Choose an oil recommended by the engine maker.

2. The mixture is too rich and running too cool in midrange. Adjust mixture very carefully, though, since overheating can ruin the motor.

3. Fuel/Oil ratio has too much oil. Use manufacturer recommendations or 2% (50 to 1) in the absence of manufacturer info.

Throttle Cable Frayed

Don’t cut the end, loop it. Here is more.