Chapter 12 of the PPG Bible has a concise flow
chart for the most common problems. This is another approach that has the
space for far more
details than would fit the book (which was already 32 more pages than planned). Whenever
specifics are known for a particular engine, they will be provided.
Contributions are welcome if you have specifics for a motor or additional
troubleshooting expertise. Thanks!
A lot can be done to prevent problems in the
first place. Much money can be saved, too, by preventing a problem from
progressing to where it tears up more expensive parts. Recommending oil is
difficult because pilots have nearly religious affiliations to whatever it
is they use. One that has earned a good reputation is Honda HP2 mc 2stroke
material, including motor-specific, will be welcome and will help the
community overall. There are many good sources, especially the yahoo
groups related to each brand. This is intended as an easy to use starting
If your motor doesn't start on the first few attempts, something is
wrong. Continued endless pulling will only provide exercise and wear out
the starting system. Start with the simple and cheap. Here are some that qualify.
Possible solution or link.
here for pull-starter
(recoil starter) issues.
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.
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).
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
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
3. There's a crankcase air leak.
4. There may be a short between the kill wire and frame triggered only
|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.
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
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
2. The plug may have fouled.
3. The decompressor valve, if installed, is stuck open or has other
(updated 2010-June-24, thanks to Lance Marczak and Alex Varv)
periodically like the kill switch is periodically activating. Mid to high throttle
This is indicative of something
interfering with the spark current since it only appears during the
lower intensity spark at idle.
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
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
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.
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.
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.
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.
|Motor dies abruptly when I throttle up from
1. It could be a Reed Valve Problem.
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
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
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).
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.
Scott Richie 2011-09-16 )
|Runs rough in the mid-range
1. It could be a Reed Valve
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
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
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).
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)
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.
a parts replacement description (as of 2010-10-03).
(Thanks to Scott Richie 2010-10-03 )
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
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
|Motor cuts out at higher RPM.
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
|Power Fades from high RPM and/or cuts out
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
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
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
3. Far less
likely is that is off, causing the spark either too early ,or too late,
|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
|Low RPM Vibration that increases as the engine
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
|High RPM vibration that increases as the engine
related problem. Many engines don't have flywheels but, if it does, it
can be source of this vibration.
runs rough in the mid range
|The engine "misses" occasionally
1. It may have an
2. There could be contaminants in the fuel although this is unlikely
since the motor is more likely to just quit.
|The motor won't shut off when I press the kill
Go here for
shutdown related problems.
|Throttle is sticky or doesn't
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
buildup on 2-stroke engines is inevitable. If it's excessive, here are
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.
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.