Motor Troubleshooting

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 oil.

This is a work in progress. Contributions of 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 place.

Made possible with expertise from: holoweb,  Alex Varv, Jeff Baumgartener, Nick Scholtes, Chris Bowles, Bob Armond, Eric Dufour and others.

General Guidelines For Motor Troubleshooting

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.

• Replace the spark plug if it's not running right. I'm amazed how often that cures a problem.
• 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. This step 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.
• Replace the Carb with a new one, a $100 quick-change that can eliminate many hours of tribulation.

Problem

Possible solution or link.

Start Problems

Go 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.

1. There's a fuel feed problem.

2. The carburetor needs adjustment.

3. There's a carburetor problem.

4. There's a crankcase air leak.

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.

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. It may be flooded.

2. The plug may have fouled.

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

Idle Problems

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.

1. It may be flooded.

2. Low idle setting on carb needs adjustment.

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.

Above Idle Problems

1. If it dies suddenly then the carburetor may adjustment.

2. It could be a Reed Valve Problem

3. There's a problem related to the motor's torquing. In one case, above about 50% power, the motor torqued enough to touch the kill wire on

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

1. It could be a Reed Valve Problem.

1. It could 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

1. The prop is too big or has too much pitch -- this is called being overpropped. Suspect this only if started happening after you've changed the prop.

2. 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.

3. 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).

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

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

2. The piston ring(s) are sticking.

Vibration

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.

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.

Flywheel

Motor runs rough in the mid range

It may have an Ignition problem.

Shutdown Problems

Go here for shutdown related problems.

Throttle Problems

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.

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