by Had Robinson
updated January 13, 2021
There are two situations when stall occurs. The pilot bends over at launch and the engine stalls. It is caused by the puddling of fuel and oil in the crankcase. Bending over will allow the flywheel to slop the puddle (usually with a higher concentration of oil) into the combustion chamber. The engine may either smoke a lot for a moment and/or stall. This is not a repairable problem. It can be mitigated by not idling the engine excessively.
In the other situation the pilot is at launch and ready, glider overhead, moving in the right direction, and goes to full or wide open throttle (WOT). Rather than surge ahead with power and away from the ground, the engine goes BLUB...CLUNK and everything stops DEAD. The glider can wind up on top of the pilot and the motor. It is probably one of the most *annoying* and embarrassing events we motorheads experience. What happened?
Typically, the engine has not been maintained properly. This includes not setting the metering lever height to the correct value, not having an inline fuel filter, failure to rebuild the carburetor at the correct intervals, clogged fuel filters, excessive spark plug gap or fouled spark plug, or secondary ignition wire failure.
All paramotors that have the fuel tank located at the bottom of the frame are *very* sensitive to fuel supply issues. The fuel pump inside the carburetor must work perfectly. Diaphragm carburetors have (2) problems when going to WOT quickly:
- Paramotor carburetors have no fuel enrichment system that kicks in when full throttle is applied quickly as in automotive carburetors or in electronic fuel injection
systems. When the throttle plate is opened too fast, the air rushes into the engine before enough fuel can get moving through the various passageways and jets in the carburetor and then into
the engine. Consequently, the engine leans out and stutters/stalls.
- The Walbro fuel pumps are *NOT* designed to pump fuel from a tank 18" (45cm) below the carburetor fuel inlet. All paramotor diaphragm carburetors were designed for obsolete chainsaws and cutoff saws that had the fuel tank level with the carburetor.
These inherent design problems ensure that the slightest fuel system malfunction will be noticeable when going to WOT. Our FSM takes care of this. However, it is still under development at this time.
It is helpful to apply throttle slowly and deliberately when launching. Sensory overload a.k.a. task saturation is very common when launching and pilots will jam the throttle unless they consciously tell themselves not to.
If the engine stalls easily at WOT, do this quick and easy test of the fuel system to rule out acute fuel starvation.
Why not install a fuel injection system on paramotor engines? It is impractical (even ridiculous) because of the number of things that must be monitored for it to work properly i.e. ambient air temperature, air pressure, air mass going through the system, engine temperature, throttle position, exhaust gas O2 content, exhaust gas temperature, etc. This is to say nothing of the electrical requirements (alternator and battery) which would also be needed. Contamination of the O2 sensor with unburned oil is another problem. On top of this, the system must be programmed with a computer for each engine. I hope pilots get the idea that fuel injection on a paramotor is impractical.
Fortunately, Southwest Airsports is working on a fix for this called the FSM (fuel system modification) that eliminates this stall problem and dramatically increases engine power, among other things. It is simple, reliable, durable, and inexpensive. It should be ready around spring 2021 for distribution after we thorough testing.