Priming Engines and Fire Risks

by Dave Sawdon

Over-priming is bad news from 2 perspectives: the engine won't start and there's a risk of fire. The fire risk comes because most Lycoming and Continental engines use "updraught" carburettors, this means that the carb is beneath the engine. Consequently any priming fuel tends to run down and out of the carb into the inlet ducting and (usually) the air filter, if the engine coughs back this can be ignited.

Over-priming can come from mis-use of the throttle as well as the primer itself. Many engines have an accelerator pump linked to the throttle, each time you pump the throttle you squirt neat fuel into the inlet. Find out if your engine is like this and, if it is, take care not to pump the throttle unnecessarily.

Cold weather starting a cold engine: The fuel will not atomise as well as normal so the engine may fire but not run. Rather than risk over-priming (and a pool of fuel beneath the carb) just prime as normal but leave the primer out. - If the engine fires but won't run then keep the starter engaged and use the primer charge to help the engine into life. - If the engine starts normally then gradually empty the primer without killing the new life in the engine.

Starting a hot engine: Do not prime before trying a start. Only try a small amount of priming if it won't start.

NOTES:

If the engine won't start how do you tell if you've over-primed it or under-primed it? The answer is to sniff the exhaust pipes (take the mag key with you!), if over-primed you'll smell fuel. If you do over-prime and the engine won't start you've got 2 choices:
- If you have an engine-driven vacuum pump you'll need to wait 10 minutes then start with mixture at ICO and throttle around half (mix rich and throttle closed as soon as the engine runs).
- If you DON'T have an engine-driven vacuum pump you can "blow out". To do this you open the throttle wide (mixture at ICO, mags off) and turn the prop backwards through a minimum of 8 blades. (DO NOT DO THIS IF YOU HAVE AN ENGINE-DRIVEN VACUUM PUMP - IT WILL BREAK!). After this it should start as normal (remember to reset the throttle, especially if hand swinging).
If you suspect a start-up fire keep the starter engaged to draw the flames into the carburettor. If this doesn't work switch the fuel off, tell the tower (if there's time), grab the extinguisher and vacate the aircraft, aim the extinguisher at the base of the fire.
If you suspect a start-up fire you should get the aircraft checked before flight. Some inlet filters are beneath the carb and are prone to damage, after a fire they need changing.