Questions about the primer for carburetor engines.

Why does the primer only spray fuel to three (sometimes just one) of four cylinders. (Like Piper Cherokee or Seminole)

And is it true that priming engine during engine start (cranking engine) may cause engine fire?

That would be great if you could answer those questions! Thank you!


3 Answers 3


The airplane maker will not normally install any more primer lines than necessary. In warm environments it's not usually necessary to prime more than one cylinder, so you will see airplanes operated in the southern US with single primer lines more often than in the northern US or Canada. My old '68 Cardinal only had a single primer port because it had been imported from the US that way (I was in Toronto at the time), and it was murder to get it started even in moderately cold weather without preheat. I had primers run to all 4 cylinders by my mechanic.

Also, you will often see one of the threaded holes at the intake port on a cylinder being used as a tap-off for a manifold pressure gauge, another reason for stopping at 3 lines.

When priming, the objective is to get a good coating of raw fuel in the intake pipes close to the port, that will take 5 or 10 seconds to fully evaporate after starting, to provide that temporary extra-rich mixture. But not too much.

Over-priming can start a fire when so much gas is squirted in it starts to collect in the intake pipes and drain down into the airbox. A backfire into the intake system during starting, because an intake valve doesn't quite seal, can ignite the fuel in the airbox, and potentially fuel that is dripping into the cowling.

That's why, when cranking and you hear a "fump!" sound under the cowling, keep cranking! You need to try to draw any ignited raw fuel back into the intake. Only stop cranking if you start to see smoke gushing out, and then it's time to leave and tell the owner he can have his plane back (or try to fight the fire if you want).

It's when you start to smell gasoline that you know you've over done it. Best to stop and wait a while for all the gas to evaporate.

A worse habit is throttle pumping to spray fuel from the carb's accelerator pump into the intake. You might do it when you start a warm engine; it's not necessary to prime with the primer and little shot of throttle while cranking is enough.

But it's critical to only pump the throttle during cranking. If you pump it while things are stopped, gasoline runs straight down into the airbox immediately. Probably most cowl fires are from people pumping throttle several times, waiting a few seconds, then starting, then it's "Fump!".... "do you smell something?".

  • $\begingroup$ Thank you for explaining and sharing your experience! $\endgroup$
    – Denji
    Apr 5, 2021 at 1:45

On a large radial engine, priming may be done without any dedicated line to a particular cylinder. It used to be standard practice in the winter time to flood the supercharger while cranking until fuel ran out of the drain, with a five gallon bucket below to catch the fuel. Apply spark, begin advancing the mixture and cracking the throttle; the priming was simply raw fuel dumped into the supercharger.

Fuel doesn't need to flow to every cylinder to prime, as the fuel source is the carburetor. This is true whether it's a small horizontally opposed engine, or a large radial. The point of priming is to get the engine to start picking up the load of the starter and to be self-sustaining. Every cylinder will come to life on its own; regardless of the engine, mixture and fuel and ignition is different in every cylinder. Not all will fire off at the same time.

Priming every cylinder can result in a fire. Typical primers in small engines in light piston singles are often plunger type primers that move a very small amount of fuel. That fuel is more useful and effective if directed to one or two cylinders, than spread among multiple cylinders. Priming all cylinders with such a small amount of fuel might result in multiple plungers of fuel being required, and less precise application of the fuel (and the possibility of overpriming.

It should also be noted that under certain circumstances, an unlocked primer can result in a rich fuel situation and spark plug fouling (by drawing fuel through the primer), whereas a failure in a primer line can result in a lean cylinder operation, or a fire. It makes sense to limit the number of primer lines to those necessary to assist engine start. Some cylinders during start operate more lean than others due to intake design, distance from carburetor, etc.

In engines that are fuel injected, each cylinder is primed, in some cases, using a boost pump and normal fuel flow, and in others through dedicated priming lines to one or two cylinders only.

In a previously reply, someone mentioned pumping the throttle, which on certain installations, invokes an acceleration pump. The acceleration pump adds additional fuel, usually as a spray, during throttle advancement, helping to reduce engine stumble, or a lean mixture. Some times this accelerator pump is called for as part of the starting procedure, but in many cases, it's used incorrectly as a means of "priming the engine." It's a good example of the need to know one's aircraft, systems, and procedures. What's true of one aircraft model or engine installation is not necessarily true of another, even when the same engines are used.


In addition to John K, it only takes one or two cylinders to fire before the engine will start. Just like when pull starting a lawn mower, once the cylinder fires once it will almost always start, similar principle here, while priming all cylinders would not hinder starting, if it only takes one of them to fire it is pointless to prime all of them.

  • 3
    $\begingroup$ "...once the cylinder fires once it will almost always start..." You really think so? Not my experience, and part of why I have an electric lawnmower now :-) $\endgroup$
    – jamesqf
    Apr 4, 2021 at 17:49
  • $\begingroup$ I know what you mean, lawnmower engines can be hard to start, but they (usually and understandably) aren't maintained to the same standard as plane engines. Often using lower quality fuel that has been sitting for months with bits of grass in them, blocked air filters, fouled plugs due to tilting so oil goes into the intake etc. Electric mowers are way easier definitely, we need electric planes @jamesqf but I haven't found an extension lead long enough and batteries aren't there yet. $\endgroup$
    – MParks
    Apr 5, 2021 at 11:29

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