Pump the throttle during start-up

[avinstructor]

I was taught that in some cases pumping the throttle just before engaging the mags can assist in starting the engine (specifically carbureted engines) - especially if you suspect a flooded engine as it is pushing air through the system and helping to get rid of some of the excess build-up of fuel.

On my multi-ride the other day the examiner told me that this should NEVER be done on a carbureted engine as it can cause a fire quite easily ... Something about a boost pump on carbureted engines when full throttle is engaged.

So which is true?

[Colonel Sanders]

Sigh.

Take the cowls off, and look at the engine. You
will see the carburetor below the engine, because
of the required thrust line.

This means that the carburetor is updraft - the air
moves up through it, when the engine is running.

This is a little bit weird. Most carburetors on cars,
motorcycles, etc are downdraft.

Now, you may have heard of this thing called gravity.

If you pump the throttle of a carburetor when the
engine is not running, the accelerator pump will squirt
fuel but instead of it going into the engine, gravity will
cause it to pool below the carburetor in the flapper box
and evaporate into a combustible mixture. If it lights
off, you're going to see flame out the intake, toasting
the filter. Not good.

That's what your examiner is talking about.

Now, if you pump the throttle while the engine is cranking,
the air (and fuel) is sucked up into the engine, actually
doing a better job of priming than your primer, which
probably only feeds one or two cylinders.

Morale of the story: shit runs downhill, which is obvious
to anyone that has worked in a corporation.

And don't pump the throttle of an aircraft engine if it's not
turning.

Again, if you know how shit works, you will be a much better
pilot.

A good pilot starts with the fundamental stick & rudder skill,
and systems knowledge. Learn your systems.

[Big Pistons Forever]

As a general rule pumping the throttle is fine as long as the prop is turning. This works quite well for an engine that is slightly under primed. If it does not start after 5 or 6 blades a few quick pumps will cause the accelerator pump to squirt in a bit of gas and the engine will start right away.

The danger of pumping the throttle before engaging the starter is if the engine back fires you can get an induction fire. This is not a problem with the primer because the primer lines inject fuel into the intake manifold just before the cylinders, not at the carb like pumping the throttle.

edited I see the colonel beat me to it, and as usual did a better job with the explanation, but I was more polite

[iflyforpie]

Opening the throttle will help with a flooded engine, not because it pumps more air in but because it allows more air in. If you pump it continuously in and out, you are only letting more air in part of the time, plus on a carburetted engine you are squirting more fuel in each time from the accelerator pump.

Pumping the throttle can cause a fire in the case of carburetted engines only. The carb is at the bottom and is an updraft carb. If there isn't enough suction from the engine to draw the fuel up (likely when it isn't running), the fuel will go back down through the throttle body and pool in the air box. If there is a back fire, this pooled fuel can then ignite. The fuel boost pump doesn't have anything to do with this.

On fuel injected aircraft, typically there is a throttle switch and if you leave the throttle advanced with the mixture rich and boost pump on, you will flood the engine and likely make a pool of fuel on the ground.... but it isn't likely to catch fire like in a carb engine.

[avinstructor]

Got it ... Thanks for the help

[pdw]

And then of course the usual precaution: keep cranking if your carb version suddenly fires in flames ... as it backfires and nearly-starts but not yet 'going'.

Have witnessed at least one episode up real close where a pilot, who quits cranking just as the fire erupts, then can't see the flame from the vantage of the cockpit : ie quickly ran up to within view of the pilot thru his window and also using hand signals ... "keep it cranking ... keep cranking !! " which vacuums up that type of fire from its source into the intake and then often also starts immediately after that.

[Rookie50]

Interesting how my PoH teaches to pump the throttle, ESP before cold starts. I have never done this on the 0 360 -- prime and start. We will see on the 0 540 -- just 2 add. Cylinders

[crazy_aviator]

Colonel sanders, you can be right and wrong,,,here is where you are Wrong ,,,

gravity will
cause it to pool below the carburetor in the flapper box
and evaporate into a combustible mixture

. Pooled fuel does NOT form itself into a combustible mixture,, it is set on fire ( a NON explosive burn) AND a relatively cool burn by the backfiring CAUSED by your other Wrong assumption

Now, if you pump the throttle while the engine is cranking,
the air (and fuel) is sucked up into the engine, actually
doing a better job of priming than your primer, which
probably only feeds one or two cylinders

As mentioned by another poster, pumping can often lead to engine fires because of the poor atomization AND ratio of fuel to air causing flames back through the induction,,, igniting PARTIALLY atomized fuel in the intake tract AND causing a slow, cool burn of the pool of fuel on the bottom of the airbox.

[crazy_aviator]

I watched an OLD graying friend of mine pump a C-180 engine in the cool morning ( post maint) AFTER I told him not to pump it ,,,and sure enough ,,it caught on fire BECAUSE of the above reasons !!!!!! Like I said before ,,,keep your primer injectors in good condition and use a technique of turning the engine over WHILE priming and you WONT have ANY problems,,,Only the ignorant who blindly follow their instructors,,prime,,then do those 9 other items on the checklist, scratch their ass, THEN turn over, have problems starting ( ESPECIALLY in cold weather/ hot starts !!!!

[CpnCrunch]

I find that most small engines (in 150/172/PA28) will start pretty much immediately hot/cold as long as you prime them properly. It does take a little experimentation to figure out the correct amount of priming for an engine in various conditions. The PA28s seem to require a hell of a lot of priming due to the short throw (and many people pump the throttle because it's easier), whereas the C150 seems to require practically no priming in warm weather (one or two at most). Any time I've had trouble starting an engine it's invariably because I haven't primed it properly.

[Antique Pilot]

It is almost 35 years since I flew a BN2A Islander. It had the 0-540 engines with carbs but no engine primers. SOP's as I was taught was to pump the throttles 3 or 4 times for a cold start BEFORE TURNING THE ENGINES OVER. It started every time, no problems.

[C-GKNT]

It is almost 35 years since I flew a BN2A Islander. It had the 0-540 engines with carbs but no engine primers. SOP's as I was taught was to pump the throttles 3 or 4 times for a cold start BEFORE TURNING THE ENGINES OVER. It started every time, no problems.

It's been 2 days since I flew my Bell-47. It has a VO-540, same procedure (no primer).

Glenn

[crazy_aviator]

The key there is BEFORE turning the engine over,,,, do that whilst turning and your chance of induction fire goes UP

[Colonel Sanders]

CA: I'm not sure you understand how an updraft
carburetor works. However, it probably doesn't
matter.

[crazy_aviator]

Im sure AND it matters to me

[CpnCrunch]

There's an article here that discusses pumping the throttle and explains how the primer works:

http://www.caa.govt.nz/Publications/Vec ... 4_May.pdf‎

[pelmet]

By coincidence, I'm getting checked out in a Cessna 162 Skycatcher tomorrow. According to the POH it has an updraft carburetor on its O-200D engine. A primer is an option in this aircraft. If no primer is installed the manual says to prime by pumping the throttle once if the engine is warm and 6 times if the engine is cold. A pump is described as pushing full in then full out.

"The engine-mounted portion of the system consists of the gravity-fed updraft float carburetor." from page 7-32

Below is from page 4-14...

IF FUEL PRIMER CONTROL NOT INSTALLED
IF ENGINE COLD
9. THROTTLE Control - PUMP (3 to 6 strokes)
10. THROTTLE Control - OPEN 1/4 INCH
IF ENGINE WARM
9. THROTTLE Control - PUMP ONCE (push full in and pull full out)
10. THROTTLE Control - CLOSED

https://support.cessna.com/docs/custsup ... 62PHUS.pdf

[Doc]

Post your thoughts on the 162, if you would. Share your impressions. I've heard it's not a big seller. Wondering why.

[triplese7en]

"Again, if you know how shit works, you will be a much better
pilot.

A good pilot starts with the fundamental stick & rudder skill,
and systems knowledge. Learn your systems.

I wish more pilots knew this... especially my 'professional' pilot colleagues I fly with.