Only prime while cranking...?

Yes, the different color of airworthiness certificate causes the laws of physics to change.

Actually, the article addresses "priming" by pumping the throttle instead of yea olde primer that is installed on yer typical pipenna.
 
Right, pump throttle five or six times, hit starter, and if it doesn't start, keep cranking but be ready to jump out and run for the fire extinguisher.

Note: Fire extinguisher in cockpit is useless against engine fires.
 
What you're thinking and what the article is talking about are two different procedures.

The technique in the article are for the home builders who elect not to have a stand alone priming system, but rather have an accelerator pump installed.

Keep in mind the C152 (and 150 if I recall) do not have accelerator pumps, so those who choose to pump the throttle during startup are not accomplishing anything. (I've seen people do it many times.)
 
tinerj said:
Right, pump throttle five or six times, hit starter, and if it doesn't start, keep cranking but be ready to jump out and run for the fire extinguisher.
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And have your insurance agent on speed dial.
 
Why not hit the starter and then pump throttle after the blade starts to turn. You start cranking lean and then fuel hits and it fires. Never again do you have to guess if your lean or flooded. Just don't pump without cranking.


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Overpriming is an issue on many aircraft. Using an accelerator pump (i.e. pumping the throttle before starting) if done incorrectly can cause major problems.

That said, on the carb'd Lycomings I used to fly, what worked really well was two pumps for cold starts and one pump for hot starts. Do that pump before cranking and it would fire right up. It's the 5-6 (or more) cranks that would cause a problem and cause backfires.

In my experience, most pilots don't know how to start their aircraft.
 
Like so many other things, this is not one of those things that applies to all engines in all aircraft.

Some aircraft can only be primed before cranking (original Stearman design with external primer comes to mind). Other aircraft must only be primed while the engine is cranking. My Beech 18 is actually placarded this way due to the Bendix fuel injection).

Read the manual and don't let internet experts fool you.
 
So I've seen accelerator pumps in action on my four stroke dirt bikes. It's simply a plunger that gets pushed down when you twist the throttle, which basically just shoots fuel into the intake.

So I assume the accelerator pumps on airplanes (that have them) are basically the same deal; some kind of mechanism that shoots fuel into the intake. So they say in the article:

You never want to pump the throttle to squirt fuel into the engine with an updraft carburetor unless you are cranking. Squirting fuel into the intake manifold while the engine is turning sucks all that fuel up into the engine, allowing all of the cylinders to get a chance to fire. The suction from all of those big pistons is actually quite amazing, and yes, if you are cranking the starter, the fuel will go up into the cylinders. But if you squirt the fuel with no suction, it will all fall back down through the carb, and collect in the air box. If you then get a backfire, you are likely to have an air box fire - and those can be very bad. By the time you know (in the cockpit) that you have a problem, the fire can really be going!

Nevermind, I get it now... this is exclusive to updraft carbs. That's why the fuel falls all the way to the airbox, and... what did somebody above say... Pipessnas aren't updrafts.

I'm hitting reply anyway, I took the time to type this carp...
 
James331 said:
Never been a fan of the pumping the throttle bit.
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Yeah, me neither. If you want to shoot a little fuel into the cylinders when cranking, no law says you can't do it using the primer plunger. Works good.
 
Chip Sylverne said:
Yeah, me neither. If you want to shoot a little fuel into the cylinders when cranking, no law says you can't do it using the primer plunger. Works good.
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For most normal GA stuff, if you prime right and everything is in good shape I've always had em fire up without any heroic measures.
 
StevieTimes said:
Nevermind, I get it now... this is exclusive to updraft carbs. That's why the fuel falls all the way to the airbox, and... what did somebody above say... Pipessnas aren't updrafts.
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For the most part they are updraft carbs - but the actual primer knob system shoots fuel closer to the intake valve so you don't have to use the accelerator pump connected to the throttle in the carburetor. Two different pumps in different locations for different purposes.
 
Most pilots don't actually understand where the primer puts fuel, and whether or not it's aerated or liquid. That knowledge would be useful to most of the pilots that try to wear out their starters each time they fire up. I've seen some guys who always crank for 30 seconds at a time due to poor technique coupled with the impulse to just keep going a little bit longer.
 
Chip Sylverne said:
Yeah, me neither. If you want to shoot a little fuel into the cylinders when cranking, no law says you can't do it using the primer plunger. Works good.
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Except my Lycoming, with updraft carb, has no primer . . . and starts well this time of year with two throttle pumps, but in winter can require 5-6, with a pause for the fuel to evaporate.
 
The problem with the accelerator pump is that it shoots a straight, unatomized stream that collides with the intake manifold's top inside surface or the throttle butterfly, and the fuel runs back down the sides or drips off the top. The primer system's nozzles, if they're in proper shape, are like those simple little antique lawn sprinklers that spin the water inside them so that it flies outward in an atomized cone shape. No moving parts. In the Lyc they're located right next to the intake valve. The smaller Continentals have them in the lower intake manifold, much closer to the carb and airbox. In many engines I find those nozzles all internally coked up from fuel that has been baked by engine heat so that they either dribble or maybe shoot a tiny stream or don't shoot anything at all. Hard starting is the result. They're really hard to clean out because of the tiny passages in them. Cheaper to replace. New nozzles aren't terribly expensive, yet too many old nozzles get installed back in overhauled engines when the engine is installed. https://www.aircraftspruce.com/catalog/hapages/primperfittingsan4022-1.php?clickkey=10951

Primer nozzles work way better than accelerator pumps because of the atomization. The key is to prime, then crank immediately. It can be hard to prime and crank at the same time in many airplanes. Just don't give the fuel time to run down the manifold and into the carb and airbox. If you just have to use the accelerator pump, understand that its piston has a spring behind it and that if you pump rapidly you're not getting all that much fuel. It can take nearly a second for the piston to move its full distance, since the throttle link just compresses that spring.

And, as someone pointed out, some carbs don't have accelerator pumps. The 152, with its O-235, doesn't, nor do some other O-235 powered airplanes. The 150 had one on its O-200.

Pumping the throttle on an injected engine just wears out throttle linkages.
 
Really some bum dope in this tread.. the C-150 and the 152 have a MA3SPA carb. it has an accelerator pump.

Primer nozzles are simply a fitting with a calibrated orifice it does nothing but squirt fuel into the intake.
The accelerator pump squirts fuel at the throttle plate.

If there is no air traveling to carry the fuel into the cylinder it will simply drain down the intake pipe and out the bottom of the system.

Never add fuel to any engine that is not cranking.

The old Radial engines had a plenum you could pump it full of fuel, it would go no where until you cranked. Old whys die hard.
 
Chip Sylverne said:
If you want to shoot a little fuel into the cylinders when cranking, no law says you can't do it using the primer plunger. Works good.
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I tried it in the 162 a couple of times and it didn't work out very well. It seems to really need 2-3 shots of prime whether you are cranking or not. If you're shooting those 2-3 shots while cranking, you're really putting a lot of extra wear and tear on the starter since it takes so long for the plunger to refill on each primer stroke. It seems to me that it's better to give it the 2-3 shots before you crank and therefore save wear and tear on the starter. If you want a compromise, give it 1 shot, refill the plunger to arm the next shot and then hit the starter while shooting the 2nd shot.
 
If you want to learn the best priming technique for an engine? Learn to hand start it. Conversely, if you're ever away from help and find yourself with a dead battery and the need to hand prop? You'd better have a good idea of how to prime it to make it start easily. In either case your goal should be to have the engine fire up with minimum rotations.
 
Stewartb said:
If you want to learn the best priming technique for an engine? Learn to hand start it. Conversely, if you're ever away from help and find yourself with a dead battery and the need to hand prop? You'd better have a good idea of how to prime it to make it start easily. In either case your goal should be to have the engine fire up with minimum rotations.
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If my battery is dead.....I'm screwed. ;)
 
Another thing to remember is that the primer system is installed by the airframe manufacturer, not the engine manufacturer.

Look to see if you have a one, two, or three cylinder priming system setup.
 
asicer said:
I tried it in the 162 a couple of times and it didn't work out very well. It seems to really need 2-3 shots of prime whether you are cranking or not. If you're shooting those 2-3 shots while cranking, you're really putting a lot of extra wear and tear on the starter since it takes so long for the plunger to refill on each primer stroke. It seems to me that it's better to give it the 2-3 shots before you crank and therefore save wear and tear on the starter. If you want a compromise, give it 1 shot, refill the plunger to arm the next shot and then hit the starter while shooting the 2nd shot.
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That's the way to do it. My Comanche on chilly mornings, but not cold enough to preheat, fires right up with 4 shots before cranking, fill the primer chamber, shoot number 5 in while she cranks. Throttle at what would yield a 1000 rpm idle.
 
Checkout_my_Six said:
If my battery is dead.....I'm screwed. ;)
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Yep, though I've heard stories that disconnecting the hot lead from the starter and MacGyvering up a couple of D cells in a flashlight tube and some alligator leads is enough to get the SOS a-sparking long enough for a hand prop. Never tried it, hope I never have to.
 
Chip Sylverne said:
Yep, though I've heard stories that disconnecting the hot lead from the starter and MacGyvering up a couple of D cells in a flashlight tube and some alligator leads is enough to get the SOS a-sparking long enough for a hand prop. Never tried it, hope I never have to.
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yup....I have a MacGyver backup....a 18V DeWalt battery and a couple of alligator clip leads. :eek:

but I have no intention of hand proping my three bladed 285 HP rig. :frown2:
 
Slightly off topic. One of my preflight checks (PA28-151) is to check all three of my primer tubings at the fitting on the cylinder. I found one broken once and a plane that sits right across from our hangar had to have the firewall forward rebuilt due to a fire from a broken primer line. Raw fuel squirting on a hot cylinder head.....poof.
 
Skip Miller said:
Another thing to remember is that the primer system is installed by the airframe manufacturer, not the engine manufacturer.

Look to see if you have a one, two, or three cylinder priming system setup.
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And this refers back to the reason this is a useful Kitplane article; builders make many of those decisions and need to understand all of them. In the end, they are writing the POH with the recommended starting procedures. Pilots of certified aircraft (and buyers of experimentals) should follow the POH but it's always good to understand more.

...what's up with this carburetor thing? I thought one just had the fuel injected into each cylinder....
 
James331 said:
Never been a fan of the pumping the throttle bit.
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Works very well if you do it 2-3 times while cranking a cold engine that has an accelerator pump.

As mentioned earlier.. doing this while not cranking can lead to an engine fire.
 
...and to think some of those old engines don't have primers. ....so, what's a woman to do? o_O
Tom-D said:
Really some bum dope in this tread.. the C-150 and the 152 have a MA3SPA carb. it has an accelerator pump.

Primer nozzles are simply a fitting with a calibrated orifice it does nothing but squirt fuel into the intake.
The accelerator pump squirts fuel at the throttle plate.

If there is no air traveling to carry the fuel into the cylinder it will simply drain down the intake pipe and out the bottom of the system.

Never add fuel to any engine that is not cranking.

The old Radial engines had a plenum you could pump it full of fuel, it would go no where until you cranked. Old whys die hard.
Click to expand...
 
Chip Sylverne said:
That's the way to do it. My Comanche on chilly mornings, but not cold enough to preheat, fires right up with 4 shots before cranking, fill the primer chamber, shoot number 5 in while she cranks. Throttle at what would yield a 1000 rpm idle.
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If you're already giving it 4 shots pre-start then I'm not entirely clear what is the benefit of doing the 5th while cranking. Is 4 not enough while 5 is too much?
 
asicer said:
If you're already giving it 4 shots pre-start then I'm not entirely clear what is the benefit of doing the 5th while cranking. Is 4 not enough while 5 is too much?
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Have you ever tried to start your engine, had it fire on a cylinder or two, only to die and you have to start the process over? This technique puts a little more fuel into the cylinder right when the engine needs it, so fuel is in the cylinder there at the next compression stroke. Or so it has worked out for me, ymmv.
 
My Cub's Continental C-85 has a Stromberg NAS3A1 carb which has no accelerator pump. The primer on a J-3 is nothing to write home about, but without it, getting it started in cold weather is next to impossible. It sprays fuel into the intake spider just above the carburetor. Even in hot weather, I prime 3-4 strokes and then pull through 4 blades with throttle fully closed and mags off. I then crack throttle slightly and turn mags on. It typically starts on first blade. When hot, it is best to simply turn mags on and prop it as it is really easy to flood when hot...and when it does it can be a royal pain to get started short of waiting 10-15 minutes. Flooded start on a handpropped airplane can be quite heart-stopping if you never done it!

My RV-4 has a Lycoming O-320 with Marvel MA3SPA with accelerator pump. Cold, I turn on fuel pump, prime 3 strokes and then as I start cranking will pump the throttle once. It typically starts within 2 blades. Hot, no prime and a single pump while cranking always works.

On our Continental W-670 engined Stearmans, pumping the throttle once the engine began to hit was pretty standard as it was hard to get sufficient prime to get them to fully start using the primer collocated with the hand-cranking point. The Lycoming R-680s didn't require throttle pumping. The R-985 on the BT-13 needed lots of pumping to get it going.

No one single method will work on all airplanes. I've frequently seen seemingly identical set-ups require vastly different techniques for successfully starting. Small Continentals all seem to have their own specific needs and you simply have to learn what works for each one individually. It can be frustrating until you learn what works.
 
James331 said:
For most normal GA stuff, if you prime right and everything is in good shape I've always had em fire up without any heroic measures.
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Yup, same here. The trick is to engage the starter *immediately* after priming. I'm not a fan of priming while cranking since I like to keep one hand on the starter and the other on the throttle. Also, I've seen a lot of pilots prime, and then slowly and methodically spend the next two minutes going through their checklist and yelling clear.. by which point the prime is gone and they wonder why it won't start. And "open 1/4 inch" doesn't mean "advance halfway so the engine immediately surges to 2,000 RPMs"

Lachlan said:
Most pilots don't actually understand where the primer puts fuel
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I know I didn't, no one could explain that well to me. It wasn't until I studied some Lycoming diagrams and carburetor cutaways that I understood what "pumping the throttle" does, IE, accelerator pump, and how the prime works

There was a thread a while ago about what we wished our instructors spent more time on. In my case it was definitely engine operation. I would say 99% of what I know about aircraft engines comes from my dad and brother (mechanical engineers, one at Pratt) and what I've read and taught myself. I think it's very important a pilot understands exactly how their engine works and how the carb heat, mixture, etc. all work together. I bet a lot of "engine failure just after takeoff" events could be avoided. I'll often hear renters at the club say "oh, NXXX runs a little rough" when it purrs just fine for me.. then I'll fly with these folks and find mixtures out of whack, completely illogical carb heat usage, and just lousy powerplant management overall
 
dell30rb said:
Works very well if you do it 2-3 times while cranking a cold engine that has an accelerator pump.

As mentioned earlier.. doing this while not cranking can lead to an engine fire.
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But if you're priming right, pre heating if needed, and you have a healthy airplane it shouldn't be needed.
 
dell30rb said:
That is true, but priming systems are often neglected.
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Fiddling with the throttle and maybe having a intake fire < Tuning up your priming system.
 
Tom-D said:
Really some bum dope in this tread.. the C-150 and the 152 have a MA3SPA carb. it has an accelerator pump.

Primer nozzles are simply a fitting with a calibrated orifice it does nothing but squirt fuel into the intake.
The accelerator pump squirts fuel at the throttle plate.
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Tom, my 152 service manual says that the accelerator pump was only added halfway through the production run (from s/n 15283592 and on, in a run from s/n's 15279406 to 15286033). in 1980 they added it. SO you're half right.

But on the subject of nozzles? You've never taken one apart. Take an old one, grind off the rolled rim on the business end, take out the little brass disc with that tiny orifice in it, and see the channels milled into the body at a tangent to the orifice. They're to spin that fuel so that centrifugal force tears the stream apart into an atomized cone. Take a good nozzle and connect it to a primer line, get someone to pump the primer, and see the spray pattern. It's not a squirted stream.
 
How common is it that the orifice for the primer gets plugged or fouled?
 
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