Priming and throttle position

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woxof

Pre-takeoff checklist
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woxof
I saw this on another forum about starting with the throttle completely closed and found it interesting

Any comments

"Why prime? When priming, two cylinders become over rich and on at least one ( and probably two cylinders)) cylinder the prime fuel runs out the open or partially open exhaust valve. The engine must be turned by the starter until the two over rich cylinders reach the proper mixture and the two lean cylinder do the same. The typical carburetor (non pressure carburetor) on 0-320, 0-360, 0-200 engines has an idle circuit and an accelerator pump. The idle circuit is fully used only when the throttle is fully closed.

A retired rocket scientist, who is now an A&P, espouses utilizing the accelerator pump by rapidly moving the throttle from fully closed to fully open one to four times depending on the temp; more for cold temps less for warm/hot. Then fully CLOSE the throttle and engage the starter. The engine will start almost instantly because the mixture is correct and it is operating off the IDLE circuit in the carb. I use this procedure on my 0-320 powered Grumman AA1-C Lynx and it starts by the second blade every time. I admit that I didn't fully understand the WHY's of this until I became an A&P and this is certainly not how I was taught but it works. When I get called because an airplane will not start immediately after refueling ( hot engine start) I always use this procedure with 100% success."
 
So is the rocket scientist saying not to prime? Just use the accelerator pump method?
 
Sure he knows better than the engineers that designed the aircraft with a primer. Hard to take engine operation advice seriously from someone that uses zeros instead of Os for the engine type.
 
I use a similar process for my "O"-320. Except I crack the throttle instead of closing it.
 
It’s a great idea if you don’t mind the occasional fire burning the plane to the ground. The fuel from the accelerator pump pools in the airbox, not near the inlet ports of the engine. If the engine backfires the fire is not so well contained as it would be if the primer had been used. Some French Aero Club aircraft long ago came with no primer and the POH recommended this procedure or close. It apparently became the French Club norm for all aircraft, still practiced to some degree. The results over time were as mentioned.

If you do use the accelerator pump for startup make sure the starter is already engaged and engine turning over, so the fuel is sucked towards the engine and doesn’t pool.
 
time it, see how long the fuel delivered, stays in the carb. that is how long it takes to end in the air box.
 
Just more internet jargon. Lycoming cautions against using the throttle for priming. Pumping the throttle in place of using the primer doesn’t affect the cylinder mixture in any significant way. Even so, the accelerator pump should only be used when the starter is engaged.
 
I owned an O-320 in a Cherokee for 14 years. I divided my cold start procedure depending on ambient temp, above 50F and below 50F. Above 50 I did basically what he suggested except that I pumped the throttle only after the prop starts turning. Throttle cracked open, starter spins, count two blades then pump and pull back once or twice on the throttle and release starter when engine starts. Priming when not spinning basically runs fuel out on the ground on an updraft carburetor mounted underneath the engine.
Below 50F I crack open the throttle and leave it alone. The crack open position needs to be enough to keep the engine running when cold but not too much. Starter spins, count two blades then pump primer vigorously until engine starts. The idea is to have the AF ratio too lean at first and force it towards rich which guarantees you will cross the optimum AF ratio to support the start of combustion. If the engine fires but goes too slow to keep running, add a little more throttle and try again. If it is really cold the engine may start and keep running a few seconds then start to die out. Quickly pump the primer enough to keep it running. You may have to do that a couple of times before the fuel starts vaporizing on its own. Hope this helps. Petehdgs
 
The retired rocket scientist who happens to be the artist who does the best work on earth on 4 seat Grummans basically advocates for no prime and to just start with the boost pump on and mix full rich. If it doesn't start, then pump once or twice with the starter engaged and it will.

Guess what? It works.

GRG55 said:
Isn't the risk of an induction fire increased quite a bit by using this technique?
Click to expand...

If you're starting quickly, not really, but it does exist.
 
Interestingly, the C162 does not have a primer. Instead, the POH says to pump the throttle. I will continue to use the primer in my 172.
 
I have seen so many variations on this in the manufacturer manuals for different types, including some which say to use the accelerator pump, I think this is truly a "read the manual" question,
 
The first thing to be clear about, is that every engine is exactly the same. So what works best on a Wright R-790 is going to be what works best on my Rotax.

woxof said:
on at least one ( and probably two cylinders)) cylinder the prime fuel runs out the open or partially open exhaust valve.
Click to expand...
How does the fuel get to the exhaust valve from the intake manifold to run out? Many horizontally opposed aircraft have the intake (where the primer typically squirts) underneath the cylinder so the fuel would have to run up-hill, past the intake valve, through the cylinder, then to the exhaust valve.That doesn't make sense.

On many engines, the primer puts fuel in the intake manifold near the valves. Some will evaporate, some will puddle. As you crank the engine, some of the puddle will move with the air flow and shear off the valve / seat and mix with the air. This is how you get enough fuel into the cylinder to make a combustible mixture. Lotsa fuel in the manifold in the hope that some will make it into the cylinder. The key to a cold start is to have enough extra fuel in the manifold that enough makes it into the cylinders. Primers, chokes, bypass circuits in the carb, that's what they do. Dump in lots of extra fuel into the manifold. If the engine is cold, there is not a lot of evaporation...

On yea olde other hande, there is the accelerator pump. Carburetors sorta mix fuel and air, some evaporates, some splashes onto the throttle plate and gets sheared off, some ends up as a liquid film (or "puddle") on the intake wall. In the auto industry, back in the olden days, a lot of effort was put into making the flow of liquid fuel on the intake walls make it somewhat equally to each cylinder. Not easy to do. A lot of the imbalance is due to the fact that the throttle plate is angled and tends to throw the fuel to one side, favoring some cylinders over others. But, in any case, a lot of the fuel that the carburetor blobs into the manifold ends up as a liquid film on the surface that partly evaporates and partly flows along with the air flow until it gets to the intake valve. How much of this film evaporates and how fast it moves tends to be a function of temperatures, manifold pressure and airflow rates. When you open the throttle, you add a lot of air and increase the manifold pressure - this means that the engine needs a lot more fuel and evaporation in the manifold is reduced - so the cylinders will not get enough fuel until the film on the manifold builds up and makes it to the intake valves. To compensate for this, most carburetors have an accelerator pump that shoots in a blob of fuel when you open the throttle in an attempt to cover up for the extra fuel ending up in the puddle and the delays in getting that fuel to the cylinders.

Now, both primers and accelerator pumps put fuel into the manifold. The difference is where. Primers typically are close to the cylinders so the fuel will get there quickly. Accelerator pumps add fuel, but much of it ends up on the walls of the manifold, and if you have an updraft carburetor much will run out of the manifold and towards the air filter. Less than ideal. In general. You also have to wonder why the engine designers would have put primers on an engine if the accelerator pump works better. Perhaps it is just tradition. Perhaps the engineers that design engines aren't as smart as a retired rocket scientist. Or, perhaps, a fuel system design engineer knows more about fuel systems for internal combustion engines than someone who is a rocket expert.

In the end, both are likely to work and it's your airplane. But were I (retired auto engine air/fuel control system guy) to be flying an airplane with a primer and an accelerator pump, I would reach for the primer.
 
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A primer nozzle is internally designed to atomize the fuel. The accelerator pump just squirts a tight stream of liquid, unatomized, since it's designed for acceleration, not priming, and acceleration happens with a lot of airflow through the carb that tears that stream apart and atomizes it on the way to the cylinders. At no or very low RPM it doesn't work nearly as well as good clean primer nozzles. It just results in puddling and a risk of fire.

I have fixed stubborn-starting airplanes by just installing new primer nozzles. They get clogged over time with residual fuel getting coked inside them due to heat. They end up either plugged or emitting a non-atomized stream.
 
tell me how fuel is carried from any cylinder when there is no air flow?

If there is no cylinder on intake stroke there is no airflow. If the engine is not cranking there is no airflow, the fuel simply drains down hill until it finds a way out.

the exception is with a engine with a plenum chamber such as the Warner 500 /Wright 985radial in these engines the fuel simply will set in chamber until it see airflow.
 
Dan Thomas said:
A
I have fixed stubborn-starting airplanes by just installing new primer nozzles. They get clogged over time with residual fuel getting coked inside them due to heat. They end up either plugged or emitting a non-atomized stream.
Click to expand...
You are supposed to clean them. Hobbes Number 9 works great.
it is supposed to this each 100 hour
 
Stingray Don said:
Interestingly, the C162 does not have a primer. Instead, the POH says to pump the throttle.
Click to expand...
It was an option. The POH says to pump the throttle if it's not installed. It also has a Continental O-200D while the discussion in the OP relates to Lycoming engines.
 
woxof said:
"Why prime? When priming, two cylinders become over rich and on at least one ( and probably two cylinders)) cylinder the prime fuel runs out the open or partially open exhaust valve. The engine must be turned by the starter until the two over rich cylinders reach the proper mixture and the two lean cylinder do the same. The typical carburetor (non pressure carburetor) on 0-320, 0-360, 0-200 engines has an idle circuit and an accelerator pump. The idle circuit is fully used only when the throttle is fully closed.

A retired rocket scientist, who is now an A&P, espouses utilizing the accelerator pump by rapidly moving the throttle from fully closed to fully open one to four times depending on the temp; more for cold temps less for warm/hot. Then fully CLOSE the throttle and engage the starter. The engine will start almost instantly because the mixture is correct and it is operating off the IDLE circuit in the carb. I use this procedure on my 0-320 powered Grumman AA1-C Lynx and it starts by the second blade every time. I admit that I didn't fully understand the WHY's of this until I became an A&P and this is certainly not how I was taught but it works. When I get called because an airplane will not start immediately after refueling ( hot engine start) I always use this procedure with 100% success."
Click to expand...
This guy is an idiot. Anyone who has to invoke an appeal to authority fallacy (especially the beaten to death 'rocket scientist' one) already loses some credibility (queue the GOT Joffrey Tywin moment "anyone who has to say he's a king is no true king".. but I digress

The accelerator pump is a very sloppy method of doing this.. it helps, sure, and you often have to give it a few squirts when cranking.. but the prime's purpose and overall function is misunderstood by this "rocket scientist". And you shouldn't be priming the living daylights out of it either. Two shots of primer ought to help, and if you *must* then one or two throttle pumps when cranking. And yes, going to town on the accelerator pump can (and has) started fires.

PS.. the people at Lycoming and Continental, IE, the actual engineers who developed these, know what they're doing (despite my oft critique of our ancient tech, they're the SME's in their field). As far as I'm aware rockets and internal combustion engines share very little in common, and while A&P's have their own street knowledge and experience (and some have god-like knowledge of our planes), they're not typically engineers, and aren't usually involved in the design and development of our engines. And for all we know this "rocket scientist" didn't deal with propulsion but worked on the trigonometry side of rockets.
 
Tom-D said:
You are supposed to clean them. Hobbes Number 9 works great.
it is supposed to this each 100 hour
Click to expand...
I've tried that. If one did it every 100 hours it would indeed work, but by the time most folks tackle it, the nozzle is so badly plugged that no cleaner can get into it. New nozzles are less than $18 each, meaning that one can replace them at less overall cost than trying to get stubborn ones cleared. I even made a pressure tank for a small amount of a powerful carbon stripper, the nozzles threaded into it and the tank pressurized to force the stuff through the nozzle. That didn't work either.

https://www.aircraftspruce.com/catalog/hapages/primperfittingsan4022-1.php
 
Dan Thomas said:
I've tried that. If one did it every 100 hours it would indeed work, but by the time most folks tackle it, the nozzle is so badly plugged that no cleaner can get into it. New nozzles are less than $18 each, meaning that one can replace them at less overall cost than trying to get stubborn ones cleared. I even made a pressure tank for a small amount of a powerful carbon stripper, the nozzles threaded into it and the tank pressurized to force the stuff through the nozzle. That didn't work either.

https://www.aircraftspruce.com/catalog/hapages/primperfittingsan4022-1.php
Click to expand...
When you clean them regularly. it's not a problem.
a Qt.of Hoppys is a life time supply
 
No accelerator pump in the carb.
No primer installed.
No mixture control.
Fuel on, throttle closed, mags off. Yank the prop until your hands hurt. Pull it twice more.
Fuel off, throttle cracked, mags hot. Yank the prop.
Sometimes it starts, sometimes you start over.

You guys are spoiled. :raspberry:
 
Shepherd said:
No accelerator pump in the carb.
No primer installed.
No mixture control.
Fuel on, throttle closed, mags off. Yank the prop until your hands hurt. Pull it twice more.
Fuel off, throttle cracked, mags hot. Yank the prop.
Sometimes it starts, sometimes you start over.

You guys are spoiled. :raspberry:
Click to expand...
My A-65 had no accelerator pump. I did have a primer that I almost never used. I had the carb float set right where the manual specifies, and with the throttle closed and mags hot, you could hear the fuel being noisily sucked out of the idle nozzles. It would catch on the fifth or sixth blade, no later than the eighth. In cold (Canadian cold) weather I would give it a single squirt of prime. Anything more and flooding was guaranteed.

Float level is critical. If it's too low the fuel has to be lifted farther up to the idle nozzles, which requires more vacuum, which requires a harder pull on the prop or you get very little fuel. It runs back down after every pull. If the float level is too high fuel will dribble out of the main nozzle. The throttle setting for idle is important, too; it it's set too high the throttle plate is farther from the carb bore's wall and the nozzles, and little suction is generated when you prop.
 
For those who love Hoppes#9... It is available in lieu of Christmas tree air fresheners... Carry on...

Oh, and my instructor for my BFR in the club 172 teaches to use throttle (2pumps) then 1/8" open for start up... I'll be checking the manual... But IIRC, it says prime....
325cf1c6d5d4cbf2b4dc1f5cf0e99e72.jpg


fly to the scene of the incident, or be recovered at the scene of the tragedy

> edit to add... Yes, the poh on the 172N says use 2-6 shots of prime... Throttle open 1/8" no prime on warm engine... I'll try same next trip. Lycoming engine.
 
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CharlieD3 said:
Oh, and my instructor for my BFR in the club 172 teaches to use throttle (2pumps) then 1/8" open for start up... I'll be checking the manual... But IIRC, it says prime....
Click to expand...

The POH trumps an instructor every time. There are way too many instructors that don't know what they don't know. One instructor can mislead 50 students, ten of which become instructors and mislead 50 each, for 500 more misled students. Wrong information spreads like wildfire and becomes so entrenched that it's impossible to correct it. Even after an engine fire on startup....
 
Dan Thomas said:
The POH trumps an instructor every time. There are way too many instructors that don't know what they don't know. One instructor can mislead 50 students, ten of which become instructors and mislead 50 each, for 500 more misled students. Wrong information spreads like wildfire and becomes so entrenched that it's impossible to correct it. Even after an engine fire on startup....
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Inconsequential... What's important is the availability of Hoppes #9 as an air freshener.

JK... I'll pass it on...

fly to the scene of the incident, or be recovered at the scene of the tragedy
 
amazing, another thread where you feel that the internet legends know more than the engineers that designed and tested something know. Do you follow anything in your POH?


N1120A said:
The retired rocket scientist who happens to be the artist who does the best work on earth on 4 seat Grummans basically advocates for no prime and to just start with the boost pump on and mix full rich. If it doesn't start, then pump once or twice with the starter engaged and it will.

Guess what? It works.



If you're starting quickly, not really, but it does exist.
Click to expand...
 
unsafervguy said:
amazing, another thread where you feel that the internet legends know more than the engineers that designed and tested something know. Do you follow anything in your POH?
Click to expand...
Ever think the engineers tried to cover all the bases and missed
 
There is much to be gained by good old RTFM. If you prime the engine when cold and it starts, why isn't that good enough? FWIW every Lycoming I've flown (that was well maintained) will start promptly when cold with around two shots of prime, and will start hot without prime and maybe one throttle pump during cranking. Certainly worked for every AA-1 or AA-5 series I have owned or flown.
 
Just a tidbit of information. The Hoppe's #9 of today is different than the one from yesteryear. I don't know when it changed, but the formula today is different. I'm guessing that one of the ingredients, was on a list of things that cause additional appendages to grow in children or something like that. The bottle that I got a couple years ago requires a bit more work to clean barrels, and injectors and primer nozzles than it used to.

Still works though.
 
Tom-D said:
Ever think the engineers tried to cover all the bases and missed
Click to expand...
Happens.

Also happens that people get away with doing things that are 100% nonsense. Just look at all the stuff in the wishful thinking aisle at the auto parts store.

I am reminded of the time that old Smitty told me the secret of long life when I dropped by while he was eating dinner - pulled a pigs knuckle (or something like that) out of his bowl - "Fat - that's what you need to keep it up...".
 
donjohnston said:
Just a tidbit of information. The Hoppe's #9 of today is different than the one from yesteryear. I don't know when it changed, but the formula today is different. I'm guessing that one of the ingredients, was on a list of things that cause additional appendages to grow in children or something like that. The bottle that I got a couple years ago requires a bit more work to clean barrels, and injectors and primer nozzles than it used to.

Still works though.
Click to expand...
I tried BONDERITE C-IC 79 AERO (FORMERLY METAL PREP 79) it works
 
donjohnston said:
Just a tidbit of information. The Hoppe's #9 of today is different than the one from yesteryear. I don't know when it changed, but the formula today is different. I'm guessing that one of the ingredients, was on a list of things that cause additional appendages to grow in children or something like that. The bottle that I got a couple years ago requires a bit more work to clean barrels, and injectors and primer nozzles than it used to.

Still works though.
Click to expand...
There's also a Hoppe's version that eats copper, which means it ruins brass primer nozzles and fuel injectors. It's intended to remove copper residue left by jacketed bullets in gun barrels. A Continental engineering rep warned us of that one.
 
Dan Thomas said:
I've tried that. If one did it every 100 hours it would indeed work, but by the time most folks tackle it, the nozzle is so badly plugged that no cleaner can get into it. New nozzles are less than $18 each, meaning that one can replace them at less overall cost than trying to get stubborn ones cleared. I
Click to expand...
Thinking this could be one of my problems starting. I've never cleaned my primer nozzles. Looked up price yesterday, $39 each. I have 1 cylinder that has less than 50 hours on it, probably the only nozzle working.

I'm going to order a set and see if that helps..

edit: when I pump the throttle the engine is turning over
 
Stingray Don said:
Interestingly, the C162 does not have a primer. Instead, the POH says to pump the throttle. I will continue to use the primer in my 172.
Click to expand...
The chief pilot (at the time) at Cessna checked me out in the 162, and told me to pump the throttle. I questioned that, and he said the carb is a horizontal draft so no fire hazard like the 150 & 172’s.
 
The POH procedure for the 172RG and 182RG is to pump the throttle. These also use side-draft carbs. The primer is only used in "extremely cold temperatures".

I was giving a rental checkout in my 150 to a pilot who had been flying a 182RG. I was not happy with the loud backfire that occurred when he pumped the throttle instead of using the primer.

Follow your POH people.
 
AKBill said:
Thinking this could be one of my problems starting. I've never cleaned my primer nozzles. Looked up price yesterday, $39 each. I have 1 cylinder that has less than 50 hours on it, probably the only nozzle working.

I'm going to order a set and see if that helps..

edit: when I pump the throttle the engine is turning over
Click to expand...
$39 apiece sound high. Do you have the AN4022-2, the 90° version?
 
DGlaeser said:
The chief pilot (at the time) at Cessna checked me out in the 162, and told me to pump the throttle. I questioned that, and he said the carb is a horizontal draft so no fire hazard like the 150 & 172’s.
Click to expand...
Cessna 162 has a 0-200 and has a MA3SPA carb.with is a vertical throat
 
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