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Old 11-24-2017, 09:20 AM
fbrewer fbrewer is offline
Join Date: Sep 2017
Location: Leander
Posts: 53
Default Prime with Throttle

Our starting checklist has the following step:

Prime with Throttle - Prime!

I think I know what is happening, but would like your comments.

What does this mean?

How do I Prime?

What happens when I Prime?
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Old 11-24-2017, 09:33 AM
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Mach.26 Mach.26 is offline
Join Date: Jan 2017
Location: Ozark, AL
Posts: 62

Probably what you think, with the fuel pump on you can pump the throttle all the way open and closed a few times to inject fuel into the engine for starting. Too much and you’re likely to flood the engine, probably will take some experience to find out how much your particular engine likes depending on how long it’s been sitting. I assumed you a talking about procedures for starting a fuel injected engine.
Adam Wright
RV-8 Builder 83611

Last edited by Mach.26 : 11-24-2017 at 04:16 PM.
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Old 11-24-2017, 09:56 AM
Canadian_JOY Canadian_JOY is offline
Join Date: Aug 2007
Location: Ontario, Canada
Posts: 1,850

A word of caution is required here, but first some background info.

Our carbs are updraft carbs - air, mixed with fuel, moves upward through the carb into the cylinders. Many carbs have what is referred to as an "accelerator pump" - a small piston pump designed to squirt raw fuel into the air flowing through the carb. Normal activation of the throttle does not invoke the accelerator pump. Rapid operation of the throttle causes the pump to squirt fuel into the carb.

Now for the word of caution. Squirting raw fuel into the carb can be a very bad thing. Fuel in liquid form will follow the laws of gravity, eventually dripping downward and pooling in the air intake. A backfire during starting provides an ignition source to light that pooled fuel. An engine fire of this nature can be very nasty indeed.

If one is going to use the accelerator pump it is best to do so after the starter is engaged. The cranking engine is sucking in lots of air - this air will pick up the raw fuel being squirted in by the accelerator pump, carrying it to the cylinders where it will do the most good.

Operating the accelerator pump before cranking is an invitation to an engine fire. Operating the accelerator pump during cranking is a recipe for good engine starts. For those of us who operate in cold weather conditions, the accelerator pump may not be a reliable enough solution so a separate priming system which injects fuel directly into the cylinder heads is often installed.
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Old 11-24-2017, 10:27 AM
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snopercod snopercod is offline
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Location: Asheville, NC
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What Canadian JOY said. In this photo, you can see the accelerator pump discharge nozzle aiming up inside the venturi. That's where the fuel squirts out when you pump the throttle:

On my plane, I had to cap off the primer because it was leaking through. Now I use the accelerator pump, but I can only get the engine started by pumping the throttle down to around 40 F. Below that, the engine is very difficult to start even by pumping furiously. I came close to draining the battery trying to get her started at 30F.
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Old 11-24-2017, 10:42 AM
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mike newall mike newall is offline
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Location: Yorkshire, England
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Have a fire extinguisher handy if you try using the throttle.....

Plenty of accounts of a dull whoomph followed by smoke rising.
"I add a little excitement, a little spice to your lives, and all you do is complain!" - Q

Donated in 2018
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Old 11-24-2017, 10:54 AM
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AndyRV7 AndyRV7 is offline
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So how does the fuel react to various throttle applications? Is it always the accelerator pump that adds fuel to the carb inlet? Does it matter how fast or much you apply the throttle? That is, can you crank the engine and just crack the throttle open a little, or does the throttle need to be opened all the way to get any fuel to squirt out? I always wondered about this.
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Old 11-24-2017, 10:57 AM
Kyle Boatright Kyle Boatright is online now
Join Date: Aug 2005
Location: Atlanta, GA
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The accelerator pump is basically a squirt gun. It only squirts fuel as you advance the throttle.

Edited to add: The amount of fuel squirted is proportional to the distance you advance the throttle. The rate at which you advance the throttle drives the velocity of the fuel spray.
Kyle Boatright
Atlanta, GA
2001 RV-6 N46KB
2019(?) RV-10

Last edited by Kyle Boatright : 11-24-2017 at 11:32 AM.
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Old 11-24-2017, 11:21 AM
Canadian_JOY Canadian_JOY is offline
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Location: Ontario, Canada
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Originally Posted by Kyle Boatright View Post
The accelerator pump is basically a squirt gun. It only squirts fuel as you advance the throttle.
To elaborate on Kyle's comment, it's like a squirt gun where the trigger is the pump - the harder you pull the trigger, the bigger the squirt.

In the case of these carb's, the pump is a piston pump which is mechanically tied to the throttle linkage. Move the throttle fast and you get a good squirt of fuel. Move the throttle slowly and the pump doesn't make enough pressure to squirt and fuel.

(Thanks for posting that great picture, Kyle - it really clearly shows the accelerator pump outlet!)
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Old 11-24-2017, 11:43 AM
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Ironflight Ironflight is offline
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Default Video!

Here's a little video I shot of the accelerator pump in action:

For a description, you can see my blog post from a few months back on
Paul F. Dye
Editor in Chief - KITPLANES Magazine
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Old 11-24-2017, 12:33 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,956

Despite what some say and as detailed in Paul's blog post, throttle pumping to prime can be done safely but it must be done correctly (I am pretty sure that induction system fires are caused by incorrect priming whether it be from throttle pumping or using an actual priming system).

- It must be done with a battery in a condition that allows normal/brisk cranking speed.
- The throttle should only be pumped (briskly / about one stroke in and out per second) while the engine is cranking (briskly).
Note - On aircraft that have a more traditional priming system (whether it be a pressure plunger or a valve that is electrically opened) I only use it while the engine is cranking as well, in order to reduce the risk of induction system fire that still exists.
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")

Last edited by rvbuilder2002 : 11-24-2017 at 12:35 PM.
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