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  #1  
Old 08-31-2010, 03:05 PM
SMO's Avatar
SMO SMO is offline
 
Join Date: Oct 2007
Location: Salmon Arm, BC
Posts: 933
Default Fuel in the Cowl

I haven't flown for 2 - 3 weeks. Last night I went out to the airport, did my walk around, pulled the -4 out of the hangar and went through my routine to start it up. Part of the routine is:

Mixture Rich
Boost Pump On
Two pumps on the throttle
Boost Pump Off
Mags On
Clear
Start

When I got to "Start" I found the battery was too weak to start the engine. Pulled the airplane back into the hangar, hooked up the battery charger, and went looking for what drained the battery. As part of my search I pulled the cowls off. That's when I found a puddle of raw fuel laying in the bottom of the cowl. The battery problem became secondary.

I ran the boost pump with the cowls off, and after a few seconds there was a stream of fuel coming from the FAB.

Today I phoned and talked to a person knowledgable about the IO-320 with the Silverhawk fuel servo and AFP boost pump. He said this was normal, particularly with an updraft servo, not to worry. He said with the engine running that fuel would get burned.

Given my reticence to get my butt burned, I thought I would ask for a second (and third and.............) opinion. Is this normal in your experience?
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1987 RV-4 Sold
2003 Super Decathlon - Sold
F1 EVO Rocket, first flight May 31/14
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Last edited by SMO : 08-31-2010 at 03:16 PM.
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  #2  
Old 08-31-2010, 09:36 PM
aerhed aerhed is offline
 
Join Date: Dec 2008
Location: Big Sandy, WY
Posts: 2,567
Default

Why would you pump the throttle on a bendix injected engine? In the first place, there's no accelerator pump. Even with a carburetor, throttle pumping is not the best priming method. I've replaced a few melted Marvel venturis.
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  #3  
Old 08-31-2010, 09:57 PM
PCHunt PCHunt is offline
 
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Location: San Diego, CA
Posts: 1,670
Default

And, if you DO pump the throttle on a carbureted engine, don't pump until after the engine is being cranked over. Otherwise, the fuel merely drips out down into the FAB.
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  #4  
Old 08-31-2010, 10:19 PM
bharral bharral is offline
 
Join Date: Mar 2005
Posts: 22
Default

If your Fuel Injection is typical, then the boost pump will push fuel into the space just behind the intake valves. If the engine is cranking or running, that fuel will be pulled into the cylinder. If the engine is not running, the fuel will run down the intake pipes and if you have vertical induction, it will then run out the bottom of the fuel servo and pool in the airbox. This is a fire hazard. If the boost pump has not lost its prime, then there is no reason to run the boost pump more than a few seconds.

I agree with aerhed that there is no reason to pump the throttle on your Fuel Injected engine. For engines with carburetors that have accelerator pumps, pumping the throttle will dump fuel into the carb intake and if the engine is not cranking or running, it will fall into the airbox, creating a fire hazard. For carb engines, if you must pump the throttle, it is best to do that only after the engine is cranking.

In the early days of flying my RV-4, I started a fire in the airbox by overpriming my FI engine. That made a big impression on me and I've not had any troubles since then
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  #5  
Old 08-31-2010, 11:17 PM
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Andrew M Andrew M is offline
 
Join Date: Aug 2006
Location: Secluded Lake,Alaska (AK49)
Posts: 359
Default Fuel Drain

Cessna 185"s have provisions to drain that fuel overboard (F.I. primer fuel)
Andrew
-9 preview
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  #6  
Old 09-01-2010, 09:02 AM
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SMO SMO is offline
 
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Location: Salmon Arm, BC
Posts: 933
Default You're right

Quote:
Originally Posted by aerhed View Post
Why would you pump the throttle on a bendix injected engine? In the first place, there's no accelerator pump.
Duh, of course you are right - I was blindly following the previous owners check list. Is pumping the throttle doing anything more than opening the air valve? Probably no harm, but no benefit either. I suppose the priming activity should be to just pressurize the system, nothing more.

Perhaps someone can explain to me why, when the engine is NOT running and the boost pump is on, the fuel runs out of the servo, but when the engine IS running it does not? I know my boost pump will flow 25+ gallons per hour and the engine never uses that much. So what mechanism stops the excess fuel from running overboard when the engine is running versus when it is not running?
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1987 RV-4 Sold
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F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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  #7  
Old 09-01-2010, 10:39 AM
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airguy airguy is offline
 
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Location: Garden City, Tx
Posts: 5,145
Default

The FI throttle body will inject fuel as a function of throttle position and air flow rate through the throttle body. I don't have personal knowledge of the SilverHawk design, but I would suspect it still moves a small amount of fuel even when there is no air flow (engine not running), which is exactly what you want for priming. If that's the case you're effectively flooding the engine and the excess is dribbling out.

On my injected C-172, a perfect cold start is achieved by mixture full rich, throttle cracked, boost pump until fuel flow gage flickers off the peg (about 2 seconds) then pump off, and crank. It will start in 2-3 blades. Hot starts require a voodoo witch doctor, but I'm getting them figured out!
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Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.

Last edited by airguy : 09-01-2010 at 10:42 AM.
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  #8  
Old 09-01-2010, 07:25 PM
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SkywayCaptain SkywayCaptain is offline
 
Join Date: Jan 2008
Location: Sheridan, IN
Posts: 94
Default

With the electric pump on and mixture rich on the SilverHawk, fuel will be flowing into the intake and start draining out of the sniffle valve (if installed) and possibly getting into the FAB (esp. on a trike). This is normal.

If it is above 55 degrees F, I don't prime mine at all.

throttle 1/4 inch
pump on
mixture rich
crank immediately

If it is cold (below 55F or so), simply wait a few seconds before cranking to prime. The colder it is, the longer you wait. (no more than about 10 seconds)

If the engine is hot and you have a short layover (less than 1 hour), the engine will probably be over-primed due to fuel being released from the "spider" into the cylinders. If you wait a few hours, this fuel will evaporate.

In this case I would start by...(caution voodoo to follow)

throttle 1 inch open
hold the stick back in taildragger with your "free" hand, er, um, legs
mixture lean (do not open the mixture at all, for any period of time prior to cranking)
boost pump on
crank the engine
when it starts, push the mixture to rich
adjust the throttle for smooth operation

if it starts and then quits, quickly move the mixture to lean. It will be pretty flooded now continue cranking, good luck.

As others have said, pumping the throttle just makes your arm tired.
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  #9  
Old 09-01-2010, 09:56 PM
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SMO SMO is offline
 
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Location: Salmon Arm, BC
Posts: 933
Default Talked to Precision

Thanks for the responses. I talked to Precision today, the mfg of the Silverhawk servo. Assuming I understood this correctly, in order to get fuel through for priming the mixture must be rich and the throttle open. The amount of fuel that is let through is restricted to the amount that would be used at idle. The servo stays on the idle circuit until the RPM gets over 1200 or so.

I have obviously been overpriming and will adjust my procedure. This has been a good learning for me.

Now to figure out why my battery did not keep its charge...................
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1987 RV-4 Sold
2003 Super Decathlon - Sold
F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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