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  #1  
Old 04-08-2012, 06:16 PM
RV10Rob's Avatar
RV10Rob RV10Rob is offline
 
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Default Loud "bang" during hot start

Twice recently I've heard a single loud "bang" during a hot (or really warm) start. Both times I realize I didn't prime as much as I usually do. What could this be (backfire, something else)? Do I need to check for damage anywhere?

I have an IO-540 w/ an impulse coupled mag and a Lightspeed plasma. I start on both (as I always have).

Thanks...

-Rob
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  #2  
Old 04-08-2012, 07:31 PM
Wayne Gillispie Wayne Gillispie is offline
 
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Default Mine has too

Rob,

My hot start procedure if shut down in less than one hour is the following: no priming, open throttle 1/2-full open, start(have to crank longer than cold starts), mixture full rich, throttle to idle. I have slick start on LH mag and RH mag is grounded during start. It backfires about half the time during my hot starts. If one has a better procedure, I would like to know too.
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  #3  
Old 04-08-2012, 07:56 PM
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Adam Oke Adam Oke is offline
 
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On my IO-550 Bonaire Ag-Truck;

Hot Starts: Mags on both, mixture rich, throttle full forward, run the boost and high/emergency until flow needle registers, once needle moves: leave boost on, turn off the high/emergency, throttle back to idle/cracked, and start. No pop, bang, or kaboom for me.
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  #4  
Old 04-08-2012, 08:00 PM
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Well for a start it is not a back fire. It is most likely an "after fire" in your exhaust.

A hot start will most likely result in for a second or so a few very lean charges going through that are not fired, and as others do the exhaust gases ignite the unburnt ones in the exhaust.

Hot start, Mixture ICO, throttle cracked about where 1700 RPM would be, I do not think the half way mark is necessary. And when it fires may well be too far, so as you crank and it first coughs into life, roll the mixture full rich, then lean back half way, turn on the boost pump to stop heat soaked pumps from vapour locking, and lean right back to peak rpm at a taxi speed.

Release brakes and taxi away.

Never failed in 600 hours
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  #5  
Old 04-08-2012, 08:17 PM
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Quote:
Originally Posted by RV10inOz View Post
Well for a start it is not a back fire. It is most likely an "after fire" in your exhaust.

A hot start will most likely result in for a second or so a few very lean charges going through that are not fired, and as others do the exhaust gases ignite the unburnt ones in the exhaust.

Hot start, Mixture ICO, throttle cracked about where 1700 RPM would be, I do not think the half way mark is necessary. And when it fires may well be too far, so as you crank and it first coughs into life, roll the mixture full rich, then lean back half way, turn on the boost pump to stop heat soaked pumps from vapour locking, and lean right back to peak rpm at a taxi speed.

Release brakes and taxi away.

Never failed in 600 hours
My hot start technique is pretty much identical and works consistently.
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  #6  
Old 04-08-2012, 08:26 PM
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Gooday DM,

You would hope so, its not like they are all radically different fuel systems

I have seen so many discussions on hot starts of various sorts, but once you understand what is going on in the fuel system, it becomes easy. Often the strange variations fluke success most of the time but fail once in a while, yet people persist with them not thinking about why.

Cheers
DB
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  #7  
Old 04-08-2012, 11:18 PM
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Thanks, guys... my starting technique is similar, so I suspect the difference is indeed that I didn't prime enough in these two instances. Any possibility of exhaust damage?

-Rob
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  #8  
Old 04-09-2012, 06:48 AM
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Rob,

NO NO NO,

WHERE DID I MENTION, OR DAVID MAIB MENTION "PRIMING"

That is for a cold start

Read my post again!!!

You priming when hot may well be the problem!!

COLD START
Prime for 3 seconds with WOT and full rich, watch fuel pressure rise to 20+PSI and count to three, close throttle and crack to a point that would give 1200 RPM and mixture idle cut off. Crank till she fires, then ramp up mixture, once stable lean to peak rpm an just past.

HOT start..... Read my post again. Do exactly what I say.....do NOTHING else.

It is that simple.

If n out go see David Maib.
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  #9  
Old 04-09-2012, 07:13 AM
Wayne Gillispie Wayne Gillispie is offline
 
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I will try it with 1/2" or so open throttle next time. I am assuming no priming is necessary due to fuel that has ran out through injectors that I hear a few minutes after shut down. Thanks Rob for asking and for the answers guys.

On cold starts, I turn boost pump on with throttle 1/4" open, mixture idle cutoff, watch for good fpres, mixture rich for 4-6 sec depending on engine temp, idle cutoff, then start. No problem on cold starts. I am sure going to get some hot start practice soon with 90F temps around the corner.
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  #10  
Old 04-09-2012, 10:05 AM
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Hot starts on an injected engine should not require priming. The fuel in the spider lines will boil on shutdown as the lines heatsoak and the fuel will squirt into the manifolds, so you are essentially starting a flooded engine.

On my IO360, hot starts are Mixture cutoff, throttle open, crank. On the first engine cough, continue crank and quickly shove mixture full rich first, then throttle back, I usually catch it right as it rises through 700 rpm. Works like a charm for me, but every engine has it's quirks.
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Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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