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george.mueller(at)aurora.
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 Posted: Wed Sep 05, 2007 6:12 am Post subject: More on the backfire |
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Just to clarify my previous message about backfiring on shut down, I was not turning the ignition off during the full power run up, but would get a backfire after doing a full power run up, backing the throttle back down to 1800 rpm, letting it run at 1800 rpm for about 15 seconds, and then shutting the engine off. The rpms would wind down after the key was turned to off, followed by a "bang" at the very end, then silence, except for the snickering coming from nearby hangers. I am doing the full power run ups to check the pitch on my prop. I found it very interesting, however, that the NASCAR folks check plugs by doing a shut down at full throttle.
I called Lockwood on this in addition to asking this list, and they said to synch the carbs. Even though I asked them to explain it to me twice, I am not entirely clear about the connection between the out of synch carbs and the backfire, but it had something to do with the extra vibration moving a needle in the carb and spilling fuel or something. So I will synch the carbs.
Another viable theory contributed by the list is stale fuel. I am using autogas from the airport, but it has been sitting in my tanks for a couple of months while I finish the airplane. I actually put Stable in the gas a while back because my understanding is that if you are going to keep autogas in your tanks more than 60 days you should add a fuel stabilizer. So I will put in some fresh gas and see if that helps.
I will also let the engine idle longer before shutting down.
George in Milwaukee
701 912UL [quote][b]
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ashontz
Joined: 27 Dec 2006
Posts: 723
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| Posted: Wed Sep 05, 2007 6:28 am Post subject: Re: More on the backfire |
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An outright backfire (not an engine knock or ping) would come from excess fuel igniting in the intake manifold. I would guess the excess fuel would be coming from the fact that even though you're turning off the engine, at 1800 rpms it's still pulling a lot of air (and fuel) through the carbs. Even so, that excess fuel should still be pulled through the cylinders even if it's not getting burned. So you have to ask yourself, what's the ignition source in the intake manifold? Could it be a stuck valve or hot carbon deposits on a valve that's causing the mixture to get an ignition source. Are the head gaskets good or could compression be leaking back and causing a pressure buildup in the intake manifold and causing a dieseling type of ignition. Does the carb have an accellerator pump that's maybe leaking and allowing too much fuel in?
How good does the engine run in general? What kind of an engine is it.
A Lycoming 0-200 occasionally backfires when you turn it off per the book (idle speed then cut the fuel off).
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planejim(at)bellsouth.net
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| Posted: Wed Sep 05, 2007 11:43 am Post subject: More on the backfire |
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George,
I too operate a 912UL 80 HP ( 553 hrs ). The "cooling down" procedure that I use is to operate the engine at about 2400 / 2500 RPM for a minute or two before turning off the ignition - the way the Rotax engine is shut down - no mixture control! I seem to remember reading about this procedure somewhere, but it isn't in the Operators Manual. I've never experienced " backfire" or "afterfire" Rotax does mention that " After a full-load ground test a short cooling run is necessary to prevent vapour formation in the cylinder head" This appears in the "Warming up period, ground test" section of the manual. This may be referring to the cooling system.
I understand the need of high power runs to set the prop but I suggest keeping the high power ground runs to a minimum and check the engine RPM / Prop Settings during a flight around the pattern. As long as all blades are the same angle and close to your final setting this has seemed to work for me.
Just my thoughts.
Jim Hoak 601HD
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cgbrt(at)mondenet.com
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| Posted: Sat Sep 08, 2007 7:30 am Post subject: More on the backfire |
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Hi George.
As you probably have found, letting the engine cool-down before shutdown eliminates backfiring on shutdown.
On amphibs I need lots of power to taxi out of the river and up to my hangar. The EGT when I park is around 1350 and shutting down immediately will cause backfires. I reduce to 2000 till the EGT stabilizes then throttle to idle until EGT is close to 1150. I then close one ignition to further reduce RPM before closing the second ignition.
If you don't have EGT gauges, idle for one to two minutes after using high power before shutdown.
Carl
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