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912 80 hp back-fire on shutdown

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ejessee

Joined: 18 Apr 2009
Posts: 60
Location: Raleigh, NC

Posted: Sat Jan 28, 2012 2:58 pm Post subject: 912 80 hp back-fire on shutdown

Hello all,

I did a search in the forum with "backfire", "back-fire", and "back fire" with no results, So here goes....



I have an 80 hp 912 UL on a Zenith 701.  When I return to my hangar after flight, unless I do a 2 -3 minute stabilization run at 2100 RPM, my 912 backfires like a shotgun when I turn it off.  I have newly rebuilt carbs, perfectly balanced carbs, plugs at .024.  The engine runs perfect, uses no oil.  My exhaust stacks are heat-wrapped.  As severely as it backfires, I am afraid I am going to damage my exhaust system.



Anyone else ever experience this?  Anybody know what is causing it.  Is something wrong or is this just the nature of the engine.



AS always, thanks for your help.  This forum is a fantastic resource.

_________________
Ernest Jessee
N4931M

mortweaver(at)sbcglobal.n
Guest

Posted: Sun Jan 29, 2012 9:12 am Post subject: 912 80 hp back-fire on shutdown

I would suggest, if you have not already checked, to verify that you do not have partial blockage of a crab idle jet.  If you had complete blockage of a crab idle jet, the result would be rough engine operation in low engine rpm range due to failure of crab float function (float bounce).  If there is momentary failure of crab float function (engine shaking will upset float function) during flight or at engine shut down, you may get a miss fire.  I found at one time, following a 912 installation, where a very small flake of fuel hose (probably scuffed off from slipping fuel hose over a crab fuel line nipple) lodged in a crab idle jet.  Think about it....grounded "P" leads from ignition switch and an available supply of fuel (caused by engine shaking - float bounce) expelled  from an engine cylinder into a hot exhaust pipe.   I had smooth engine operation in mid and high rpm range and rough idle.  I would get one loud back fire from one exhaust pipe on shut down.  Thinking that one crab must be flooding out, I pulled plugs and found nothing unusual.  I had to use a fine wire bristle from a wire brush to force the blockage clear of the idle jet.  With carburetor equiped 912 I have notice that upon engine shut down at too high of an idle rpm may cause the engine to run on a few extra revolutions.  I get the smoothest shut down when pulling the idle rpm down to about 1700 and then turn the ignition switch off.  Although not frequent, I have noticed that when flying in rough air that just for an instant engine operation is interrupted by a slight hic-up.  Not a miss fire, but just a momentary change in rpm.  I suspect that is due to carb  float bounce and have learned to accept perfection is not to expected in all things mechanical.

 Dave Weaver



--- On Sat, 1/28/12, Richard Girard <aslsa.rng(at)gmail.com> wrote:



  	  Quote: 	
		  

From: Richard Girard <aslsa.rng(at)gmail.com>

Subject: Re: 912 80 hp back-fire on shutdown

To: rotaxengines-list(at)matronics.com, kolb-list(at)matronics.com

Date: Saturday, January 28, 2012, 11:33 PM



 There are two distinct and opposing conditions that could cause your engine to backfire. Too rich an idle mixture letting unburned fuel slip by the valves into an exhaust system already heated by high EGT's when the power is pulled back on approach. Or the mixture is too lean and it's on the verge of running on after the ignition is shut off.(my Harley used to do this on warm up when I pulled the choke off too early).  What is your idle RPM before the stabilization run? Is it different after the stabilization run?

 If your heart can stand the strain, let it backfire one last time and immediately look at the plugs. Do the same after a stabilization run and note if there's any difference.

 



 Rick Girard



 On Sat, Jan 28, 2012 at 4:58 PM, ejessee <eejessee(at)us.ibm.com (eejessee(at)us.ibm.com)> wrote:

  	  Quote: 	
		  --> RotaxEngines-List message posted by: "ejessee" <eejessee(at)us.ibm.com (eejessee(at)us.ibm.com)>



Hello all,

I did a search in the forum with "backfire", "back-fire", and "back fire" with no results, So here goes....



I have an 80 hp 912 UL on a Zenith 701.  When I return to my hangar after flight, unless I do a 2 -3 minute stabilization run at 2100 RPM, my 912 backfires like a shotgun when I turn it off.  I have newly rebuilt carbs, perfectly balanced carbs, plugs at .024.  The engine runs perfect, uses no oil.  My exhaust stacks are heat-wrapped.  As severely as it backfires, I am afraid I am going to damage my exhaust system.



Anyone else ever experience  this?  Anybody know what is causing it.  Is something wrong or is this just the nature of the engine.



AS always, thanks for your help.  This forum is a fantastic resource.



--------

Ernest Jessee

N4931M





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=365064#365064



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 Zulu Delta

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 Thanks, Homer GBYM

 



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 [quote][b]

ejessee

Joined: 18 Apr 2009
Posts: 60
Location: Raleigh, NC

Posted: Mon Jan 30, 2012 6:11 am Post subject: Re: 912 80 hp back-fire on shutdown

Dave,

Thank you for the information.  This "crab Idle jet"... is that in the carburetor?

_________________
Ernest Jessee
N4931M

mortweaver(at)sbcglobal.n
Guest

Posted: Mon Jan 30, 2012 6:40 am Post subject: 912 80 hp back-fire on shutdown

No....that discomfort which some may suffer when flying was not what I was referring to.  The typo was not intended to cast me as a comedian.  I will strive toward better editing in the future.

 Dave Weaver



--- On Mon, 1/30/12, ejessee <eejessee(at)us.ibm.com> wrote:



  	  Quote: 	
		  

From: ejessee <eejessee(at)us.ibm.com>

Subject: Re: 912 80 hp back-fire on shutdown

To: rotaxengines-list(at)matronics.com

Date: Monday, January 30, 2012, 8:11 AM



 --> RotaxEngines-List message posted by: "ejessee" <eejessee(at)us.ibm.com (eejessee(at)us.ibm.com)>



Dave,

Thank you for the information.  This "crab Idle jet".. is that in the carburetor?



--------

Ernest Jessee

N4931M





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=365207#365207





onics.com/Navigator?RotaxEngines-List" target=_blank>http://www.matronics;                     -Maww.matronics.com/contribution" =======





	
 [quote][b]

dashwoodlock(at)yahoo.ca
Guest

Posted: Mon Jan 30, 2012 6:58 am Post subject: 912 80 hp back-fire on shutdown

if you have to ask this question. you have to get an engine mechanic to do the work...



--- On Mon, 1/30/12, ejessee <eejessee(at)us.ibm.com> wrote:

  	  Quote: 	
		  

From: ejessee <eejessee(at)us.ibm.com>

Subject: Re: 912 80 hp back-fire on shutdown

To: rotaxengines-list(at)matronics.com

Received: Monday, January 30, 2012, 9:11 AM



 --> RotaxEngines-List message posted by: "ejessee" <eejessee(at)us.ibm.com (eejessee(at)us.ibm.com)>



Dave,

Thank you for the information.  This "crab Idle jet"... is that in the carburetor?



--------

Ernest Jessee

N4931M





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=365207#365207



http://www.matronics;                     -Maww.matronics.com/contribution" =======





	
 [quote][b]

Jason Omelchuck

Joined: 07 May 2009
Posts: 120
Location: Portland Oregon

Posted: Mon Jan 30, 2012 8:20 am Post subject: Re: 912 80 hp back-fire on shutdown

Technically a back fire is through the carbs and an after fire comes out of the exhaust.  It might be possible that the heat wrapping on the exhaust allows the exhaust pipes to get hot enough to ignite a fuel air mixture.  When you turn the engine off, it pumps some un-burned mixture through the engine where it is ignited in the exhaust.



my $.02 worth

Jason



 	  ejessee wrote: 	
		  Hello all,

I did a search in the forum with "backfire", "back-fire", and "back fire" with no results, So here goes....



I have an 80 hp 912 UL on a Zenith 701.  When I return to my hangar after flight, unless I do a 2 -3 minute stabilization run at 2100 RPM, my 912 backfires like a shotgun when I turn it off.  I have newly rebuilt carbs, perfectly balanced carbs, plugs at .024.  The engine runs perfect, uses no oil.  My exhaust stacks are heat-wrapped.  As severely as it backfires, I am afraid I am going to damage my exhaust system.



Anyone else ever experience this?  Anybody know what is causing it.  Is something wrong or is this just the nature of the engine.



AS always, thanks for your help.  This forum is a fantastic resource.

ejessee

Joined: 18 Apr 2009
Posts: 60
Location: Raleigh, NC

Posted: Mon Jan 30, 2012 9:55 am Post subject: Re: 912 80 hp back-fire on shutdown

Thanks.  I have suspected the heat wrap as perhaps adding to the problem.  



As for the implication that I should get a mechanic... Hey.. I just missed the joke.  I conduct my plane maintenance with humility and try to get someone to check my work, but I am very capable.  I have rebuilt my airframe, and have worked on virtually every sub-system on my engine.  The A&P who helps me with conditionals says my plane is one of the best maintained he has every worked on.  I am not intimately familiar with the name of jets in the carbs.



Sorry for taking a bit of offense.  I just hate condescension.

_________________
Ernest Jessee
N4931M

dashwood

Joined: 24 Jan 2009
Posts: 73
Location: sw ontario canada

Posted: Mon Jan 30, 2012 6:55 pm Post subject: Re: 912 80 hp back-fire on shutdown

sorry if i sounded condescending. my humblest apologies. i was on a tight schedule at work and didn't have time to elaborate.  I agree the problem is likely in the carb jetting /blockage. at the idle setting.   it sounds like the idle rpm is correct.  is there any exhaust leaks hidden by the wraps.. i was always told that backfiring had to have oxygen induced to ignite the unburnt gasses. backfiring opens the hole to cause more backfiring to cause bigger hole .. and around we go.   the unburnt fuel has to be coming from somewhere though.??one bad carb and one good one would be hard to notice. maybe a little more vibration. but a partial flooding of one side would only show on the plugs at that point of shutdown... 

remember that you will find a variety of blackness inherent in rotax engines b/c of the poor intake manifolds dispersement. Pleas keep us informed on what you find...

upon rereading the original post my comment "it sounds like the idle rpm is correct." is not right... a 2100 cool down rpm is what i would call it... then pull throttle to idle of 1550 to 1600 just before you shut off mags. i do both at the same time but one after the other is more accepted practice..

one has to be careful not to get the idle setting too low so you don't accidentally kill  the engine on approaches. my plane will not sit at idle with out someone on the brakes. without holding the throttle it will idle at 1700  to 1900 and as all standard 701 linkage design will creep higher. i have to pull throttle and hold it to get down to 1600 to 1700 and continue pulling till engine stops... if i don't the stopping engine shake will open the throttle and all kinds of noise and sputtering/ runon happens.

_________________
Ross Alexander: CH701 driver 912ul 788tt
It only takes two things to fly, airspeed and money

Last edited by dashwood on Tue Jan 31, 2012 7:19 pm; edited 2 times in total

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Tue Jan 31, 2012 6:00 am Post subject: Re: 912 80 hp back-fire on shutdown

Here are a few things to check and try. First the header wrap isn't causing the problem. You are running a little rich and this could be caused by your shut down procedures. Is the air mixture screw on the bottom turned out only 1.5 turns? This needs to be double checked before the next test. Are you venting any fuel out the vent tubes? Slow the engine down to 1650-1750 before shutdown and turn one mag off for 2 seconds and then the last one. The  2100 rpm shut down may be part of the problem. Open the gap a little on the plugs to .025 -.026. (not my first consideration)

.023 -.027 is the normal gap, but sometimes having a small gap gives an incomplete burn. You just may have more than one thing adding to your problem. If the above suggestions don't help then you may need to pull the carbs back off. This is what changed since the problem started and someone may have done something wrong on the carb rebuild. It could be something as simple as the float arm at the wrong height. My bet is it will end up being a carb issue.

Have you done a compression leak down test? (This would be my last thing to do, but would remain in the back of my mind).

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

ejessee

Joined: 18 Apr 2009
Posts: 60
Location: Raleigh, NC

Posted: Tue Jan 31, 2012 6:34 am Post subject: Re: 912 80 hp back-fire on shutdown

Thanks to everyone for replies.



As to the problem starting after the carb rebuild... it didn't start then.  It has been happening for some time.    I will check all the things suggested.  I have been shutting down at about 2100.  I will begin shutting down at 1700-1800 RPM.  I already do the 2 second per mag thing at shutdown.  I will be doing a compression check as my conditional is coming up.



I just changed the plug gap about 2 months ago to ease in cold starting.  the regap helped in cold start, but again... the backfire/after fire was happening before that change.



I have checked the exhaust stacks... they are tight.  There is no rust on the exhaust, so small leaks are unlikely... but possible.  I'll check that as well.



Thanks again to everyone.

_________________
Ernest Jessee
N4931M

donaho1(at)verizon.net
Guest

Posted: Tue Jan 31, 2012 7:48 am Post subject: 912 80 hp back-fire on shutdown

I joined this thread late, so maybe you already discussed this, also 

I have NO experience with the 912. However, aren`t backfires and 

ignition timing related?  I fly a Kolb Firestar II Rotax 503. 

Lanny   At 09:34 AM 1/31/2012, you wrote:

 	  Quote: 	
		  



Thanks to everyone for replies.



As to the problem starting after the carb rebuild... it didn't start 

then.  It has been happening for some time.    I will check all the 

things suggested.  I have been shutting down at about 2100.  I will 

begin shutting down at 1700-1800 RPM.  I already do the 2 second per 

mag thing at shutdown.  I will be doing a compression check as my 

conditional is coming up.



I just changed the plug gap about 2 months ago to ease in cold 

starting.  the regap helped in cold start, but again... the 

backfire/after fire was happening before that change.



I have checked the exhaust stacks... they are tight.  There is no 

rust on the exhaust, so small leaks are unlikely... but 

possible.  I'll check that as well.



Thanks again to everyone.



--------

Ernest Jessee

N4931M





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=365298#365298

dashwood

Joined: 24 Jan 2009
Posts: 73
Location: sw ontario canada

Posted: Tue Jan 31, 2012 7:21 pm Post subject: Re: 912 80 hp back-fire on shutdown

i would also put much weight / consideration into Mr Lee's comments, thanks for jumpin in as always... its hard to sit by and see someone misdirected.    

thanks

_________________
Ross Alexander: CH701 driver 912ul 788tt
It only takes two things to fly, airspeed and money

JohnE

Joined: 31 Mar 2012
Posts: 11

Posted: Mon Apr 02, 2012 8:38 am Post subject: Re: 912 80 hp back-fire on shutdown

Curious if you solved the problem? Was it the carb's?

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