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  #1  
Old 07-10-2017, 07:03 PM
GoN4Broke's Avatar
GoN4Broke GoN4Broke is offline
 
Join Date: Aug 2011
Location: Fort Mohave, AZ
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Default Black Spark Plug

Hey all. Doing my very first annual. Took the Rotax Repairmans inspection class 8 months ago. All went very well with the structural stuff but when I turned my attention to the engine I came up with an issue I hope someone will tell me is no big deal. Some background; my RV-12 first flew last Aug (2016) and has accumulated 49.9 hours total time. For the last 20 hours I have used 100LL exclusively. When I was inspecting the engine I noticed a slight grease/oil stain emanating from the #1 exhaust port. Nothing much and I figured it might just be the exhaust manifold bolt might have been a little under tightened. When I pulled the spark plug out of the #1 cylinder it was fairly dark and there was a slight bit of residue on the threads and it was dark as well but not as dark as the #1 plug and it didn't appear shiny. I checked the #2 & #4 plug and they were both more or less grey with no residue to speak of. There is no other visible external leak and I have not experienced any vibration, misfire, high oil temps or anything else that would indicate loosing oil. The oil was first changed at 20 hours and then again at 46 hours with no noticeable loss in fluid between changes.


I checked the Rotax Line Maintenance manual section 12-20-00 and the findings indicate the following - possible valve stem seal, misfiring, too much oil in the combustion chamber or worn cylinder or piston rings. I have less than 50 hours on this engine and am hoping the situation is not that dire. Any thoughts ??
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RV-12 N811JK Completed May 2016

Last edited by GoN4Broke : 07-11-2017 at 03:53 PM. Reason: Fix Picture Link
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  #2  
Old 07-10-2017, 07:27 PM
BigJohn BigJohn is offline
 
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Location: Gloversville, NY
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Jim, it appears that your Cyl 1/3 carb is running rich. Have you checked the float on that side to see of it might be heavy?
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  #3  
Old 07-10-2017, 07:36 PM
DHeal DHeal is offline
 
Join Date: Sep 2006
Location: Windsor, California
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Black sooty #1 plugs are fairly common on 912s due to intake design. It helps if you run the engine up before checking the plugs to burn off excess carbon. Do you use the appropriate thermal paste as specified by ROTAX on your plugs' threads? Clean up the plugs, gap them (.023" Winter - 027" Summer -- I use .025" in NorCal), use thermal paste, and monitor. Lots of info re: sooty plugs on www.rotax-owner.com site forum.
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Last edited by DHeal : 07-10-2017 at 07:51 PM.
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  #4  
Old 07-10-2017, 08:53 PM
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Tony_T Tony_T is offline
 
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Not able to see your plugs on Photobucket. Do your plugs look sort of like these?

Others I have seen look similar.
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  #5  
Old 07-10-2017, 09:30 PM
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DonFromTX DonFromTX is offline
 
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Good grief Tony! If mine looked that bad I would be shopping for a new engine.
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  #6  
Old 07-11-2017, 08:21 AM
Jolly Jolly is offline
 
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Got this from Kodiak.
Normally dry black sooty plugs come from idling and or too wide a gap. Reduce gaps to .025 and either run the engine up at 4K rpm for 5 min. or go fly and when you land do not sit and idle. The black dry soot will burn off during normal operating rpm's. The black dry soot is very normal from idling.

p.s.
We shouldn't get carried away with the thermal paste either. A little is more. Too much is not good. Just a thin coat on the top 1/2 threads near the crush washer and not down by the electrode. Allowing the silicone paste to get on the electrode will cause a misfire
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  #7  
Old 07-11-2017, 12:46 PM
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DonFromTX DonFromTX is offline
 
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I hear what you are saying, but the lack of uniform soot is what bothers me. I would suggest that an engine that has carbs in sync and proper mixtures,
should be able to idle without smoking SOME OF the plugs. Does everyone with a Rotax have plugs that always look similar to the ones Tony showed us?

Quote:
Originally Posted by Jolly View Post
Got this from Kodiak.
Normally dry black sooty plugs come from idling and or too wide a gap. Reduce gaps to .025 and either run the engine up at 4K rpm for 5 min. or go fly and when you land do not sit and idle. The black dry soot will burn off during normal operating rpm's. The black dry soot is very normal from idling.

p.s.
We shouldn't get carried away with the thermal paste either. A little is more. Too much is not good. Just a thin coat on the top 1/2 threads near the crush washer and not down by the electrode. Allowing the silicone paste to get on the electrode will cause a misfire
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AF1 (was RV12) EAB Air Force One, SN 461, N461DG VIKING/HONDA ENGINE POWERED! AND N756TS ROTAX POWERED
A&P, PP-SEL, Pathological Flier, EAA Technical Counselor
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  #8  
Old 07-11-2017, 01:56 PM
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Jetguy Jetguy is offline
 
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Nothing to see here, move on.
Ops normal .
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  #9  
Old 07-11-2017, 01:56 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by DonFromTX View Post
I hear what you are saying, but the lack of uniform soot is what bothers me. I would suggest that an engine that has carbs in sync and proper mixtures,
should be able to idle without smoking SOME OF the plugs. Does everyone with a Rotax have plugs that always look similar to the ones Tony showed us?
It has to do with the shape of the intake manifolds.
The runners for the back cyl make a rather abrupt turn immediately after the induction flow exits the carb on each side so the back cyl. run just a little bit leaner than the front.
Any engine design that has multiple runners from a single fuel delivery source has to overcome this. Part of the reason for using taller performance intake manifolds on carburated auto engines.

On the 912, this imbalance is small at normal operation throttle settings, but it will show signs on the plugs if the engine is idled much before the plugs are removed.
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  #10  
Old 07-11-2017, 04:07 PM
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GoN4Broke GoN4Broke is offline
 
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Sorry about the previous lack of pictures. I hate Photobucket. Anyway you should now be able to see the plugs in the original post. It was probably bad lighting when I first looked at them as I didn't notice that much blackening on the other plugs, but after looking at the pictures it appears that all of them are blackened to a degree. I also wanted to relay that most of my flights have ended back at my own airfield (A20). That means is a slow 1/2 mile taxi at 1800 - 2000 RPM to my hanger after every flight. Could that be contributing to the sooty residue ?
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