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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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 Posted: Sun Nov 25, 2007 6:25 pm Post subject: Dry Black velvet Plugs |
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Hi All,
FYI
I was having an issue on cylinders 1&3 spark plugs running with a dry velvet black coating. That means the mixture was a little rich. Not from idling too long as some A&P's suggested. I rebuilt the carbs and the rich mixture problem went away. The carb on that side evidently had an issue. Now both sides are a nice light tan.
I must have had 4 Rotax people tell me I was idling too long, one suggested to change the mixture screw which I did with out any good results. No one suggested rebuilding a carb or that a carb might have had a leak (it was not the float level, that was checked early on).
If you are having a velvet black coating on one side of your engine then rebuild the carb. It's easy. Two gaskets and a few "O" rings, then sync the carbs.
The morale to the story is to have good differential diagnosis skills. Think of all that might cause an issue and not just the easiest or most common.
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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Don G
Joined: 10 Jan 2006
Posts: 156
Location: Central Illinois
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| Posted: Thu Jun 19, 2008 10:11 am Post subject: Re: Dry Black velvet Plugs |
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I came up with this post in a search on black plugs, and I would like to re-invigorate the subject with a question.
ON my 80 hp 912, I experience similar plug inspections, except it is always on both forward cylinders, thats right, both cylinders closest to the prop. The back 2 run clean as a whistle. I inspected the plugs yesterday, at 50 hours. And as usual, both forward plugs have a really good coating of dry black velvet, and both rear sets looked close to as new.
NO other issues, Engine runs great. Make wonderful power and tops out at WOT at 5500 in flight. I generally cruise at 4800 or so...95 to 100 mph
5 k yields about 105 to 108 when I am alone.
As far as idle time or low throttle time after landing to taxi to hangar. Not very much. Hanger is in the front row and about 300 ft from runway. with 200 ft paved ramp. So Taxi time is part throttle until in front of hangar and I always shut it down right away. depending on which end of the 2700 ft turf runway I land on...I have to taxi on active until the 300 ft, at part throttle setting. generally at low idle for the last 100 feet is about all.
About the only thing I can figure is, the long manifold run to to forward cylinders is what make these plugs so different when inspecting.
I would like to hear of others experience here.
thx
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Don G.
Central Illinois
Kitfox IV Speedster
Luscombe 8A
RV9A |
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Thu Jun 19, 2008 10:33 am Post subject: Re: Dry Black velvet Plugs |
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Good Point Roger,
When I was in my trade school training I was always taught to imagine
myself in a shack in the desert southwest (USA). When I hear the
beat of hooves, it can not be horses, at my school it was always zebras!
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Ira N224XS |
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Fri Jun 20, 2008 3:30 am Post subject: Re: Dry Black velvet Plugs |
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Don G.
I had the black dry plug problem on both plugs on #1 cylinder only and it took me a very long time to determine the cause. I had very similar operating conditions too. I finally found a small gage white wire somewhere in the ignition system that was not completely broken but partially so. I repaired it and that fixed the problem. I'm still not sure where schematically the partial break was in the diagram but repairing it did fix the problem. It is still a mystery to me how that single white wire could cause BOTH plugs on a single cylinder to blacken.
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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Rich L
Joined: 23 Feb 2008
Posts: 63
Location: North Idaho
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| Posted: Fri Jun 20, 2008 4:06 am Post subject: Re: Dry Black velvet Plugs |
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I have had the same problem on my 912uls since 0 hrs. I also screwed the mixture in 1/4 turn with no change. I have balanced my carbs twice now and the two forward cylinders still have the "black velvet plugs". It continues to run great with 150 hrs on it now and all the numbers are well within specs (EGT,CHT, Oil & Water Temp, etc).
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lucien
Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM
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| Posted: Fri Jun 20, 2008 6:04 am Post subject: Re: Dry Black velvet Plugs |
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| Rich L wrote: | | I have had the same problem on my 912uls since 0 hrs. I also screwed the mixture in 1/4 turn with no change. I have balanced my carbs twice now and the two forward cylinders still have the "black velvet plugs". It continues to run great with 150 hrs on it now and all the numbers are well within specs (EGT,CHT, Oil & Water Temp, etc). |
I had this problem on my 912uls and there were 2 causes:
- overly rich idle, I have to run my idle mixture screws in 1/2 turn from the recommended setting (1 turn out overall).
- carburettor vent lines running outside just tucked into the float bowl spring. The vent lines have to be connected either to the intake airbox if you have one or routed into the air filters if you run those. The carburettors will run rich (and very much so at higher altitudes) unless the vents are at the same atmospheric pressure as the intakes in front of the venturis. This requirement is in the Rotax documentation. I bought some brass fittings at lowes and put them into holes in the back of my air filters. Fixed the rich running completely.
The cylinders closer to the gearbox do run a little richer than the other two on my motor as well, probably due to slightly different lengths of the intake runners for the two banks of cylinders......
LS
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LS
Titan II SS |
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Fri Jun 20, 2008 1:48 pm Post subject: Re: Dry Black velvet Plugs |
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Hi Guys,
The length of the intake manifolds combined with each of our different engine setups in different planes can cause our plugs to look different and even have that dry black velvet coating. You won't have that when you are running at operational speeds (i.e. 4800-5200rpm). This is usually caused in the low rpm range. There can be more than one cause. Try one cure at a time until it goes away. If it doesn't don't panic your engine still runs good as many people will attest to.
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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