Van's Air Force
Don't miss anything! Register now for full access to the definitive RV support community.
Rotax 25 hr. inspection num. 1 & 2 plugs look rich
- Thread starter redmark
- Start date
the_other_dougreeves
Well Known Member
Have you checked your carb balance?
TODR
TODR
MartySantic
Well Known Member
Similar condition here BUT only for the #1 cylinder. The plugs for #1 were black (carbon and not oil). Changed all plugs at 90 hours and will re-check them after another 25 hours. Let me know if you solve the issue.
MartySantic
Well Known Member
A possibility in my case. The way the cowl is designed, maximum cooling is effected on the #1 cylinder.
the_other_dougreeves
Well Known Member
This isn't that unusual. We see this in a number of CTs 912 installations. It's probably the front cylinders getting more cooling.
TODR
TODR
From an AVWeb article:
Dull, smooth, black deposits indicate lead/carbon fouling. Obviously, incomplete combustion is the usual suspect, but improper operation of a cold engine (like jamming the throttle open) can produce the same thing. Don't be surprised to find these sorts of deposits after prolonged taxi (without leaning) particularly on a cold, wet day.
I think sitting around on a cold day waiting to get your oil temps up to 122 doesn't help. It's one of the reasons I put a Reiff heater on mine (as did Marty)...plug it in at night and your oil temps are 90 degrees when you wake up...not the usual ambient air temp.
Dull, smooth, black deposits indicate lead/carbon fouling. Obviously, incomplete combustion is the usual suspect, but improper operation of a cold engine (like jamming the throttle open) can produce the same thing. Don't be surprised to find these sorts of deposits after prolonged taxi (without leaning) particularly on a cold, wet day.
I think sitting around on a cold day waiting to get your oil temps up to 122 doesn't help. It's one of the reasons I put a Reiff heater on mine (as did Marty)...plug it in at night and your oil temps are 90 degrees when you wake up...not the usual ambient air temp.
It could be the intake manifold design, or the cooling shroud. Hard to verify without CHT/EGTsensors on all 4 cylinders. Air cooled V twin motorcycles can have this same sort of problem with the rear cylinder running warmer.
I would like to think that the Rotax intake manifold had some balanced flow designed into it. I don't know if other applications have had this trouble-Perhaps a call to one of the Rotax distributors would yield some suggestions. One could make a redesigned shroud with a plenum in it to ensure equal air flow to the front and rear cylinders, but again you would want some data to go by from sensors.
I would like to think that the Rotax intake manifold had some balanced flow designed into it. I don't know if other applications have had this trouble-Perhaps a call to one of the Rotax distributors would yield some suggestions. One could make a redesigned shroud with a plenum in it to ensure equal air flow to the front and rear cylinders, but again you would want some data to go by from sensors.
Clean Plugs
I just checked mine after a 3 hour flight (5,500' ASL) and they had a slight brown tint to them with the rear cylinders being cleaner. It may be if your doing short hops around the pasture in cold weather it may be causing the sooting on the plugs. My flying has been on the Texas coast where its warmer 80 - 50 deg F.
My tail pipe is sooted though, probably from warm up. It may take extended run time to burn the plugs clean. Just a thought before any adjustments are made.
46 hours on the clock waiting for a new engine driven fuel pump.
Gary
I just checked mine after a 3 hour flight (5,500' ASL) and they had a slight brown tint to them with the rear cylinders being cleaner. It may be if your doing short hops around the pasture in cold weather it may be causing the sooting on the plugs. My flying has been on the Texas coast where its warmer 80 - 50 deg F.
My tail pipe is sooted though, probably from warm up. It may take extended run time to burn the plugs clean. Just a thought before any adjustments are made.
46 hours on the clock waiting for a new engine driven fuel pump.
Gary
We found when testing with EFI on the 912s that the carbs when perfectly set up, had a max EGT spread of 140F at one rpm. Mixture distribution is far from perfect in these engines with the carbs. EFI brought the spread down to 35 degrees and the power up about 6hp as well.
Spark Plug Reading
One condition for spark plug reading is that to get a proper reading, the engine must be shut down from "cruise power". The low rpm and idling during landing and taxi will tend to give a more rich reading.
Also, don't try to "read" the exhaust pipe when running auto fuel. Auto fuel always results in a "sooted" exhaust pipe.
One condition for spark plug reading is that to get a proper reading, the engine must be shut down from "cruise power". The low rpm and idling during landing and taxi will tend to give a more rich reading.
Also, don't try to "read" the exhaust pipe when running auto fuel. Auto fuel always results in a "sooted" exhaust pipe.
soot in exhaust pipe
Just adding to Mel's post, I also have a fair amount of black soot inside my exhaust pipe and I was told the same thing- this is very typical for an engine running auto gas- whether it be a Rotax or a Lycoming/Continental. Was told not to be concerned.
My plugs looked about the same cylinder to cylinder at 25 hours.
Jeff
Just adding to Mel's post, I also have a fair amount of black soot inside my exhaust pipe and I was told the same thing- this is very typical for an engine running auto gas- whether it be a Rotax or a Lycoming/Continental. Was told not to be concerned.
My plugs looked about the same cylinder to cylinder at 25 hours.
Jeff
Exhaust Pipe Soot
The soot in the exhaust pipes is accumulating during cold / idle operation or whenever else the combustion is rich. This soot will never burn off during cruise etc. like the spark plugs since they will never reach reaction temperature with free oxygen at the same time. The oxygen is consumed within the cylinders during combustion therefore the carbon in the tail pipe has nothing to react with. I also agree that autogas has nothing to do with carbon build up and that the precision of EFI would prevent the carbon deposits from forming in the first place. Carbon deposit is an indication that incomplete combustion is taking place leaving "free" carbon to precipitate on the cooler surfaces. Unlike the spark plugs that have an oxygen source during cruise, once the carbon is deposited in the exhaust system, it won't go away. By the way Edison found out that carbon could be used as a filament in his light bulb as long as it was void of oxygen (vacumn). It just glowed didn't burn!!!!
Gary
The soot in the exhaust pipes is accumulating during cold / idle operation or whenever else the combustion is rich. This soot will never burn off during cruise etc. like the spark plugs since they will never reach reaction temperature with free oxygen at the same time. The oxygen is consumed within the cylinders during combustion therefore the carbon in the tail pipe has nothing to react with. I also agree that autogas has nothing to do with carbon build up and that the precision of EFI would prevent the carbon deposits from forming in the first place. Carbon deposit is an indication that incomplete combustion is taking place leaving "free" carbon to precipitate on the cooler surfaces. Unlike the spark plugs that have an oxygen source during cruise, once the carbon is deposited in the exhaust system, it won't go away. By the way Edison found out that carbon could be used as a filament in his light bulb as long as it was void of oxygen (vacumn). It just glowed didn't burn!!!!
Gary
Last edited:
Interesting comments about auto gas and sooty looking exhaust pipes. I don't see this on any of our EFI equipped engines- Subaru, Rotax, Suzuki or Lycoming. Pipes always look nice medium tan colored around here.
Your company doesn't produce EFI for Rotax anymore...why not?
Seems to be normal according to my 912 mechanic friends and my own experience. My 912ULS and the other on our field do the same thing.
The stock idle jetting of 1.5 turns is a bit rich out of the box too, so it might be worth it to work on it if you're out tinkering one day. After my recent carb rebuild and rebalance, I spent some time with mine. Once said and done, I ended up at about 3/4 of a turn out on each with idle EGT's between about 1070 and 1100F. Idle is noticeably smoother too.
My field is at 6300' MSL so that probably affects it a fair bit, but I'd say the half hour or so of work on the idle mix is worth it. Also, the carbs seem to normally have a rich spot in the 2200-3500 rpm range anyway, so if you spend a fair bit of time in that range (i.e. when taxiing) you're going to get the telltale soot in the pipe and black plugs.
I always check for a nice antique-white baffle in my exhaust too which indicates adequately lean running at full power, and just ignore the soot on the pipe outlet best I can
LS
The stock idle jetting of 1.5 turns is a bit rich out of the box too, so it might be worth it to work on it if you're out tinkering one day. After my recent carb rebuild and rebalance, I spent some time with mine. Once said and done, I ended up at about 3/4 of a turn out on each with idle EGT's between about 1070 and 1100F. Idle is noticeably smoother too.
My field is at 6300' MSL so that probably affects it a fair bit, but I'd say the half hour or so of work on the idle mix is worth it. Also, the carbs seem to normally have a rich spot in the 2200-3500 rpm range anyway, so if you spend a fair bit of time in that range (i.e. when taxiing) you're going to get the telltale soot in the pipe and black plugs.
I always check for a nice antique-white baffle in my exhaust too which indicates adequately lean running at full power, and just ignore the soot on the pipe outlet best I can
LS
steve wyman
Well Known Member
I agree with the above post. I ran a 912 for about 300 hours in my Rans S-7, before I sold it and purchased the RV-12 kit. The plugs always seemed to look that way - darker up front, consistently. The engine always ran like a top. Stop worrying and go fly. Steve
Little demand for the system in reality. Before we did all the R&D, we had a dozen people lined up to buy it. When the kit was tested and all ready for sale, not one of those people actually bought. Lesson learned, next time I'd collect non-refundable deposits.
We've had more luck selling the bits to convert Jabirus over to EFI lately with several flying down under now very successfully. The Jabs appear to have more serious mixture distribution problems than the 912s. The EFI cuts fuel consumption a few liters per hour for the same TAS.
Last edited:
A little more info...
The plugs in my front cylinders looked black after just a few minutes of ground running. So I guess it is just normal. Doesn't seem as though it would be a temperature thing, because the cowl was off and all cylinders should have been equally cold. I'm guessing there is a slight mixture difference due to the position of the carbs and intake manifolds.
The plugs in my front cylinders looked black after just a few minutes of ground running. So I guess it is just normal. Doesn't seem as though it would be a temperature thing, because the cowl was off and all cylinders should have been equally cold. I'm guessing there is a slight mixture difference due to the position of the carbs and intake manifolds.
