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Spark Plug Diagnosis

Tacco

Well Known Member
In the middle of my annual. Take a look at my plugs. I chock it up to the front cylinders running much cooler than the back. Any other explanations out there?
 

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Not to me. The 1/2 bank is running much richer at least at idle/taxi.

Sure, that’s what I thought as well. But how is that possible if they are fed by different carbs and they each share the carb feeding one of the rear cylinders? I.e. 2 & 4 are on the same carb, and 1 & 3 are on the other. What am I missing?
 
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Plugs

That's normal for the carbureted ULS, the cylinder closest to the carburetor will run richer. Particularly if the engine idles for a while. The intake runner is a bit shorter for those cylinders and that's why it's richer. I would not expect a 912is engine to soot the plugs like that, even after extended idling.
 
Ahhhh...except that the cylinders running richer, are the ones further from the carb. Cylinders 1&2 have a longer intake run than 3&4 which are clearly the lean ones.
 
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could be the picture quality, but #2 looks like oil deposit and #1 typical over-rich carbon deposit. Rich mixtures usually leave a matte, dust like look and oil leaves a semi-gloss, shiny residue. If I am correct, you may want to peak at the top of the #2 piston, as it would also be coated with oil residue if you are burning oil in that cyl. You can also see a bit of the oil residue on the end of the threads on 3/4, where there is not enough heat to burn it off. Just checking, but are the 1/2 plugs the same heat range as 3/4?

I have never seen a rotax so can't comment on possible causes of over rich on one cyl.

Larry
 
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Common for carbureted four-cylinder Rotax. You are looking at remnants of idling. Intake manifold runner lengths and sharp radius bends make uneven fuel/air distribution at idle. Mid-range and cruise have equal flow and all four plugs would have a good appearance if possible to examine prior to idling on the ground.
 
I agree it is common.
Particularly if there was a some extended idling time from a long taxi prior to engine shut down before the plugs were removed.

The intake manifold runners for the two rear cyl. make a sharp turn downward immediately after the carbs but it is a more gentle sweeping curved path to the front two cyl. This causes the rear cyl to run just a bit leaner than the front at some throttle settings.
 
Another thought…. Carbs on Rotax 4-cylinder engine jump around a lot at idle causing float level to be all over the map. This is aggravated because carbs are located at cylinder heads instead of near centerline of engine like on a Lyc or Cont. Carb float bowl on an aircraft engine is a very wild environment with +/- G’s, uncoordinated turns, etc.
 
Thanks guys. I’m thinking it is indeed common and do to the front cylinders running cooler at idle & taxi rpms.
 
I vaguely remember from LSRM school it's possible the carb piston can get indexed incorrectly via the diaphragm. When it's not aligned correctly it can create turbulent flow which can exacerbate the richness differential between the banks at idle. Very easy to check by sighting down the carb intake so it's worth a quick look to rule it out.

Of course I don't remember anything about when the jet-needle and needle-jet are active and not, so take my opinion for what it's worth...free.
 
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