Rotax 912 ULS cylinder 3 running lean

[macedonranges]

We have just done our 200 hour service and changed spark plugs and found the two spark plugs in cylinder 3 were very lean compared with the other cylinders ..

 

Anyone have any ideas on when this would be - as cylinder 3 and 4 are fed from the Right carbie.

 

Engine is running well and have had no problems.. so not sure about why it is happening

 

Picture of spark plugs - Left side is from cylinder 4 and right side is form cylinder 3

 

Thanks

 

ROB

 

 

 

[cscotthendry]

Rob:

 

I'd be very interested in the answer as mine runs the same.

 

 

[crashley]

Hi did you let the engine idle before you pulled the plugs I was told by a rotax dealer that two cylinders always run very rich at idle and the only way to check the plugs it to run it flat out for about 30 minutes and then cut the mags and then pull the plugs and check them How you do this I don't know

 

 

[Kyle Communications]

Have a look at a thread I put up with the same about 8 months ago will see if I can find it..my 2 front plugs left and right looked the same and the 2 rear plugs looked the same as well but they were different the front was different to the back

 

 

[Kyle Communications]

http://192.69.222.91/threads/advice-wanted-on-912uls-plugs.65805/#post-309382

[macedonranges]

Rob:I'd be very interested in the answer as mine runs the same.

I spoke to Gary from Bert Flood imports - distributors of Rotax in Australia and he said the following

 

The spark plug that “looks lean” actually looks OK, and the other one is a bit rich.

 

Did you run the engine at high revs before looking at the spark plugs ? as they do tend to go black and sooty with prolonged low rev running.

 

We normally see different colored spark plugs on the same bank, this is due to the design of the intake manifold and not anything to worry about.

 

When I did the change of spark plugs I had warmed the engine to facilitate the engine oil change so it was probably only at idle for 5-6 minutes so interesting comment

 

- the engine has been running fine and no problems - so will be interesting to hear other peoples comments about what they have found when doing spark plug changes in the Rotax 912 ULS

 

cheers

 

ROB

 

 

[crashley]

running at an idle for 5 minutes will make two plugs very black on my 912 two plugs will go black very quickly if idling

 

 

[Ultralights]

one looks too rich to me, the right one looks normal.

 

 

[facthunter]

The firing order of each bank fed by an individual carb, is not even. I believe it is absolutely normal for these engines to show the indications you describe. Idling will soot them up. A plug is chosen to run nearly red hot on the insulator at high output. That is the only time it "cleans itself". It must be the correct heat range to achieve this. Don't fiddle with the maker's recommendations on this engine in that respect. The engine is not troublesome left alone. Just check your connections (rubber) for cracks. now and again. Nev

 

 

[macedonranges]

Thanks Nev .. I see you are from New Gisborne .. Where do you fly out of ? Riddells creek or SUNBURY ? Cheers Rob