redmark Posted January 9, 2011 Report Share Posted January 9, 2011 I did my 25 hr. Rotex inspection today and my forward cyls. 1 & 2 both had rich looking plugs but my number 3 & 4 plugs looked fine. Any ideas what the problem could be? Or is it a problem? Thanks, -Ron Link to comment Share on other sites More sharing options...
Roger Lee Posted January 9, 2011 Report Share Posted January 9, 2011 Hi Ron, It's not a problem and will occur in most Rotax 912's. It is part an idle symptom and part a difference in air intake manifold length. It isn't present at higher off idle speeds. Go enjoy and don't worry about it. Make sure you did a good job documenting the 25 hr. in the engine logbook. You could go to the trouble of making a small air mixture adjustment, but it isn't needed or worth the time. You may notice that the dry soot black on those plugs may end up on another set next inspection. Link to comment Share on other sites More sharing options...
redmark Posted January 9, 2011 Author Report Share Posted January 9, 2011 I've also been told that this could be a sign of number 1 & 2 cyl. running cooler. It's been one of the 5 coldest December here and I've had a hard time keeping the oil temp up during the 25 hr. period. Thanks for the help, -Ron Link to comment Share on other sites More sharing options...
Roger Lee Posted January 9, 2011 Report Share Posted January 9, 2011 Hi Ron, It happens when the OAT is 95F too. Link to comment Share on other sites More sharing options...
Jeremy MacGregor Posted January 9, 2011 Report Share Posted January 9, 2011 Make sure your prop isn't over loaded, should be able to turn at least 5300 in level flight, preferably more, @ WOT. Slightly darker color is nothing to worry about, sooty consistently at 25hrs and I would investigate further. Link to comment Share on other sites More sharing options...
Roger Lee Posted January 9, 2011 Report Share Posted January 9, 2011 Howdy Jeremy, I think Ron just looked at these after an idle only. I don't think his issue pops up at higher rpm settings. Many of us have that dry black soot after idling a while on a plug or two, but goes away at higher rpms. It even will change from one plug to another once in a while. Most of these guys only see this after they have taxied in or idled their engine for a bit and pulled the plugs. If they ran it up to 4000 rpm and killed the engine it would not be there. It is more of a function of the air mixture at idle rpm. The problem is if you adjust the idle mixture screw on one carb it effects all the plugs on that side and other plugs may not be affect like the one or two you see that time or the dry soot plug or plugs will move around at times. Best to leave it as is because it isn't hurting anything and isn't there over idle rpms. p.s. Jeremy, Give me a call at home when you get a chance. Link to comment Share on other sites More sharing options...
Tom Baker Posted January 9, 2011 Report Share Posted January 9, 2011 Also if you are burning car gas some of the additives to boost the octane burn sooty, so the soot on the plugs does not always mean it is to rich. Tom Link to comment Share on other sites More sharing options...
Runtoeat Posted January 10, 2011 Report Share Posted January 10, 2011 Make sure your prop isn't over loaded, should be able to turn at least 5300 in level flight, preferably more, @ WOT. Slightly darker color is nothing to worry about, sooty consistently at 25hrs and I would investigate further. Jeremy, you know a lot more about this than I do, and we've certainly talked a lot about this on this forum and the other, but don't we want to see rpm's more towards 5600 to 5700 rpm in WOT operation during level flight? I'm seeing 5200 @ zero flaps 65kt WOT climb leaving the runway and 5700 rpm WOT level flight. Wouldn't 5300 rpm WOT @ level still be on the low side for rpm and result in the engine being lugged down below 5000 during the climb outs? Link to comment Share on other sites More sharing options...
Roger Lee Posted January 10, 2011 Report Share Posted January 10, 2011 Hi Dick, If you are running 5700 rpm WOT at your normal altitude then you could afford to add a little pitch back into the prop and get a little closer to 5500. Probably 3/4 of a degree. 5500-5600 is a good place to be, 5700 you have a good climb prop, but are loosing some cruise and fuel economy. Jeremy says "preferably more". If you have black plugs at cruise rpm you have a real problem. All most all only see their plugs after idle and those can be a black dry soot sometimes. Link to comment Share on other sites More sharing options...
redmark Posted January 10, 2011 Author Report Share Posted January 10, 2011 Thanks for all the help... I feel better about the rich plugs now... the 25 hr. inspection required an oil change and I idled the engine that morning for at least 10 min. getting the oil temp up so that I could change it. -Ron Link to comment Share on other sites More sharing options...
Runtoeat Posted January 11, 2011 Report Share Posted January 11, 2011 Hi Roger. It would be nice to be able to drop in and see the "prop doctor" and get my prop re-pitched but Tucson is a little too far from Michigan. My plane flies so well that I hate to mess with it but there are a couple of guys around here who know how to adjust the pitch on CT's. With the cold air temps, I do have tremendous climb capability and the pitch does work well for the short grass fiields I fly into. My plugs were a nice tan color at the last annual. Although I like the good climb performance, I'll consider your recommendation to go with a little more pitch. Link to comment Share on other sites More sharing options...
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