need help with high egt temps/#3 cylinder plug burning very white

[yellow fox]

Can any one figure this out. Engine 100 uls, carb needle clips set on 3rd notch from top and plugs gaped to .025. temp readings provided by dynon skyview. During level flght cruise 4900 rpm and higher egt runs from 1525to 1555 temp. After landing plug inspection revealed all cylinders were black in color except #3plugs were very white in color ,showing very lean. Enine runs great,mag ck very good,mo gas 93 oct. Also oil temp at 185 and cht at 127 cruise. Engine has 30 hrs turning a ivo inflight adjust prop set to 5450 climb can any one help

[Av8r3400]

EGTs are much higher than I see with my 912UL. I rarely see temps over 1200°. But, my motor is the lower compression 80hp version.

Oil and coolant (sensor in the corner of the head) temps seem very cold to me. Oil temp should be at least 230° during normal operations. Coolant (CHT) should be at least 180° during normal operations.

[1SeventyZ]

Carbs sync'd properly?

[jtpitkin06]

We really need more information to answer the question. Are the indications something new, or is it just the first time you have noticed them?

You have one cylinder that is now running lean and showing white plugs. This indicates the fuel is completely burning in the#3 cylinder and not in the exhaust. That should give a lower EGT than normal on that cylinder. However, you say your other plugs are black, not tan… indicating too rich in the other cylinders and fuel is burning in the exhaust showing a higher than usual EGT. If all the plugs were the same black, tan or white it would be much easier to diagnose.


The significant information in your post is the color of the #3 plugs. Your number 3 plugs are both indicating an imbalance with the other cylinders. This says you have one or more of the following:

An intake manifold air leak
Carburetor mount rubber sleeve leak
Carburetor sync problem
Mismatched carburetor jetting/needles
Sticking float valve causing carburetor to go lean
Dirt or debris in float valve
Carburetor float adjustment too low
Clogged carburetor fuel passage
Sticking diaphragm slide
Wrong plug installed
Clogged fuel filter input to carburetor



Your indication of high EGT by itself is meaningless without other information.



Did your EGT normally run a lot lower and this high reading is something new?



Were the readings always in this range and you are simply comparing it to another engine's indication for the first time? (Which is also meaningless)

So, we really need to know if you’ve been diddling with the carburetor settings or this is something that just popped up on its own without external input.


John

[Russell320]

You may also want to review Rotax SL-912-016 for engine speeds and manifold pressures. These engines are designed to run best at 5800 for takeoff (5 min) and 5500 the rest of the time. Don't forget about the manifold pressure when using an ifa prop, especially at 4900 rpm.

[yellow fox]

Thanks for the replies, spent the day on the engine ck the carb sink and was good . Last fall when the plane was first flown it seemed to be running very rich so we changed the carb needle clip to the 2 from3 and that worked well and flew 25hrs before storing. This spring before flight drained all fuel and carb bowls change plugs. After take off to cruise egt went over 1500, so landed and moved clip back to3 from top and flew no change. Went through all fittings ,carb boots compression ck all ok ,only thing not done was carb bowl level. Cht temps right now it was a dynon problem and fixed oil temp by closing down some of the oil cooler surface area.

Yes from day one that cylinder has always been much leaner just not that lean and gets have alwas been in the high 1400s

If one of these clues is the problem then why is the no1cylinder not lean and this still does not fix the over all high gets on both sides

[jtpitkin06]

The EGT is not the problem. (See posting on "EGT short course")



You have one cylinder running lean and 3 running rich. None are running correctly with a nice tan color. The quest is to find out why. I would suggest going leaner on the side with two black plugs going back to clip 2 from 3.



The other side with one white and one tan is a bit problematic. Even though two cylinders run off the same carburetor it is possible to get an imbalance. Usually that shows up in engines with lots more time as things wear out. Your engine has just 30 hours so worn parts are not likely.


I'm still betting on a small intake leak. Check manifold to cylinder connections. You can also idle the engine on the ground and shoot some throttle body cleaner around the intake system. If there is a leak the engine will stumble or quit when you hit the leaky area.


While the cowling is off and the engine is running. Check if the carbs are vibrating. A shaking carb can go lean.



If you do find any discrepancy, fix it and match the needles on both carbs to position 2.


As a long shot - check the valve adjustment on cyl #3 intake valve. Also check the valve for sticking.
And you might do the check on float bowl height.



This one is a bit of a mystery. I'd like to know what you find.
Keep us posted.


John Pitkin
Greenville, TX

[szicree]

I have no familiarity with the Rotax ignition system, but have you considered timing issues? Retarded timing will certainly cause high egt all around and sooty plugs. As for the one lean plug, I vote for intake leak.

[moosepileit]

Pancake K&N Air Filters not mounted in the proper orientation per the service letter/instruction? Says you can get bad fuel/air distro.

[jtpitkin06]

Follow-up post...

Have you found anything suspect or are we still searching?

I'm interested if you go back to the clip 2 position and it doesn't run as rich, does that help the distribution to the lean plug at all or is it still lean?

John