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Question about early J3300 - carbon in rocker chambers

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Question about early J3300 - carbon in rocker chambers

Postby Marco » Mon Sep 23, 2019 6:39 pm

First of all, this is my very first post in this form. My name is Marco, I'm from Germany and I have been flying a microlight "Wild Thing" with a Jab 3300 engine for 5 years now and roughly 250hrs. I consider myself as a very confident autodidact for this engine, like for most other things in daily life. I am doing ALL the maintainance by myself.
The engine has a S/N of 5xx (early solid lifter) and has around 670hrs of total flying time. Unfortunately, the last 2 years I accumulated only 50hrs of flying time in this aircraft with more than 100 landings, making an average flight time of less than 30mins. Due to several airfield floodings, long holidays etc. there were also quite some periods (~ 4-6 weeks long each), where I did not fly the aircraft at all.

Now, I am confronted with something new.
When doing the 50hrs check recently I found two new things in the oil filter (see photo attached): First, some metal flakes (non-magnetic), which I think is aluminum from the pistons due to the many cold starts and some "dry starts" . This corresponds with slightly elevated aluminum levels in the oil analysis (23ppm) so I think this is cause by the rare use during the last 2 years. I now, that not flying an engine is abuse and I promise to perform better in the future. ;-)
But additionally, there was quite a large amount of black stuff in the filter, "crispy" to the touch. I think this is carbon. When adjusting valve clearance, I found in all rocker chambers a thin layer of carbon on the cylinder-side surface, i.e. the "bottom" of the chamber. Not much, just a thin layer. I can't recall, if I also saw this in previous maintainances. But what I can surely remember, that the rocker chambers never had the "shiny metal" look, they were dark coloured all the time. But I cant remember the carbon...
Due to some modifications, I think I have oil and head temperatures very nicely under control. On a warm summer day, I reach an oil temperture of 95oC, but only with two persons onboard and an extended climb (like direct to 4000 feet). Head tempertures of the hottest head during the climb reach AND STAY and 180oC. During cruise, oil is at 85oC (thermostate controlled) and the hottest head (#2) at 150oC. All other heads are around 110-130oC during cruise. So I think I dont overheat the oil. Hence I am concerned that the outlet valve guides may be worn and have to be replaced. I use oil Aeroshell 15W-50. Compression of the "worst" cylinder is 78/80 when hot.
I usually fly the oil 50hrs (opposing Jabiru's recommendation of 25hrs), since oil analysis has always proven the oil to be in good shape after 50hrs. This time, oil oxidation is quite high and viscositiy at the "upper limit of allowed tolerance", which may also point to overheating the oil... But the oil sat for 2 years in this engine... May this be the reason for high oxidation? There is absolutely no significant nickel in the oil analysis (1ppm) which I would expect with worn valve guides. And would the valve guides in ALL cylinder start to leak at the same time...?
So, I'm not sure how to go on. Just flying is one option. Perhaps with an additional just-to-be-sure oil change after 25hrs. I also think about cleaning the rocker chambers and see how fast the carbon builds up again.
I would be glad getting some advice on this topic and happy to discuss it with the forum.

Always safe flights,
Wild Thing WT-02 (nose wheel version)
Jabiru 3300
Posts: 1
Joined: Tue Sep 10, 2019 3:25 pm
Aircraft: Wild Thing

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