Hello Jarek
I'm not sure if this will help, but you did ask for suggestions :-)
One way that one cylinder could be leaner than the other is from an air
leak around the intake structure, after the carburettor. So your carb
might be set a little too rich, but the No 2 cylinder would then be
leaned out by an air leak local to that cylinder, which could dilute the
fuel mixture.
Another possibility is that the compressions in the two cylinders are
different, so one sucks better than the other. The air leak could be
via the exhaust valve. Do you have compression readings available?
I had to spend quite a bit of time with my Dimona getting the mixture
settings correct. When I first purchased it - straight after a top
overhaul - the CHT readings in flight were wildly different, and one
side did produce blackened plugs. I very gradually adjusted the carb
needles using the special tool (1/4 turn at a time) and I finally got
the plugs all nice and powdery grey and the CHT readings within a few
degrees of each other. On one side the front and back plugs are still
slightly different in appearance, so I tuned for one slightly blackened
plug to ensure the other cylinder was not too lean.
Another possibility is your spark plug HT lead might be damaged. An
intermittently-firing plug might cause the vibrations you report, plus a
fouled spark plug due to partly-burned fuel. On my aircraft the HT
leads look like they came from Noah's Ark, so to speak, and I'm
wondering when they will need replacing. Or a magneto fault? The
contact leading to that spark plug could be weak.
Hope this helps! I look forward to reading what others write.
With warmest regards
Martyn
New Zealand
On 12/04/19 8:36 AM, Jarek Steliga wrote:
Hello,
I am at the end of my tether. Please help.
I screw in 4 brand new Limbach spark plugs. The engine runs smoothly.
After a while I detect vibrations, then check all the plugs and
consistently one on cylinder 4 is black (all the others look fine).
When I screw the sooty plug in on a different cylinder and run the
engine, the plug will clean itself up (e.i. burn all the soot and look
like new). Conversely, a clean plug mounted on cylinder 4 will foul up
again.
Has anyone had a similar situation?
While checking the fuel mixture with Gunson colourtune
https://www.youtube.com/watch?v=xEZ0-uN0NA4 , the mixture on cylinder
4 is slightly richer (e.i. slightly too rich) than on cylinder 2. How
can that be when both cylinders are supplied by the same carburettor?
Looking forward to someone's suggestion
Regards
Jarek
