In addition to Martyn's suggestions here's another one to add to his list of things to check... If your fuel is not being atomised efficiently it comes out of the carb as fine droplets rather that a mist. The heavier droplets lose momentum quicker and then get sucked into the first and closest cylinder. This can be caused by 1- The needle spring is worn and the needle is not tracking smoothly up and down the jet. 2- The float level is too high and the needle adjustment has been wound down to compensate. 3- Worn needle or worn jet
regards Rob Rob Thompson 0429 493 828 (Please note that my new email address is amessageto...@gmail.com) On Fri, Apr 12, 2019 at 12:06 PM Martyn Cook <mar...@actrix.co.nz> wrote: > 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 > > > > > >
