I should have said that it was still rough after refitting the balance pipe. It never used to be rough like it is now so something has changed. The weather is not really suitable for working on the plane this time of year so I will probably have to live with it until next year.
Start the engine and check to see if the mag drop is fairly even or if there is a large difference. Pull the carb bowls and look for debris. If the carbs have sat a long time with fuel in them or they are fairly old they may need to be pulled apart and cleaned or even rebuilt. The small jets may have some debris with a partial reduction of fuel that makes it rough, but not enough to cause a more pronounced problem. This sounds like fuel in nature so I would start there. Check the bowls and jets first.Also check the all areas around the carb for any air leak.
You can check this last, but it sounds like a fuel delivery issue.
While you're at it don't forget to rule out something electrical in nature. Something like a spark plug cap, a loose spark plug, thermal paste on a plug or a bad plug gap.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
Sounds like you may have found the problem. A leaking float valve could definitely overfill the carb bowl and seep past the bowl gasket. The trash you found could have been holding the float valve partially open. Although the mixture on that carb would have been overly rich, the manifold pressure would not have changed. As floats get older, they will sometimes begin to flake off black residue that ends up in the bottom of the bowl but this does not cause a problem with the float valve; just the main and needle jets. Let us know what happens, but I would bet you've already fixed the problem.
I have the 80hp 912 and has 78 hrs on it now since new. I too am experiencing vibration on throttle back in a certain rpm range also. It has only recently started doing this. It will clear as quick as it starts, particularly when I work the throttle. Have checked carb balance and is ok. Will pull carbs off and check float bowel for debris. Had to install drip trays recently as right carb was dripping fuel onto exhaust pipe so may have same problem as you! Closing throttle would result in immediate drop in fuel demand but not a drop in fuel pressure? Maybe my right carb float needle is not doing it's thing. Interested to hear if you have fixed the issue. My prop is wood, fixed pitch.
I don't seem to have got any further. I flew the plane a week ago and the problem seemed worse than ever. I have checked the synchronisation again and it seems OK.
I have stripped the carbs again and they look fine, there is no evidence of anything wrong.
The only thing I have found is the overflowing float bowl on one of the carbs which I mentioned in a previous post. This appeared to have been fixed at the time but I am wondering if this is still going on in the air and is causing an excessively rich mixture on one side. Other than change the float needle I cannot see what else to do. That said the existing needle looks absolutely fine.
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