It is amazing how many solutions there are to fix a problem, or rather many problems, that all have the same symptom. Just goes to show that one should be careful of just blindly applying cookie-cutter solutions.
Blue skies, Paul, hope it still gives you many years of faithful service.
Yesterday I had the opportunity to lower the main needles on my carbs by one notch to make the mid range mixture leaner.
There was perhaps a slight improvement in the vibration but fundamentally the problem persists. I am not sure what to do next. I cannot think of any other simple diagnostic tests that I can perform to identify the problem.
What I would like is to have some measure of the exhaust gases on each bank of cylinders along with the throttle angle. The latter I may be able to implement with some simple throttle position potentiometers.
If anyone has any more ideas they would be gratefully received.
Did you ever change your engine mount rubbers? I did and it pretty much fixed the vibe. I can still sense the difference in engine feel in that same rpm range on throttle back but it is not transferred into any real palpable vibe. I am guessing that zone is something inherent in the motor design as others have suggested too. Also have found that vibe can be a symptom with multiple causes as varied as you can get, I found recently as we are now in winter that too cool running makes the engine run rough at low rpm. I tried a 3 blade composite prop prior to changing engine mount rubbers and this improved overall smoothness but still had that rpm zone of vibration exactly the same. Like you I tried the needles in a leaner position but it made no difference except caused the motor to backfire on shutdown. I have also run a fine hole return fuel line which seems to have helped stop some fuel overflowing the carb vent on descent to land but I think the bigger cause of this flooding was roughness/engine shake causing the needles to unseat and allow xs fuel into the carb bowl. This issue has been frustrating and have found myself going round in circles wasting time and money before arriving at a solution. Hope you find yours soon.
I am still convinced that I have a fuelling problem. Yesterday, as before, when I throttled back slightly in the cruise the engine went from being very smooth to rough. If I then raise the nose to create more of a load for the engine, the roughness disappears. The RPM is different to when it was initially smooth and it is mainly the load that seems to affect the roughness.
I will take a closer look at the engine mounts when I have the covers off next.
I also noticed that it was only when unloading the motor on throttle back that the vibe was bad, if i reduced the throttle in the climb it didn't do it. I found my engine mount rubbers to be a generic (no markings) with a inner metal sleeve that was too soft allowing it to compress/mushroom when tightening the engine mount bolts. This ovaled the inner aspect of the rubber allowing too much engine movement on the mounts. The new rubbers are true Barry Mounts of the grade recommended for my plane and I noticed a difference as soon as I started the motor on the ground. It seems all paths lead to the one symptom of vibration on this motor :-) I did think for a while that it was excess fuel pressure hence me fitting the return line. I have the new version mechanical fuel pump which I hear other people say puts out a bit more pressure then then old style. The only thing I didn't do was look in the gearbox. My motor only has 170 hrs on it now so wasn't keen to take that apart. A friend with a 1000hr 912 has just sent his gearbox away for service/check as he has some vibe issues he thinks are attributable to that. Will wait to hear if that fixes it.
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