Shudder

[FredG]

Dick, you asked above, "In other words, does uneven balance of the carbs stop being a factor when the high speed portion of the carb circuit comes into play?" and I never saw an answer.

 

From my experience and knowledge of other engines with multiple carburetors (2-4 carbs on cars and motorcycles), balance is most important at low RPM (mostly closed throttle) and little to no factor at high RPM (mostly open throttle). If the throttle valves are fully open at WOT, then carb balance as done by the usual procedure has no effect (it can't, the throttle cables are slack at that point). Of course, there can still be reasons for imbalance having to do with other carb defects (e.g., sticky slide, damaged diaphragm, crud, etc). But, considering just the standard balance procedure and carbs that are working properly, the more open the throttle, the less relevant standard pneumatic carb balance becomes.

 

Just the opinion of a non-mechanic and not meant to be guidance for maintaining airworthiness.

[Anticept]

One of the things we do when balancing the carbs is to balance them at two different RPMs, and then advance and retard the throttle to make sure the needles are moving together. Had a problem with N566FD where one carb was lagging and got it fixed.

[Runtoeat]

Fred, thanks for the reply. I visualize the inner workings of the carb in the manner you do. Since my vibration is in I seem to have concerns in mid-range engine rpm and all clears up at WOT, I am probably dealing with some sort of restriction in a secondary passage. Although icing isn't normally a problem with our Rotax engines, this could have been a factor due to the low temps at the time. Also, the low torque of my gearbox might also be a factor, especially when the low torque is combined with cold temps.

 

Anticept, what caused the "lagging". A sticky slide or cracked bellows? Just thinking of things to check when my carbs get opend up for inspection.