Runs rough on mid range RPM

Roger Lee wrote:

A fuel issue is not a likely cause at just that rpm.

It could be a couple of things. Do the cheap, easy and most common things that cause issues first. Do one thing at a time. I would first sync the carbs around 3300 - 3500 and see if it goes away. Plus don't forget to sync at idle too. This is one reason I never just sync at 2500 as the book states. It's just too far down into the idle circuit and not very representative of air and fuel flow. When I used to try 2500 for the high rpm sync the needles on the sync gauges would split apart again as you applied more throttle. Plus as you get up into the higher rpms the crossover balance tube isn't functioning like it does in the idle rpm range. I do not have that issue when I sync up around 3300 -3500 rpm. 

It could be the gearbox and or prop and it can even be wind direction impacting the prop especially as you unload the prop. 

Very much support the comments from Roger, IMO Rotax should revise the line maintenance manual to encourage sync at the higher RPM. Also agree strongly with the comment on wind strike, found this not so long back, initially thought I had a prop balance issue, changed my heading by 15 degrees and issue disappeared, went back to original heading came back. 

Vibration or roughness when throttling back can be due to work splines in the gearbox. I had this and had to replace the propshaft and drive dog arrangement and the problem was solved. It wasn't a cheap fix, about 1000GBP!

I am led to belive that the problem is more prevalent on gearboxes without a slipper clutch.

Thanks to everyone for the suggestions.

I will sync the carbs as Roger suggested and also look at the prop and other areas.  I also had not thought or had anyone mention heading and wind.  All three flights were using the same heading.  

Thanks again for this great advice.  

I occasionally have rough running at speeds in the range 2500 to 3000 because one of the two throttle cables sticks slightly. Cycling the throttle in and out once or twice usually fixes it. Twiddling the angle of the cable sleeve terminator can help, so can lubrication. I try to make sure that terminator is secured firmly so that it doesn't move, and the cable stays straight through the whole length of its travel from closed to open.  

Many times:

Vibrations can be tracked back to throttle cable systems in different aircraft that do not work evenly every time you use the throttle. I see this even in carb syncs where the carb sync differs every time you use the throttle.