EGT 200 f increase and 500 RPM drop on mag check

I was fighting a high rough mag drop that previous owner had had put up with. Had new ignition modules with soft start installed and ignition system values  checked by experienced Rotax mechanic. Finally had to replace left carb due to being unable to get needle and seat to seal. Mag check is now smooth and within drop specs. Don't know why but very pleased.  Don't have EGT and after seeing all the worry about them on this forum almost glad I don't. We never had EGT gauges on small certified engines and never missed them.    

Here’s another piece of data on this issue that Paul didn’t include: engine mag checks fine at full throttle and idle. Issue only appears in mid range. 

Nicholas Wilcox wrote:

Here’s another piece of data on this issue that Paul didn’t include: engine mag checks fine at full throttle and idle. Issue only appears in mid range. 

Carbs are fully vacuum synchronized at midrange though 

Nicholas Wilcox wrote:

Garrett Wysocki wrote:

I helped a friend a year ago with excessive mag drop after ruling out ignition it turned out the carbs needed cleaning and new o-rings. Just cleaning the idle jets helped some but a rebuild solved the problem. So the excessive rpm drop was mixture related.

Wouldn’t carb related be on both mag sides and not just one, given wiring setup? 

Actually the drop which was not out of Rotax's acceptable range and was mainly on one mag side but the owner of the aircraft wanted the mag check to be as perfect as possible. When doing a mag check the distance inside the combustion chamber from the spark plug compared to how the fuel air charge comes in and swirls around is unique since the plugs are top and bottom yet the fuel air charge always comes in from the top. His aircraft was used/stored with auto fuel and sat in a hot Florida hanger for up to two months at times. The carbs were slightly gummed up with oxidized fuel. We used/swapped known good ignition components from my aircraft to rule out ignition issues. Again carb cleaning and new o-rings cleared up this mag drop issue.

Makes most sense that intermittent running at midrange flying would be Carbs. 

Makes sense that mag drop on upper verses lower would be different based on fuel/air entering top but typically ROTAX specs should be reasonable to hit with this situation.

Makes sense that one mag running good and one bad would be electric since the carb is operation great with one mag side.

Just talked with a pilot who was having the same problem. I advised him to switch the electronic module boxes out and try it. (Now I am wondering how this was done because when I look at it closely it does not look like they can be easily switched? A top and B bottom clearly labeled and flipping over would cross the in/out wires) He did mag check and same mag was bad. He switched both boxes out with new ones and it fixed problem. This makes no sense.

Has anyone had any problems with the ignition coils? I have had two go bad with my 912iS.

Thanks for all the help.....