The C85 engine in my 415C developed a pronounced stumble at low idle and at 1000 RPM. Cleaning the plugs and adjusting the idle screw did not help, so I was convinced I had a blockage in one of the Stromerg carburetor passages. My mechanic, however, suspected the magnetos. I had my doubts since the engine was smooth above 1000 RPM and had a less than 50 rpm drop when performing the standard magneto check. When we pulled the magnetos, however, my mechanic performed a simple test which revealed one mag throwing a spark 3/4 inch while the second throwing a spark only 1/8 inch. Since both mags were over 600 hours old we decided to replace them. Wow, what a difference! My wallet is now lighter but my engine purrs.
Jeff DeGange 415C
