MAG check side A lower RPM than side B

Hi Thomas,

I just did a quick (rough) review of the video you referenced -

You haven't commented on engine running ie smooth/rough, when you do your Mag check.

Also - how many hours has the engine done? 😈

A 100 rpm drop on a mag check is expected and an indication that it’s running normally.

The engine runs fairly smooth both at idle and higher.  However, it does run rough when checking each of the mags.  340 hours on the Hobbs meter

I think the experts must be on holiday.

I can only comment on my experince/observation.

Current 912ULS 183hrs - last one 900+hrs.

My tacho readings do not stay right on the 4000 rpm mag test setting. There are natural/expected fluctuations due to wind, slight engine speed changes and probably sensor error. I have never taken much notice but would not be unduly alarmed by a 10-20 rpm fluctuation ie 3990 - 4010 rpm.

My A & B "mag" checks show similar fluctuations ie a precise mag drop (100 rpm) almost never happens- its within an acceptable range. I would probably not notice a 130 rpm drop as long as the engine remained relatively smooth.

With each mag off, there is is a slight reduction in smooth running, I would not describe it as rough.

When I do my mag checks I am looking for :

Set aproximately 4000 rpm - engine smooth - all systems as expected

Posative off - instant reduction in rpm - shows that ignition "ground" contact good.

RPM drop acceptable around 100. Engine still running well - shows that active ignition functioning correctly.

Mag on. Posative (no hesitation) return to around 4000rpm - engine smooth, all systems as expected.

Repeat on second ignition.

Note: Further warming of engine, due to high rpm, may see engine speed creep up  ie second check may result in higher speeds.on both A&B .😈

UPDATE:

Thanks for all the advice.  I had thought it was an ignition side issue but for giggles I checked the carbs and they were slightly out of balance.  After balancing my mag check is back to normal.