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mag check at idle

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prkaye

Well Known Member
I have always done a mag check (L-Both-R-Both) as part of my runup. I don't recall ever having done a mag check at idle. I did that today and i noticed that the engine runs pretty rough at idle on just the mag. I have an electronic ignition on the other side, and it idles much better on the EI (or on both). The RPM drop when i switch to just the mag at idle is also much bigger than the RPM drop during a mag check at higher power settings.
Does this sound normal?
 
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Phil,

I sometimes do a mag check at a bit above idle (1000 rpm +/-) after start because I have to cross the runway to the runup area and would rather discover a dead mag in front of my hangar.

I taxi well leaned on my carbed O-360. If I do my runup that lean, I may see a 300 rpm drop on my mag and 20 rpm drop on the EI. Richen it up, and the mag drop is only around 50 rpm. EI's really do ignite lean mixtures well.
 
Remember that magnetos run on a fixed timing, usually 25 BTDC for most 4 bangers. So the magneto is a compromise timing wise & is probably optimized for between climb & cruise, not so much for idle.

Your EI (you don't mention which brand) would likely change it's timing utilizing an advance curve varyied by rpm & MAP.

You could mark your flywheel timing mark in increments up to 40 deg BTDC & use a timing light (cautiously!) to see what your EI is doing.
 
Idle is a vague number. If you have a metal prop and set your rpm at 700 and run on one mag, the engine should be smooth. Watch your egt's to figure out the bad plug/wire etc.
 
Also remember mags put out lower voltage at lower RPM's. You may have plugs with gaps just a bit too large, or the mag itself may be on its way out. As someone else said, check the EGTs on just the mag.
 
prkaye said:
I have always done a mag check (L-Both-R-Both) as part of my runup. I don't recall ever having done a mag check at idle. I did that today and i noticed that the engine runs pretty rough at idle on just the mag. I have an electronic ignition on the other side, and it idles much better on the EI (or on both). The RPM drop when i switch to just the mag at idle is also much bigger than the RPM drop during a mag check at higher power settings.
Does this sound normal?
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This is normal if the EI is running more or less advance at the RPM used during the check. We generally don't do mag checks at idle, because at lower RPMs, the roughness of running on one ignition is more significant. Higher RPMs and higher MAPs create more challenges to lighting the charge and therefore more likely to identify a weak ignition. Therefore higher RPM mag checks will identify more issues than lower ones.
 
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So for a typical mag check at runup RPM, you should expect to see an EGT rise on all cylinders when you switch from both to either mag alone. Is the same behaviour expected on airplanes with one mag and one EI? Would you still expect to see an EGT rise when you switch from Both to either the mag or the EI alone?
 
The egt rise is the result of a less efficient combustion process when removing one ignition source. The resulting “unspent fuel” is burned on the way out the exhaust port raising the egt. EI is more efficient than a magneto so you should see a higher egt rise when removing it.
 
If the EI provides significant spark advance at runup power, then no, there will be a very small, hard to detect, egt rise when operating on the EI only. The mag is pretty much along for the ride, firing after the burn front has gone by. Of course, running on the mag only should show an egt rise.
 
prkaye said:
So for a typical mag check at runup RPM, you should expect to see an EGT rise on all cylinders when you switch from both to either mag alone. Is the same behaviour expected on airplanes with one mag and one EI? Would you still expect to see an EGT rise when you switch from Both to either the mag or the EI alone?
Click to expand...

Yes, egt should rise when one ignition is dropped, unless one of them is excessively advanced. The amount of rpm loss or egt rise will depend on the advance setting of the individual systems. It is typical for an ei to show different results as they are often using something other than 25 , that the mag is using, at runup rpms and map.
 
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rmarshall234 said:
The egt rise is the result of a less efficient combustion process when removing one ignition source. The resulting “unspent fuel” is burned on the way out the exhaust port raising the egt. EI is more efficient than a magneto so you should see a higher egt rise when removing it.
Click to expand...

Less than optimal advances generally doesn’t result in uncombusted fuel. It is more about WHERE it combusts relative to piston position. At the optimal advance point, more of the combustion energy results in greater pressure and therefore more heat converted to energy and more heat transfer to cyl head. When the advance is late, some of the combustion doesn’t convert heat to energy and instead just makes the combusted air hotter and this shows as higher egts. Combustion is complete before BDC. Amount of un burned fuel in exh is all about afr or mixture, generally speaking; mismatched oxygen and hydrocarbon molecules. The above is the basis for the inversely related egt / cht movement when changing advance.
 
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Airplane engine camshafts are profiled to work best at operating speeds. That is why they don't exactly run great at idle.
 
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