Engine sort of runs with mags off....

Salty

Salty

Touchdown! Greaser!
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Salty
After a two hour flight and short taxi, then 10 minutes of cool down, restarted the plane and taxied it to another location, maybe 5 minutes at most. Then turned off both mags while full rich. Engine keeps popping, acted like it was firing on one cylinder, just barely turning over, kept doing it for 5 or 10 seconds before I turned the mags back on and shut off the mixture.

mag test acted normal after next start. Ran fine on each mag by themselves.

My theory is a hot exhaust valve firing one cylinder. Very odd.
 
Salty said:
After a two hour flight and short taxi, then 10 minutes of cool down, restarted the plane and taxied it to another location, maybe 5 minutes at most. Then turned off both mags while full rich. Engine keeps popping, acted like it was firing on one cylinder, just barely turning over, kept doing it for 5 or 10 seconds before I turned the mags back on and shut off the mixture.

mag test acted normal after next start. Ran fine on each mag by themselves.

My theory is a hot exhaust valve firing one cylinder. Very odd.
Click to expand...
I would suspect a ignition switch that is starting to have a high resistance short.

disassemble clean re-assemble
 
'dieseling' in a hot gas engine - usually along with some carbon build-up - is a possibility. I had a car (1968 vintage) years ago that would do that, when shut off hot. This was back in the carbureted - distributor - no computer days. A few years later, the auto manufacturers added an 'anti-diesel' solenoid that would close the carburetor butterfly completely (below idle position) when the key was turned off.
 
I've had a couple of auto engines that would diesel occasionally. Air-cooled engines are likely to be worse since they run hotter. IIRC, air cooled Volkswagens since 1960 all had a solenoid valve to shut down the idle circuit to prevent this. I suspect this is one reason we are taught to shut down aircraft engines with the mixture control, the primary reason being safety.
 
40YearDream said:
'dieseling' in a hot gas engine - usually along with some carbon build-up - is a possibility. I had a car (1968 vintage) years ago that would do that, when shut off hot. This was back in the carbureted - distributor - no computer days. A few years later, the auto manufacturers added an 'anti-diesel' solenoid that would close the carburetor butterfly completely (below idle position) when the key was turned off.
Click to expand...

Growing up we had a car that would diesel in the summer on the cheap gas dad bought. When it did it I'd just turn the key back on, put it in drive, and shut it off in gear. The drag from the torque converter was enough to stop the engine.
 
Yup. Dieseling. I had a '78 Dodge pickup, the worst vehicle I ever owned by a long shot, that would do that. I had to shut it off and let the clutch grab to stop it. I took the head off and cleaned all the carbon off the pistons and head, put it back together, and it was fine for a couple of months and then started dieseling again.

Never bought another Chrysler product. Having to fix something on that stupid truck nearly every weekend did that to me.

My old Auster would diesel. Being a British airplane, it had some strange features, like a mixture control that was full rich when it was all the way back. The throttle and mix controls were levers, side by side, and there was a tab on the throttle lever that pulled the mix to full rich when you closed the throttle. So you couldn't lean it to stop it, and leaning likely wouldn't have killed it anyway. It probably had a back-suction mixture system like the old Stromberg carbs. They don't shut the fuel off altogether.
Anyway, you shut it down with the mag switches, and if it was hot it would diesel on for a long time. One had to shove both the throttle and mixture full forward and wait a bit for it to quit. Or shut off both tanks and let it idle, ignition on, until it died.
 
Tom-D said:
I would suspect a ignition switch that is starting to have a high resistance short.
Click to expand...
^that

It *could* be dieseling, but the reason you do a mag check on the ground with them off is to check the grounding. More likely the issue is there, opposed to the engine dieseling.
 
Tantalum said:
^that

It *could* be dieseling, but the reason you do a mag check on the ground with them off is to check the grounding. More likely the issue is there, opposed to the engine dieseling.
Click to expand...
I'd agree if it didn't run normally on each mag by themselves. 100 rpm lower than both in both cases, but running smoothly. On "off" it was not running smoothly in any way.
 
Salty said:
I'd agree if it didn't run normally on each mag by themselves. 100 rpm lower than both in both cases, but running smoothly. On "off" it was not running smoothly in any way.
Click to expand...
Right. Not likely the mag switch. It could be. though, a loose P-lead connection at one mag. Engine vibration will make it an intermittent connection and rough running.
 
over running is not a mag problem, other than it being badly timed. most the aircraft do not have compression ratio to do that.
if the engine runs fine, it is not dieseling, it is not completing is not shut off completely.
 
Dan Thomas said:
Right. Not likely the mag switch. It could be. though, a loose P-lead connection at one mag. Engine vibration will make it an intermittent connection and rough running.
Click to expand...
depend upon where the short is, old bendix are famous for it. wires, mag connection, mag switch..
easy trouble shooting. remove the old bendix connector, it should not run.

to a run a old Bentix you must have a connector installed.
 
Tom-D said:
over running is not a mag problem, other than it being badly timed. most the aircraft do not have compression ratio to do that.
if the engine runs fine, it is not dieseling, it is not completing is not shut off completely.
Click to expand...
The dieseling doesn't need a spark. Just needs a sharp little bit of carbon in the cylinder to set the mixture off near the top of the compression stroke. That's why my old Dodge and my old Auster did it with the ignition turned right off. Neither of them had much for compression ratios.

A diesel engine relies on the heat of compression to ignite the fuel. A gasoline engine, as you say, doesn't have the compression ratio to do that, but glowing carbon will do it for you. In the old days some guys would clean their car's cylinders out by dribbling a little water into the carb as they revved the throttle. The steam formed in the cylinders would blow that carbon out.
 
Salty said:
After a two hour flight and short taxi, then 10 minutes of cool down, restarted the plane and taxied it to another location, maybe 5 minutes at most. Then turned off both mags while full rich. Engine keeps popping, acted like it was firing on one cylinder, just barely turning over, kept doing it for 5 or 10 seconds before I turned the mags back on and shut off the mixture.

mag test acted normal after next start. Ran fine on each mag by themselves.

My theory is a hot exhaust valve firing one cylinder. Very odd.
Click to expand...

understand, after running, and dieseling. it seems the same but it is not.
 
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