Eh...the engines not shutting off

popaviator

popaviator

Well-Known Member
My student needed 10 stop and goes last night so we headed off to AFW and took full advantage of the multi million dollar runways they had there. Coming back to DTO when we began the shut down checklist and pulled the mixture to idle and the engine just started to cough and it was like a "very rough idle". After this continued to go on for about 15 seconds I decided to increase RPM to 1700 and then pull back the mixture to idle. After that it shut off....WTF? :confused: I'm trying to figure out why this would happen?
 
I'd say have mx check the idle cutoff adjustment on the mixture control.

That said, if it happens again, leave the mixture at idle cutoff and open the throttle. That should kill it.
 
Mixture idle cutoff stop might need adjusted, or maybe the mixture control valve in the carb is leaking (that would likely require a carb rebuild).

Also, set the engine to about 1000 RPM before pulling the mixture to ICO.
 
to make fire you need oxygen fuel and a spark... if it runs with mixtures in cut off position the engine is still getting fuel. you could check this by turning on the master and electric fuel pump leaving mixtures at cut off and look to see if fuel is still coming out. either way the airplane need to go into MX so it doesn't really mater what you do.
 
popaviator said:
My student needed 10 stop and goes last night so we headed off to AFW and took full advantage of the multi million dollar runways they had there. Coming back to DTO when we began the shut down checklist and pulled the mixture to idle and the engine just started to cough and it was like a "very rough idle". After this continued to go on for about 15 seconds I decided to increase RPM to 1700 and then pull back the mixture to idle. After that it shut off....WTF? :confused: I'm trying to figure out why this would happen?
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My best guess as to why this would happen: When you push the throttle forward you open the butterfly valve allowing much more air (as well as fuel) into the carburetor. When you then pull the mixture to cutoff you should cut off fuel, but in your case it sounds as if you merely reduce the amount of fuel so... the fuel mixture to O2 ratio is too small to maintain combustion. When you have the throttle at idle, the butterfly valve is partially closed to limit air mixing with smaller amounts of fuel keeping it at a combustible ratio. In your case, it appears that when you cut off the fuel mixture it still allows enough fuel to create a combustible mixture.

A/P folks? This sound right? Or am I just off my rocker (again!):D
 
Bumblebee said:
My best guess as to why this would happen: When you push the throttle forward you open the butterfly valve allowing much more air (as well as fuel) into the carburetor. When you then pull the mixture to cutoff you should cut off fuel, but in your case it sounds as if you merely reduce the amount of fuel so... the fuel mixture to O2 ratio is too small to maintain combustion. When you have the throttle at idle, the butterfly valve is partially closed to limit air mixing with smaller amounts of fuel keeping it at a combustible ratio. In your case, it appears that when you cut off the fuel mixture it still allows enough fuel to create a combustible mixture.

A/P folks? This sound right? Or am I just off my rocker (again!):D
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That's pretty much it! That's why running up to 1000 RPM helps. It seems to me i've flown a few carb-ed planes that had slightly leaky mixture valves and wouldn't quite shut down with the throttle all the way at the idle stop. Either that or they took a while to use up the remaining fuel.
 
popaviator said:
It was a carb'd 152. Our checklist cals for power to 1000RPM before shut down.....that's why I found it so strange.:cool:
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In that case it may actually be normal. If you pull mixture to cutoff, then throttle idle it will probably sputter for awhile. Like bumblebee said, if you increase the air, then there may not be adequate fuel for combustion... So, throttle up to 1000 RPM, and it should shut down.
 
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