Good Morning,

If you have flown the Champ on a cold day you probably know what I’m referring to….

The oil sump heater has been on, you  warmed up the engine to get 100 degrees oil temp, the run up was good, you are all set.

You line up on the runway advance the throttle….then it happens, the engine stumbles, it shakes, now do you abort or continue? What happened? Everything was normal…and it still is.

When you advance the throttle, you are adjusting the butterfly valve in the carburetor to let in more air to mix with the fuel being metered by the carburetor.   

The carburetor has an accelerator pump that injects more fuel into the carb to keep up with the increase amount of air that is being drawn into the carburetor as the throttle is advanced in high power demand scenarios like take off, rejected landings or and other times you move the throttle at a fast rate.  The accelerator pump does a good job of keeping up with the in rush of air in temperatures above 35ish degrees. 

When the temperatures fall below 35ish degrees the air is denser and even the accelerator pump cannot keep up with the in rush volume/density of the cold air.  So as the throttle is advanced forward on a cold day, you create a lean mixture that results in the engine stumbling and shaking.  Very unnerving if you are not anticipating it!

The remedy is to slow down the rate at which you advance the throttle.  How slow?  Slow enough so the engine does not stumble.  I recommend counting to 5 to advance the throttle from idle to full power, slow and steady.   If the engine begins to stumble, hesitate the advance, once is “catches up”, continue your slow, smooth advance.  Once you get to full throttle,  the engine should be turning up to 2200 rpm and running smooth.  If it is not at 2200 or its still rough, abort the  take off.

What do you expect to see in terms of normal indications on EVERY take off?  What is your criteria to abort the take off on the runway?  Do you have a pre-take off briefing that sets criteria for abort on the runway, land straight out, turn back to the airport? 

As a follow on comment, if the engine stars stumbling in the high power scenarios in warm weather,  that could be an indication of the accelerator pump not working.  Let me know if this happens in spring/summer.

Takeoff briefing

• Take off roll: verify the oil pressure is in the green, tach is at 2200 rpm, airspeed is indicating, in that order.  If not, abort on the runway before mid field.
• Engine loss below 1000′ MSL (600′ AGL): pitch to 60 MPH, land straight out
• Engine loss above 1000′ MSL (600′ AGL): Pitch for 60 MPH, return to the runway
• I recite this out loud before I line up on the runway, and verbally identify passing each portion of the brief.

Enjoy the Champ,

Mike G.