M20C Cold Weather Start

[Vance Harral]

1 hour ago, Hank said:

Fast pumps are easier with my throttle quadrant compared to push-pull controls

Always nice to meet another member of the tribe equipped with the superior throttle quadrant. 

[Jackk]

27 minutes ago, Vance Harral said:

When done carelessly, yes.  But this is sorta like saying, "Flying a taildragger is a great way wreck an airplane in a ground loop".  We accept certain risks in order to operate certain airframes, and that's just the way it is.  For better or worse, many carbureted airplanes - including the Mooney M20C and M20G - have no way to prime the engine for start other than throttle pumping, and explicitly prescribe the practice in their operating manual.

 I fly taildraggers that I hand prop, even they have a primer, crazy to think the Mooney doesn’t have a proper primer.

 

 Is that something you can add?

[Ragsf15e]

2 hours ago, Hank said:

Fast pumps are easier with my throttle quadrant compared to push-pull controls. 

Oh boy, here we go! Johnson bar’s next!

[47U]

33 minutes ago, Vance Harral said:

1 hour ago, Jackk said:

Pumping the throttle is a great way to burn your airplane down

When done carelessly, yes.  But this is sorta like saying, "Flying a taildragger is a great way wreck an airplane in a ground loop".  We accept certain risks in order to operate certain airframes, and that's just the way it is.  For better or worse, many carbureted airplanes - including the Mooney M20C and M20G - have no way to prime the engine for start other than throttle pumping, and explicitly prescribe the practice in their operating manual.

That’s why this little drain on the bottom of the airbox is so important.  To prevent any excess fuel from pooling in the bottom of the cowling, which if that occurs certainly presents a hazardous condition.  The lower cowl has a hole with a grommet for the tube to drain excess fuel, should it occur, on the ground and hopefully away from any source of ignition.

Like @Jackk mentioned, I suppose a primer system could be retrofitted to the O-360, my O-320 Cherokee had a primer.  I think a primer system isn’t without its own set of risks, though.  The best solution is for pilot’s to understand how their system works, whether it’s a primer or accelerator pump on the carb, and don’t ask it to perform outside of its design parameters.

[Hank]

15 minutes ago, Ragsf15e said:

Oh boy, here we go! Johnson bar’s next!

No, I enjoy my electric gear. With my shoulder, I couldn't operate manual gear as often as I cycled the electric gear during IFR training . . . . It's better than it was, but it's still not right. Surgery made it only hurt sometimes instead of all the time. I blame it on overwork from my job in my late 20s:  taking apart, repairing, changing over and reassembling plastic injection molds that were mostly 8 to 18 tons of steel, probably averaging ~26,000 lbs..

[Fly Boomer]

12 hours ago, Hank said:

Sadly, our planes don't have a primer. Just like my old pickup in college, in winter stomp on the gas pedal a few times before turning the key. Or in my Mooney, pump the throttle a few times; the colder the temps, the more times.

As I was typing that, I was reminded of an old pickup I had (bought as salvage) that had no choke — automatic or otherwise.  I could get it started in single-digit temperatures, but I just about wore out the accelerator pump.  Fifty pumps with key off, start cranking, and continue pumping.  It would come online one cylinder at a time, and eventually smooth out.  Fun times, no money.

[Hank]

3 hours ago, Jackk said:

Pumping the throttle is a great way to burn your airplane down

Not my Mooney. Been doing it every start for 18 years.

3 hours ago, Jackk said:

Does your POH say to pump the throttle for a carbureted engine?

This is from my Owners Manual, already posted above, but highlighted for those who cant remember it from a page back.

[Skates97]

Similar to what has been posted above. The only difference I have when it's cold is to wait about 15 seconds from pumping the throttle (3x) before starting. Haven't had any problems at 35°F. Lycoming says "cold weather" is 10° F or colder and needs an engine heater. I tried once to start when the cylinder and oil temps were mid 20's from being cold soaked and the cold oil wouldn't let it turn the prop fast enough to fire. Plugged in the engine heater (which also has a heat pad on the oil sump and oil cooler) and got temps above 30° F and started right up.

[ziggy122]

@C.J. @Hank  Why do you gents turn off your boost pump?  Do you turn it off for normal starts as well?  The dark art of starting a carbureted engine outside of normal temps has been a greek tragedy of trials, tribulations, and sadness thus far for me.  An engine heater is the only easy answer I've found.  I know it needs more gas, but that hasn't made it easy yet.

Thanks for sharing your techniques.  I'm going to work in those next time I am starting without engine heat.  It usually takes me at least 2 full 30 min starter cycles and a lot of curse words.  

[Hank]

1 minute ago, ziggy122 said:

@C.J. @Hank  Why do you gents turn off your boost pump?  Do you turn it off for normal starts as well?  The dark art of starting a carbureted engine outside of normal temps has been a greek tragedy of trials, tribulations, and sadness thus far for me.  An engine heater is the only easy answer I've found.  I know it needs more gas, but that hasn't made it easy yet.

Thanks for sharing your techniques.  I'm going to work in those next time I am starting without engine heat.  It usually takes me at least 2 full 30 min starter cycles and a lot of curse words.  

Yes, I turn off the fuel pump for every crank, ever since attending my first MAPA PPP the month after finishing my insurance dual back in 2007. 

The key to cold starts is to give the cold fuel time to vaporize before turning the key. If it fires but doesn't catch, pump the throttle quickly while continuing to crank. "Pump" means full stroke, Idle to WOT and back to Idle, and keep pumping while cranking. After about 15 seconds, if it's not running, give it a rest, then try again with the whole thing if you haven't preheated the engine; if you did preheat, just pump the throttle several times, leave it cracked open, wait a bit (watch the yoke clock, 30 seconds under 40°F, 60 seconds if below freezing) and crank.

[Andy95W]

3 hours ago, ziggy122 said:

@C.J. @Hank  Why do you gents turn off your boost pump?  Do you turn it off for normal starts as well?  The dark art of starting a carbureted engine outside of normal temps has been a greek tragedy of trials, tribulations, and sadness thus far for me.  An engine heater is the only easy answer I've found.  I know it needs more gas, but that hasn't made it easy yet.

Thanks for sharing your techniques.  I'm going to work in those next time I am starting without engine heat.  It usually takes me at least 2 full 30 min starter cycles and a lot of curse words.  

With our carbureted engines, the fuel pump doesn’t do anything once the fuel bowl in the carburetor is full.  Once the engine starts, the engine driven fuel pump keeps the bowl full so the electric pump isn’t needed.  The reason it needs to be on for takeoff and landing is just in case the engine driven pump fails at a critical time.

The issue, like Hank said, is giving the fuel a chance to vaporize so it can be pulled into the cylinders to ignite.  The reason engine and oil preheat fixes the problem is because the carburetor is bolted to the bottom of the oil sump, and the warm metal encourages faster vaporization.

The only other thing that helps for cold starts is to pump the throttle while cranking the engine.  The sprayed fuel is sucked into the cylinders, but once it fires, it’s not happy.  And if it’s so cold that you need to resort to that, you should probably be preheating anyway.

Good luck!

[ziggy122]

Thanks gents. @Hank @Andy95W Appreciate the thermodynamics/mech engineering behind it as well.  (I fought hard for a C- in thermo, which made me realize I wasn't cut out to be an aeronautical engineer...)

[Hank]

1 hour ago, ziggy122 said:

Thanks gents. @Hank @Andy95W Appreciate the thermodynamics/mech engineering behind it as well.  (I fought hard for a C- in thermo, which made me realize I wasn't cut out to be an aeronautical engineer...)

Thermo was easy. An awful lot of it boils down to 

Then again, I'm an ME with a Master's Degree.

[C.J.]

On 1/4/2026 at 8:05 PM, ziggy122 said:

@C.J. @Hank  Why do you gents turn off your boost pump?  Do you turn it off for normal starts as well?  The dark art of starting a carbureted engine outside of normal temps has been a greek tragedy of trials, tribulations, and sadness thus far for me.  An engine heater is the only easy answer I've found.  I know it needs more gas, but that hasn't made it easy yet.

Thanks for sharing your techniques.  I'm going to work in those next time I am starting without engine heat.  It usually takes me at least 2 full 30 min starter cycles and a lot of curse words.  

Hi @ziggy122

Sorry you're having cold stating issues. As far as me turning off the boost pump, what @Andy95W said regarding this is also my reason. I'd only add I want all the juice from the battery being used by the starter (other than my anticollision beacon which uses next to nothing in amps). Throttle pumping as @Hank points out, is definitely effective under the conditions he outlined, however I've never needed to do that. Lots of good advice from other MS'ers here as well. In summary start with a well charged battery, some decent pre-heating (I highly recommend the "Reiff XP System with the Oil Cooler option") with an insulated cover and adequate priming & wait a minute for good vaporization.

Best regards, C.J.