PearlPilot

I had no problems at all starting a good old carbureted 172 on a warm summer day. But I have the hardest time trying to start it when it is very cold. Example: This morning it was around 45ish and last night it was below freezing. I thought the engine is rather cold, but not cold enough for a pre-heat. So I primed it a couple of times and tried cranking no luck. Waited a few minutes, pumped throttle a couple, tried again, and repeated the procedure for a good 10 minutes. Since there wasn't anybody around to help me I had to call it quits and sacrifice flying in a very beautiful, calm, crystal clear morning. I was very disappointed, needless to say. This is what I have been taught. If it is below freezing, definitely pre-heat. Around 32-45, 2 shots on the prime, more than 50, pump throttle. This is not the first time I've had trouble starting in cold weather. I have read on numerous accounts that pumping the throttle is something that you should not do, but nevertheless, it was taught to me and I do it for that reason (although I am hesitant toward doing it). Pumping the throttle, sends raw fuel into the air filter and this could potentially cause a fire (right?). Obviously I am very confused about the proper way to start an engine. It is very embarrassing to not be able to start your plane and not be able to fly. I am really worried about running into such a situation when I do take passengers. I am not a mechanic, and all the mechanical knowledge I have is very limited to the textbook and the POH. Any help is GREATLY appreciated!!

xjcaptain

It depends on weather you think you are getting too much fuel to the engine or not enough. Smell is one way to tell, or look below to see if fuel is pooling below the engine. If too much...leave the mixture out, crank and slowly push the throttle in about 1/4 - 1 inch. When it catches, push in the mixture. Not enough fuel, prime more (3-4 depending how cold). I've started 172's down to about -45F. When in doubt though, preheat. At a minimum it will be better on the engine. Probably the most important part is SLOWLY advancing the throttle until you find the sweet spot where it wants to run. Not enough is no good, just as too much is no good. Just like the three bears....it has to be just right.

NoyGonnaDoIt

Go back top the POH for a moment. What does it say about how many shots of prime to use? If it's like the one I have, it says 2-6.

If yours says the same, think about it: You are trying to start an engine that was sitting all night in below freezing temperatures on a day where you are just above the guideline for using preheat, and trying to start with the bare minimum amount of priming.

It could be a weak battery or a weak starter - those tend to worsen with cold temperatures. But your first step should be to use an appropriate amount of prime for the temperature of the engine.

IC ALL

I would have primed it a bit more. One trick you might try is prime two or three shots. Pull the prop through by hand 10 times. Then prime it two or three more times. Then give it a try.

shdw

If you are pumping the throttle without cranking the engine you might as well get out of the plane and pat it on the tail before you get in, they do approximately the same thing...nothing. Pumping the throttle only has an effect when the engine is turning, the effect is the same as continuing to prime the engine as though it isn't getting enough fuel to begin with. In most cases it is completely pointless.

As others have said, prime it more and when in doubt preheat. As for moving the throttle, put it in a half to an inch and just leave it. In my personal experience, people playing with the throttle seem to have all kind of starting problems. I personally do this and have only had one carb engine not start on me, due to bad spark plug in the #3 cylinder. Temps ranging from -25 to 80 F and all of these were horribly maintained 152s, 172s, and grobe gliders, one of the reasons I left that job.

These are the steps I follow, regardless of outside temp:

1) 2 primes, throttle in 1/2 inch and crank for about 10 seconds. If it doesn't sound like it catches stop and

2) 2 more primes, throttle to 3/4 inch and crank again for 10 sec. If it doesn't sound like it catches stop and

3) 2 more primes and throttle to an inch in, again 10 seconds of cranking.

If at any of these steps I hear it catch but fail to start I continue cranking past that catch point for about 5 seconds. This is to clear out whatever fuel was in there. Next I give it 3 shots of prime and repeat. If the same thing happens I give it 4. If it happens again this is where it gets a little tricky, I give it 4 shots then bring the primer out for the 5th shot but don't push it in. When I hear it catch I slowly feed in the 5th shot to keep it firing until it starts.


If all this doesn't work I assume it is flooded, but to be sure I give it 5-6 more shots to make it flooded. Then I push the throttle to about 2 inches in, almost half way and put the mixture at idle cutoff. I then crank the engine and do a start just like a fuel injected, shoving the mixture in when it fires. If it fails I return the mixture to idle cutoff and crank another 10 seconds to ensure all fuel is gone and go back to step one.

Hope that helps, good luck.

PearlPilot

Thank you very much, I seem to have a much better understanding now...I gotta review these procedures with my instructor and hope to save another embarrassing situation. In a way I am glad it happened, because now I know what to do. It made me review the POH and the chapter on engines.

These methods work for the first start of the day. What about subsequent starts and time period in between? Does it make any difference?

Left Handed

Just be very careful that you have the ignition off when you do this. And yes, primer is your friend.

X Rated

Shdw makes the assertion that your carburetor has no accelerator pump. And while there are some aircraft that do not have an accelerator pump in their carbs, there are many aircraft that do.

I've had plenty of luck pumping the throttle while cranking--and prior to cranking. I believe it was the old Piper manuals that recommended pumping the throttle twice before cranking. I've also had luck with a shot of prime while cranking the engine in cold temperatures--down to about 15 degrees.

As each carburetor on every airplane is adjusted differently, they tend to have their own personalities. The difficulty is in learning the personality you're dealing with.

X

esa17

Carburated Mooney's are notoriously difficult to start in cold weather so I learned a neat little trick good for most carburated engines. Believe it or not there is a way to get a hard starting plane to fire nearly every time.

Mixture - rich
Throttle - cracked
Prime the engine by pumping the throttle three to five times.
Bump the mags quickly not letting the blade go a full turn.
Mixture - ICO
Throttle - half
crank until firing then ease the mixture in and the power out.

It's not to dissimilar from the Cessna three hand start and doesn't involve pulling any blades through by hand.

joepilot

If you are pulling the prop thru or hand propping with the intention of starting the aircraft, treat it the same way. Yes, you make sure that the mags are off, but you assume that they have a short and are live. If you are alone, be sure that the aircraft is tied down. Get instruction from somebody that has real world experience in hand propping an aircraft. Make absolutely sure of your footing and retreat path ( NO snow or ice). Don't curl your fingers around the back side of the prop. Use a proper leg swing and back away from the aircraft after every pull on the prop.

This can be a dangerous operation, still, people were flying for twenty years before self starters became at all common, and there are many aircraft flying today that must be hand propped to start.

Joe