Hot start why don’t we prime?

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Joffreyyy

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Joffreyyy
Why don’t we prime if It is a hot start in most 172s just a simple answer to understand the mechanics of it would be greatly appreciated thanks !
 
Lots of variables to consider such as injected or carb. Some engines/airplanes require it and some don't. It isn't a hard and fast rule to NOT prime when the engine is warm. Some people don't, some people give it one squirt.
 
Joffreyyy said:
just a simple answer to understand the mechanics of it would be greatly appreciated thanks !
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hahahahahaha

with my Cherokee I give it 2 shots of primer when hot starting. Just seems to work better.
 
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And I give mine one. Just goes to show ya OP that every airplane is different.
 
And for you Cherokee guys that are priming a hot engine, I hope you lifted the cowlings and checked the primer lines before you do so.
 
I just looked at my PA28-151 book:

Starting cold:
1/4 inch throttle
fuel pump on
full rich
engage starter
if it doesn't start within 5-10 seconds, disengage starter, prime with 1-3 pumps and try again.

Hot:
1/2 inch throttle
fuel pump on
full rich
engage starter

Flooded:
full throttle
fuel pump OFF
idle cutoff
engage starter
when it starts, advance mixture control and retard throttle

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I do have my own checklist, but I don't have it with me so I don't know if I've modified those procedures for my own circumstances. It's been a while since I've flown the Cherokee, but I remember that I generally prime when cold, and don't when it's hot. If I prime when it's hot I seem to end up in the "flooded start" section.
 
Cars with carbs used to have automatic chokes, or far enough back, manual chokes. The primary reason is that with everything cold, less fuel vaporizes, and more sits around not participating in bringing the fuel/air up to a combustible ratio. Priming (or choke plates) throws enough extra fuel into the intake that it is more likely to have enough vaporized to get into the combustible range, without vaporizing so much that you no longer have enough oxygen present, and are now "flooded"
 
EppyGA said:
If one breaks, they break off at the fitting, you'll be squirting raw fuel on a hot cylinder. It makes a great fire.
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Ah gotcha, nope dont do that. My lines are new though ( I understand new can break too), I replaced them a few years ago because they work hardened and were not allowing fuel to pass through them. Sure was a pain changing them out too under the dash!
 
I had problems hot starting and flew up to my A&P and asked him to show me. Mixture rich, pump the throttle half way twice and crank.
 
simtech said:
Ah gotcha, nope dont do that. My lines are new though ( I understand new can break too), I replaced them a few years ago because they work hardened and were not allowing fuel to pass through them. Sure was a pain changing them out too under the dash!
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Not gonna tell you how to operate your plane. I have a friend with a Warrior, he got to rebuild it firewall forward after priming on a hot start. Fortunately someone was close by to pick up the fire extinguisher. I check them every time.
 
Nope throttle pump is a mortal sin in a Cherokee, so ive been told. Hear about a good many fires from throttle pumps in a Cherokee.
 
EppyGA said:
Not gonna tell you how to operate your plane. I have a friend with a Warrior, he got to rebuild it firewall forward after priming on a hot start. Fortunately someone was close by to pick up the fire extinguisher. I check them every time.
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If I had to pull my cowling off every flight I wouldnt fly. Its not hard but its not as easy as a warrior.
 
Joffreyyy said:
Why don’t we prime if It is a hot start in most 172s just a simple answer to understand the mechanics of it would be greatly appreciated thanks !
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It’s always about the fuel/air ratio. The colder and the air in the combustion chamber, the more fuel it takes to achieve the proper ratio, and vice-versa. So, for any given airplane, you just do what it takes to find the sweet-spot for the condition.

On a hot engine, or a cold one, if you get the fuel/air mixture correct, assuming spark timing is fairly close to what it should be and the fuel supply is working normally, it will start.
 
simtech said:
Nope throttle pump is a mortal sin in a Cherokee, so ive been told. Hear about a good many fires from throttle pumps in a Cherokee.
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Remember, I said half way.

If you have a method that works, I’m not saying to change. Just saying what works for me.
 
EppyGA said:
And for you Cherokee guys that are priming a hot engine, I hope you lifted the cowlings and checked the primer lines before you do so.
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Because Cherokees are the only aircraft with primer lines that can break or leak right?
 
Joffreyyy said:
Why don’t we prime if It is a hot start in most 172s just a simple answer to understand the mechanics of it would be greatly appreciated thanks !
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If the fuel lines still have fuel in them, priming might lead to a flooded start. I'm assuming that the engine was shut down with the mixture control.

Bob
 
Clip4 said:
Because Cherokees are the only aircraft with primer lines that can break or leak right?
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Certainly not, it was Cherokee guys commenting on using the primer on a hot start so I was passing along some info that I had knowledge of.
 
In my new Lycoming clone I prime about half the duration when hot as when cold, then go full throttle with mixture at lean cutoff. It fires right up.
 
EppyGA said:
Don't pump it all the way
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Pumping the throttle, when not cranking, in many planes can lead to a fire if you have a backfire.... Look at your carb and intake and see where the fuel will go/run if you were to inject raw fuel at the point of the carb.
 
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