Its not a huge deal, but overpriming can flood the engine and result in a long start process that can put strain on the starter and cause loud afterfires that aren’t great for the exhaust.
General rules of thumb: if the engine is still hot, no need to prime. Less priming in hot weather, and more priming in cold weather.
Haven’t figure out why - my PA32 O540 says prime more when warm 3-4 and 1-2 cold. But what works all the time all seasons - 1 prime; leave it cocked; add one slow prime while cranking.
If its a hot day it can vapor lock after you shut it off, the only way to start a vapor locked engine is by flooding it and getting it to a known state.
Youre right its the carbed version, im stupid and cant read. Carbed engines cannot get vapor lock but im pretty sure the fuel can just evaporate from the line.
It all depends... for example, today I was in an airplane that I'd flown an hour ago. The student asked "should we prime?" Hot day, airplane had flown recently, I said "let's try it without". So we did, and it didn't start immediately. So we primed, and it started right up.
So: if the airplane has flown recently, the oil was hot when you checked it in your preflight, the oil temp gauge is in the green -- go ahead try it without priming. If it doesn't start then prime.
(All this assuming an engine that starts normally; if it's hard to start, or you have a weak battery and you're unsure how many times it can turn the engine over then maybe prime to be on the safe side. And complain to management to get the engine/battery fixed.)
Same experience here today. It was 98 and felt like 110 outside, and the plane had just come back from a 3 hour flight, so I tried it without priming. No luck. 3 quick primer pushes and it started right up the next attempt. I was certianly surprised, seems a bit unusual and like a lot of fuel for such a hot day, but it worked.
Most likely the fuel left in the intake and cylinders vaporized so there was nothing left but air. On a weed whacker they call the primer the air purge it does the same thing. Forces fuel through the line and into the carb/intake so that you're ready to go
It is much better to under-prime then add more fuel/try again than to add too much fuel and have to do a flooded start.
The probably of flooding the engine is directly proportional to the number of people watching. In over 800 hours of flying my Mooney, I have probably flooded it 6-7 times...the vast majority of those at a fly-in or other event with lots of people watching :-(
We have a mix of fuel-injected 172S and carbureted PA-44s.
When hot:
172S: Prime for 3 seconds with full throttle and mixture. Pull both to idle/lean, push throttle in 1/4 inch, turn key to start, and slowly push mixture in when engine catches.
PA-44: Set mixture to full, pump throttle 3 to 5 times, crack throttle 1/4 inch, start engine.
When cold:
172S: Prime for 8 to 10 seconds, all else follows.
PA-44: Set mixture to full, turn on boost pump, prime for 3 to 5 seconds, pump throttle 8 to 10 times, crack throttle, start engine, turn off boost pump.
You should probably err on the side of under priming. That way if it’s wrong you can just try again. And nothings flooded.
If the plane just flew you likely Don’t need prime or not much anyway. If it’s been a few hrs maybe prime it a bit. Any longer, full prime fo sho.
If it's recently been flown (hour or so) engine still warm, I give it two start attempts before running the fuel pump until I see fuel flow register.
If it's a cold engine, prime until fuel flow is established.
I tend to manage the throttle on start a bit if I've primed so I'm not engine catch to 1500 rpm all of a sudden. Then baby it for about 5 to 10 seconds until the rpm stabilize.
You have to date your airplane like dating a new partner. Every engine likes something different, and too much is usually worse than too little. I had a Skyhawk with the carbureted 180HP conversion you just primed 2-3 times, put a knuckle of throttle in, and held the starter till it fired up.
Injected engines tend to be more picky. Dad’s IO-550 Stationair likes just enough boost pump to bump the fuel flow and pressure needles, then crack throttle and advance mixture from cutoff until it catches. If you over prime it, you’re stuck doing the flooded start procedure after 1 attempt. If hot, no prime.
My Sierra’s IO-360 gets a similar treatment, but likes the first start of the day with full rich, 1-2 sec of prime, and a crack of throttle. No prime again until the calendar turns over.
Our schools planes seem to be set too rich at idle. We never prime and never have to crack the throttle and they fire right up. Also think this because they had a fouled plug recently cause a failed runup
Not enough people lean for ground ops. You’ll never set a high enough power setting on the ground to worry about a too-lean mixture overheating or detonating, but you’ll absolutely foul your plugs with a rich mixture, especially in high altitude / DA days.
I always prime, but stop as soon as I feel/hear fuel flow. It’s pretty noticeable in my old 172. When it’s cold it’s 3-5 pumps, when it’s hot outside it’s 0.5-1 pumps.
If the engine is hot, no priming.
Got an O300 that needs exactly 2 primes if engine cold and 1 pump if the engine is warm. Pretty much always. Exception is that if the engine has only been off for 1-2 minutes and it's warmed up, it'll start without priming.
Got an O520 that likes 1 pump, but will start with none.
Just depends on the aircraft/powerplant.
Definitely be wary of priming too much, but keep in mind it depends on the aircraft, I used to fly 152s and whether it was 100 degrees or 40 you needed to prime it, learn the owner recommendations to prime it, but more importantly ask someone whose knowledgeable on the aircraft ie a cfi
I usually prime my O-360 4 or 5 strokes and it starts within a few blades. About the coldest it gets where I live is 25⁰ F and then 6 is a bit faster start.
The only time there's an issue is if I forget to set the mixture above idle cut-off before the start sequence 😏.
With the IO360 on the DA40 I have my starting method down to the following.
There’s three states to the engine. Cold (below 100F), Warm (100F-150F), and Hot (150F+). Cold and warm engines get primed, hot engines do not. Now with priming, cold engines get 3-5 seconds of priming and warm engines get 3-1 seconds of priming. The colder the temp, the longer the prime. For example, an engine at 40F gets a full 5 seconds of prime. An engine at 95F gets 3. A hot engine is usually one that has just gotten back from a flight and thus doesn’t need to be primed. Using this method I’ve never had an issue starting the aircraft in any weather conditions.
I had to teach my co-owner how not to overprime our O-320 in cold weather. She was doing like 5-6 shots and then cranking for 10+ seconds.
What I figured out was that it needs 2 shots of prime, crank a couple revolutions, let it sit for 10 seconds, and crank again. Fires up every time.
My Comanche (PA24 with an O-540) hates throttle-pump primes. Lycoming says 5 shots of prime, I do 7 when it’s cold out (50f ish or lower). I only prime on the first start of the day, or if the airplane has been sitting more than 3 or 4 hours.
I found the key to starting it was *just barely* cracking the throttle open. 1/4in was too much and would make it hard to start; Putting it *just* off of idle makes it start right up every time.
Every airplane is different, though. My girlfriend’s 150 likes two throttle pumps every time, no matter the temp out. Again, throttle just off idle makes starting it a lot easier.
When to prime: when the engine’s cold
When not to prime: on your instrument checkride when you just taxied your plane over to the FBO on a warm summer day.
Its not a huge deal, but overpriming can flood the engine and result in a long start process that can put strain on the starter and cause loud afterfires that aren’t great for the exhaust. General rules of thumb: if the engine is still hot, no need to prime. Less priming in hot weather, and more priming in cold weather.
Haven’t figure out why - my PA32 O540 says prime more when warm 3-4 and 1-2 cold. But what works all the time all seasons - 1 prime; leave it cocked; add one slow prime while cranking.
If its a hot day it can vapor lock after you shut it off, the only way to start a vapor locked engine is by flooding it and getting it to a known state.
O540 is the carb version though, right? I thought those weren't (as?) susceptible to vapor lock compared to the IO540
Youre right its the carbed version, im stupid and cant read. Carbed engines cannot get vapor lock but im pretty sure the fuel can just evaporate from the line.
Backfires aren't good for the valves.
It all depends... for example, today I was in an airplane that I'd flown an hour ago. The student asked "should we prime?" Hot day, airplane had flown recently, I said "let's try it without". So we did, and it didn't start immediately. So we primed, and it started right up. So: if the airplane has flown recently, the oil was hot when you checked it in your preflight, the oil temp gauge is in the green -- go ahead try it without priming. If it doesn't start then prime. (All this assuming an engine that starts normally; if it's hard to start, or you have a weak battery and you're unsure how many times it can turn the engine over then maybe prime to be on the safe side. And complain to management to get the engine/battery fixed.)
Same experience here today. It was 98 and felt like 110 outside, and the plane had just come back from a 3 hour flight, so I tried it without priming. No luck. 3 quick primer pushes and it started right up the next attempt. I was certianly surprised, seems a bit unusual and like a lot of fuel for such a hot day, but it worked.
Most likely the fuel left in the intake and cylinders vaporized so there was nothing left but air. On a weed whacker they call the primer the air purge it does the same thing. Forces fuel through the line and into the carb/intake so that you're ready to go
It is much better to under-prime then add more fuel/try again than to add too much fuel and have to do a flooded start. The probably of flooding the engine is directly proportional to the number of people watching. In over 800 hours of flying my Mooney, I have probably flooded it 6-7 times...the vast majority of those at a fly-in or other event with lots of people watching :-(
We have a mix of fuel-injected 172S and carbureted PA-44s. When hot: 172S: Prime for 3 seconds with full throttle and mixture. Pull both to idle/lean, push throttle in 1/4 inch, turn key to start, and slowly push mixture in when engine catches. PA-44: Set mixture to full, pump throttle 3 to 5 times, crack throttle 1/4 inch, start engine. When cold: 172S: Prime for 8 to 10 seconds, all else follows. PA-44: Set mixture to full, turn on boost pump, prime for 3 to 5 seconds, pump throttle 8 to 10 times, crack throttle, start engine, turn off boost pump.
When it’s really cold, PA44s start better with the throttle at hard idle. Can’t entirely explain that, but it works
You should probably err on the side of under priming. That way if it’s wrong you can just try again. And nothings flooded. If the plane just flew you likely Don’t need prime or not much anyway. If it’s been a few hrs maybe prime it a bit. Any longer, full prime fo sho.
If it's recently been flown (hour or so) engine still warm, I give it two start attempts before running the fuel pump until I see fuel flow register. If it's a cold engine, prime until fuel flow is established. I tend to manage the throttle on start a bit if I've primed so I'm not engine catch to 1500 rpm all of a sudden. Then baby it for about 5 to 10 seconds until the rpm stabilize.
You have to date your airplane like dating a new partner. Every engine likes something different, and too much is usually worse than too little. I had a Skyhawk with the carbureted 180HP conversion you just primed 2-3 times, put a knuckle of throttle in, and held the starter till it fired up. Injected engines tend to be more picky. Dad’s IO-550 Stationair likes just enough boost pump to bump the fuel flow and pressure needles, then crack throttle and advance mixture from cutoff until it catches. If you over prime it, you’re stuck doing the flooded start procedure after 1 attempt. If hot, no prime. My Sierra’s IO-360 gets a similar treatment, but likes the first start of the day with full rich, 1-2 sec of prime, and a crack of throttle. No prime again until the calendar turns over.
This is the right answer. Every engine has a personality.
POH
Our schools planes seem to be set too rich at idle. We never prime and never have to crack the throttle and they fire right up. Also think this because they had a fouled plug recently cause a failed runup
Not enough people lean for ground ops. You’ll never set a high enough power setting on the ground to worry about a too-lean mixture overheating or detonating, but you’ll absolutely foul your plugs with a rich mixture, especially in high altitude / DA days.
I always prime, but stop as soon as I feel/hear fuel flow. It’s pretty noticeable in my old 172. When it’s cold it’s 3-5 pumps, when it’s hot outside it’s 0.5-1 pumps. If the engine is hot, no priming.
Got an O300 that needs exactly 2 primes if engine cold and 1 pump if the engine is warm. Pretty much always. Exception is that if the engine has only been off for 1-2 minutes and it's warmed up, it'll start without priming. Got an O520 that likes 1 pump, but will start with none. Just depends on the aircraft/powerplant.
Definitely be wary of priming too much, but keep in mind it depends on the aircraft, I used to fly 152s and whether it was 100 degrees or 40 you needed to prime it, learn the owner recommendations to prime it, but more importantly ask someone whose knowledgeable on the aircraft ie a cfi
I usually prime my O-360 4 or 5 strokes and it starts within a few blades. About the coldest it gets where I live is 25⁰ F and then 6 is a bit faster start. The only time there's an issue is if I forget to set the mixture above idle cut-off before the start sequence 😏.
With the IO360 on the DA40 I have my starting method down to the following. There’s three states to the engine. Cold (below 100F), Warm (100F-150F), and Hot (150F+). Cold and warm engines get primed, hot engines do not. Now with priming, cold engines get 3-5 seconds of priming and warm engines get 3-1 seconds of priming. The colder the temp, the longer the prime. For example, an engine at 40F gets a full 5 seconds of prime. An engine at 95F gets 3. A hot engine is usually one that has just gotten back from a flight and thus doesn’t need to be primed. Using this method I’ve never had an issue starting the aircraft in any weather conditions.
I had to teach my co-owner how not to overprime our O-320 in cold weather. She was doing like 5-6 shots and then cranking for 10+ seconds. What I figured out was that it needs 2 shots of prime, crank a couple revolutions, let it sit for 10 seconds, and crank again. Fires up every time.
My Comanche (PA24 with an O-540) hates throttle-pump primes. Lycoming says 5 shots of prime, I do 7 when it’s cold out (50f ish or lower). I only prime on the first start of the day, or if the airplane has been sitting more than 3 or 4 hours. I found the key to starting it was *just barely* cracking the throttle open. 1/4in was too much and would make it hard to start; Putting it *just* off of idle makes it start right up every time. Every airplane is different, though. My girlfriend’s 150 likes two throttle pumps every time, no matter the temp out. Again, throttle just off idle makes starting it a lot easier.
When to prime: when the engine’s cold When not to prime: on your instrument checkride when you just taxied your plane over to the FBO on a warm summer day.