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Carb Heat

Old 30th Nov 2021, 19:44
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by utilizing the accelerator pump you can prime the engine for starting and this is often the preferred way.
I disagree. If the airplane has a hand primer, it should be used rather than the accelerator pump. The reason being that the hand primer will atomize the fuel to a much finer mist, being drawn in with the air better, greatly aiding combustion, and reducing draining back as a liquid. That's why they're hard to push, you're pumping the fuel through a tiny orifice to atomize it, instead of shooting a stream into the carburettor throat. And, the primer nozzle is further downstream in the inductions system, reducing draining back. Drained back fuel is a source for a carb fire. Pumping the throttle does help a stumbling engine at start, but is not preferable over a primer. The Cessna and Piper POH's do say to use the primer....
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Old 30th Nov 2021, 20:27
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Originally Posted by ahramin View Post
Checking the certification requirements that might be applicable to a similarly equipped certified aircraft it requires the carb heat to be able of generating a 100F rise. I've never seen a system that can deliver this much heat but we have a carb t probe on the way and we'll see what it can do.
The carb heat on a normally aspirated B model Aztec can give a 120F rise. I've used that ability in icing conditions (long ago).
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Old 30th Nov 2021, 20:57
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Pilot DAR, your comments are of course valid but not the whole story. I've given a description of many priming installations that are problematic. The accelerator pump also provides fuel downstream from the carburettor. As to both engine and carburettor fires I've seen too many and due to over-priming using the installed primer. Whatever the technique used if the engine doesn't start then stop after 3 - 4 rotations. Consider why the engine failed to start. It might have nothing to do with the fuel/air mixture. Don't keep turning and priming although I have seen this advised in some POH. If the engine (exhaust) is hot you have a high chance of an exhaust fire from over priming.
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Old 1st Dec 2021, 00:37
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The accelerator pump also provides fuel downstream from the carburettor.
Well, the accelerator pump discharges right in the carb, where the primer nozzle may be just downstream of the carb, or actually at the cylinder intake. The point which is more important is that the product of the primer is a mist of atomized fuel, which ignites very easily, and is less likely to flow backward and accumulate as a liquid. The accelerator pump discharge is something like a water pistol squirt, just a stream of gasoline. If it is not taken up with the induction air, it will flow backward into the airbox, and reside there, being a potential fuel source for a carb fire (I've had two). Sure, the primers can get really stiff, that means that they should be maintained. Primer nozzles do clog, and the primer plungers need occasional lubrication. There's special gasoline proof grease for this.Allowing the "normal" use of the accelerator pump as a primer is setting the student up for a carb fire.

I had a carb fire last winter in my 150 on a very cold day, preheated start. It did not start the first time, and fired and stumbled during the the second attempt until it stopped again. I had been priming it as it stumbled to run, as the POH says. I saw the smoke coming up the windshield defroster vent, and realized what was happening. I moved the mixture to ICO, and cranked the engine until the battery was discharged. With that I jumped out, a fire extinguisher completed the job. My wife was phoning me to tell me that she could see the fire from the kitchen window - I had to ignore her just that once - I was really busy. It was still damaging and expensive.
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Old 1st Dec 2021, 04:16
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Originally Posted by Fl1ingfrog View Post
Yes, the fuel provided from the accelerator pump will be the same. The fuel supplied by the pump bypasses the carburetor so it adds to that supplied by the carb. In fact, by utilizing the accelerator pump you can prime the engine for starting and this is often the preferred way. In some types it is the only way of priming the engine to start. Amazing to me is that the hand primers fitted to the 150/172 series may only provide fuel to just one or two cylinders. In this case I always find that using the accelerator pump by pumping the throttle is preferable.
Bypasses the carburetor? I'm guessing you've never had the cowl off an aircraft let alone a carburetor apart? The accelerator pump is part of the carburetor.

Yes, there are a few engines out there without a primer and using the accelerator pump is the recommended way to prime them. It's not an ideal situation though and they didn't add primers to all the later engines because they wanted them primed with the accelerator pump did they?

Ask PilotDAR about carb fires, he's an expert .
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Old 1st Dec 2021, 08:13
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We used to operate both 152s and 150s at a flying school I used to work for. By the time winter came around, we always had to do a briefing on the differences in the priming systems as the students were very much used to the 152s, where the priming fuel is deposited a lot closer to the cylinder than on the 150s. To make matters worse, someone thought it a good idea to have all the dual flights use the 152s as they had a bit more power, while the 150s were scheduled for students flying solo. Anyway, that was a whole different discussion.

The cold starting procedure on the 150 (if my memory is correct) has you turning the prop over by hand while priming, so that the suction will draw the fuel up, from the point down in the carb where it is injected, towards the intake manifold. On the O-200, the carb is of course down below the engine and you've got an airbox at the bottom, the carb directly above it and from that point the intake manifold goes straight up and turns outboard to end up underneath the cylinders. Gravity will of course always take over and fuel will inevitably drain down again towards the air box. Because of this sometimes the procedure was modified to turning the engine over on the starter while priming, but this was not in accordance with the POH. It did get the job done though. We also had the inevitable carb fire at times, one time when I was on board. The first clue was a burning smell and smoke coming up through the vents. My student was very quick and starting shutting things down before I could tell him to keep cranking so the sensible next thing to do was: get out. Fortunately, there was no big fire, but you could tell something had gone wrong, see below.



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Old 1st Dec 2021, 08:46
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For 15 years or so I have never had a problem starting a cold soaked 0-200 with no primer (OK Southern England, so never that cold). I use a heater to warm the induction system for about 30 minutes, pump the throttle 6 times, turn the prop through 8 blades, another 3 throttle pumps for luck and it never fails to start instantly. Relating to the earlier discussion about carb icing, the engine sometimes stops again before there is enough heat to stop icing which, as I said earlier, tends to happen a lot on the ground unless one uses carb heat.
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Old 1st Dec 2021, 11:59
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Originally Posted by pulse1 View Post
I use a heater to warm the induction system for about 30 minutes
That is called cheating!
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Old 1st Dec 2021, 15:28
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The cold starting procedure on the 150 (if my memory is correct) has you turning the prop over by hand while priming
This was a similar technique shown to me by the engineers of a very prominent UK school. They were required to start and warm 30 PA28 aircraft every morning throughout the cold mid winter. The technique was: set a piston at the top of compression and then using the accelerator pump give two strokes of the throttle and pull through. Do this for each cylinder plus two more for luck and start without delay. This worked every time; no flattened batteries, no flooded engines and no fires. I've done this successfully for over twenty years.

An engine fire is only a risk if you over prime. If you do not over prime then you will not have an engine fire, be it exhaust or carburettor, whatever starting technique you employ. If the engine is hot you probably do not need to prime. Never assume, in the first instance, that the fuel mixture is too lean but rather it is too rich.

A common reason for a difficulty when starting a warm engine is that the previous flight has not been shut down correctly. A common fault is to treat the mixture as if a switch. Rich to shut off; in a milli-second. Shut down with care allowing the engine to burn off all excess fuel and to leave the spark plugs clean

Last edited by Fl1ingfrog; 1st Dec 2021 at 20:18.
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Old 1st Dec 2021, 18:13
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https://www.lycoming.com/content/tips-start-your-engine

"Lycoming engines of more than 118 HP have a throttle pump which can be used for priming under moderate ambient temperature conditions while turning the engine with the starter."

Lycoming says it's ok and it works very well. Don't knock the technique if you have not actually evaluated it.

Last edited by EXDAC; 1st Dec 2021 at 20:37.
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Old 1st Dec 2021, 19:21
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Anyone familiar with this?
For Lycoming 160, in very cold air, I was told:
5 primer pumps.
Leave primer open.
Throttle set.Turn over X times, close primer to lock and open throttle as normal when it starts. (X~≥10? - memory)
If it doesn't start, delay then repeat.
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Old 1st Dec 2021, 20:13
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From the same publication that EXDAC has provided is the following bulletin:

Article-Knowledge Base

Service Instruction No. 1148 C

Use of Carburetor Heat Control
Supersedes Service Instruction No. 1148C October 12, 2007 MODELS AFFECTED All Lycoming engines using float type carburetors. TIME OF COMPLIANCE During engine operation. Under certain moist atmospheric conditions (generally at a relative humidity of 50% or greater) and at temperatures of 20 to 90F it is possible for ice to form in the induction system.
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Old 1st Dec 2021, 23:11
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Lycoming says it's ok and it works very well.
I take a different meaning from the same Lycoming instruction;

Priming can be best accomplished with an engine priming system, as opposed to use of the throttle.
And goes on to [correctly] advise about engine fires resulting from excessive "throttle" priming. And, it's worth noting that where there is a conflict between the engine manufacturer's published information, and the airframe manufacturer's flight manual, the flight manual shall prevail. Lycoming has no design control over the airplane manufacturer installing a hand primer as a part of the type design, so something is better than nothing, but they are saying that a primer is the best way to prime. The POH's for a Lycoming powered 172 and 177 do not mention using the throttle to prime at all, though give instruction to use the primer.

It is possible that a Lycoming engine could be installed in an approved airplane type, for which there were no dedicated primer system, and then, yes, throttle priming would be better than ruining a starter motor trying to start an unprimed engine, but the fire risk would still be there. That'd probably be why every Lycoming carburetted engine I've ever flown has a dedicated priming system.

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Old 1st Dec 2021, 23:38
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Originally Posted by Pilot DAR View Post
And, it's worth noting that where there is a conflict between the engine manufacturer's published information, and the airframe manufacturer's flight manual, the flight manual shall prevail.
I suggest you read the 1975 Piper PA-28-180 starting procedure. It is total garbage. It would have me crank for up to 10 seconds with no prime before attempting to use the primer. I can let my airplane sit a month and start it in less than 2 seconds with one or two cycles of the throttle lever. I've been starting it this way for 20 years. I know the airplane and I know what works.

There is zero risk of an induction fire if the throttle is pumped one to three times while cranking. All the fuel is ingested by the engine. In contrast, excessive prime with the engine not turning will pool fuel in the induction system.

The local flight school probably instructs the student to use the Piper procedure. The continuous grinding of starters is painful to hear. Some people never develop any feel for engines. They blindly follow a procedure with no understanding of what is happening or why it doesn't work.

As I said earlier, don't knock the technique if you have not used it. Well, have you used it?

(This thread is starting to look like a re-run of a recent SuperCub.org thread)
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Old 1st Dec 2021, 23:59
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Well, have you used it?
Yes, I have, in three of the four planes I have owned, and others I have flown. It works. But, with a reduction in effectiveness and safety. My present carburetted Lycoming 360 starts best when cranked for 5 to 10 seconds, and then a primer prime introduced. I have perfected this technique over the 550 hours I have flown this plane since 2008, as there is no stated factory procedure for starting it (that was actually mine to develop for it, but the project was not finished to STC approval, so no flight manual was ever developed for it). I have a particular interest in consistent good starts, as some of them are floating. I have certainly flooded it with the primer, and tried pumping the throttle during other experimentation. Using the primer sparingly after the engine is already cranking works best. Certainly, running the battery flat, or ruining a starter motor is to be avoided, as hand propping this plane would be impossible, and help may be a long way away at some destinations.

I certainly agree that it's possible for a pilot to get to know a plane so well, that even a non approved technique results in consistent success. I used to have the 310R I flew down pat. Anyone reading this, who knows their plane that well, knows this. But, to suggest that a student pilot should start developing their own starting techniques invites carb fires, or overheated starter motors.
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Old 2nd Dec 2021, 00:44
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Originally Posted by Pilot DAR View Post
My present carburetted Lycoming 360 starts best when cranked for 5 to 10 seconds, and then a primer prime introduced.
You use the starter more in one start than I would in 5 starts and it seems that's for the same or similar engine. How can you assert that a technique that requires about 10 seconds of cranking is more effective that a technique that requires about 2 seconds of cranking? You offer no justification for the claim of an increase in safety and I believe that claim is nonsense.

13 hours short of 1,000 hours in my PA-28-180 and I consider my starting technique to be well proven. I would only consider using the primer if operating in much colder conditions than I normally operate in.

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Old 2nd Dec 2021, 03:40
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How can you assert that a technique that requires about 10 seconds of cranking is more effective that a technique that requires about 2 seconds of cranking?
The starter has a 30 second duty cycle. I'm within that - as long as I don't flood it. A ten second crank, with priming as I have established gives me reliable starts. A reliable first start prevents the need for a second attempt, and greater risk of something going wrong with that start. Were I to have a carb fire in my 360, I would have no way whatever of knowing that, so preventative technique is the best for me. I had a carb fire on my other plane last year, having flown it more than 3000 hours over 34 years (so I kinda knew it). It was a surprise to me, I thought I had that plane figured. It was an expensive fix. If I run the battery down with failed start attempts, I have a bigger problem to solve. So, I've learned to be patient, start the way it works, even if it take a few second longer, get a predictable first start, and go flying. Typical ambient flying temperatures for me range from 90F to -15F, so varied preheating and priming techniques are appropriate. One technique will not cover that range. The safest start is the start which worked smoothly the first time, even if it required a little more patience.

Certainly starting my O-200 was a very different priming technique from my O-360. The O-200 was generally purring after a 2 - 5 second crank. No so the O-360 as installed in my plane, believe me, I've tried!

You do what works for you, it's your plane. As a person who writes flight manual supplements and issues STC's on GA planes for a living, I respect the techniques provided in existing approved flight manuals and POH's, as they are a product of testing and approval. They are not always the only way, they are the way preferred by the manufacturer of the plane. I can hardly advocate to deviate from the type of documents I create and approve, using methods I have tested myself, and understand. But yes, I have tried other ways....
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Old 2nd Dec 2021, 16:04
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Originally Posted by EXDAC View Post
You use the starter more in one start than I would in 5 starts and it seems that's for the same or similar engine. How can you assert that a technique that requires about 10 seconds of cranking is more effective that a technique that requires about 2 seconds of cranking? You offer no justification for the claim of an increase in safety and I believe that claim is nonsense.

13 hours short of 1,000 hours in my PA-28-180 and I consider my starting technique to be well proven. I would only consider using the primer if operating in much colder conditions than I normally operate in.
EXDAC the increase in safety comes from a reduced risk of fire as already pointed out. No one collects stats on carb fires but the engine and aircraft manufacturers have knowledge of them and a few people here discussing this have knowledge of them. Personally with your aircraft I would just use the primer (that's why it's called a primer) but pumping the throttle will work just as well or better under your conditions with little risk. No one is telling you that your technique is not acceptable for starting your aircraft in Arizona.

You on the other hand are suggesting that PilotDAR's technique for starting an engine consistently in a range of temperatures from Arizona like down to double digits with a minus in front of them while drifting downriver are not as good as your technique.

I have found in aviation that the best results come from knowledge of both the system and the manufacturer's procedures coupled with experience. The problem with pilots finding their own homemade procedures that work under certain conditions is that they do not know the risks of these procedures under different conditions. It's a common cause of accidents.

My Lycoming IO-540 manual has a starting technique that works consistently whether the engine is warm or cold in a wide range of temperatures. Having seen many pilots following their own perfected technique fail to get that engine started in less than perfect conditions I follow this technique with the exception of cold starts on normal temperature days. I have a slightly different technique that works well under those conditions and start the engine faster. 95% of my starts are done this way but I don't go running around telling everyone that I'm smarter than Lycoming and my technique works better. When my partner started flying the aircraft, I taught him the Lycoming technique first and then once he had experience with the engine, taught him the modified technique.
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Old 2nd Dec 2021, 16:14
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The way that always worked for a cold start for my 0- 320 in a Super Cub was three strokes of primer, pull through eight blades by hand, one or two more primes depending on how cold it was, lock the primer, pump the throttle once, set one finger width of throttle and turn the key. (Yes, it had a key start) Keep it running with delicate use of throttle.
I was at one point complaining of poor starting when I discovered the coil for the shower of sparks magnetos was dead. A chuffing miracle that it started at all, really.
There was nothing in the airframe manual at all about starting procedures. (Or about anything much else, really)
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Old 3rd Dec 2021, 12:47
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Nice thread drift. Never had any issues starting a Warrior (O-320) the standard way, from memory it was several primes if cold, mixture full rich, throttle at the 1500 rpm position, crank and when it kicks over retard throttle to idle rpm. A Hughes 300 IO-360 was another beast altogether, open throttle a tiny bit, prime if cold, fuel pump on, mixture full rich for 3 seconds then mixture full cut-off, fuel pump off, crank, when engine fires quickly move the mixture to full rich and set throttle at idle rpm. If you did it right it would start on the first crank, if you did it wrong you'd be cranking for ages with rest gaps in-between screaming "start ya' [email protected]'.

Re carburettor icing, I had a situation once when practicing engine failures under instruction. The carby heat was confirmed working during run up and was applied during the engine failure simulation, however the simulation probably went longer than usual as a better field was selected. Upon reaching the safe lower altitude and applying full throttle the engine started shaking the plane quite vigorously with only just a tad over 1000 rpm indicated at full throttle. Still, you could climb a Warrior with that limited power, a little flap and pointing into wind, and we managed to gain enough altitude for a straight in approach (rather glide from the reporting point). The reason for the carby icing was not doing any engine revs during the prolonged simulated engine failure to bring back some heat into the exhaust manifolds that feed the carby heat unit.
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