A few things you should know:
The thing about turning the engine backward is based upon the mag impulse couplings not engaging, and making a spark, so the engine should not accidentally start. Instead of a weak attempt at safety, simply never turn the prop by hand, ever, at all, unless the plane is secured from moving, and you are prepared for the engine to fire and run as a result of your turning the prop – it’s simple. If you don’t want the engine to run, don’t turn the prop.
The vanes in a vacuum pump are most commonly set at an angle, such that turning the engine backward greatly increases the risk of breaking one off inside the pump, or shearing the sacrificial driveshaft. It’s up to you, you won’t hurt the engine, but you’re risking an expensive vacuum pump, is it worth it?
A fuel pump in a carburetted fuel system will do nothing other than provide fuel to the carb, you cannot prime with it as you might with an injected engine.
Yes, most O-200’s will have an MA-3 carb with an accelerator pump. Yes, pumping the throttle will pump fuel into the throat of the carb. However, doing so is not primping the engine, it’s dumping a poorly controlled volume of not atomized fuel into the carb, where it would be delighted to catch fire. When it does, there’s a chance that you’ll get that fire extinguished, and perhaps not even know that you had the fire. BUT, that carb fire may have burned the delicate venturi in the carb, which is held in with three little steel clips. When the venturi is burned, and mis-shapened, it will cease to be the aerodynamic feature in the carb which makes a carb work properly. So, the carb won’t work properly, and will be expensive to repair. In case people are unaware, there is an AD for MA-3 carbs to inspect annually for venturi damage. I have found it during inspections. In contrast, a pump type primer produces an atomized mist of fuel in the induction system, either up the induction spider, or at the induction ports in each cylinder, depending upon primer nozzle location. That atomized mist of fuel is much more eager to burn in the cylinders, than the pool of fuel now dribbling through the carb into the airbox. So, use a primer to prime, and leave the accelerator pump on the carb to do what it should do in the background, rather than misusing it.
That said, as a pre start check on O-200’s I don’t know, I always slowly move the throttle to fully open, than closed once, but not associated with priming. This is because some older MA-3 carbs have the throttle arm connection to the throttle shaft of the carb which is a friction type, rather than a positive mechanical engagement. If the previous flight landed with carb ice, and closed the throttle that way with this type of MA-3 carb, the throttle plate could have become repositioned. (The story of how I know is this is sad and long, but I know it!). So if you have normal and full throttle control motion, you’re safe to fly, if the throttle seems to have an unusually short travel, there is a problem. Once you know the plane, and throttle arm set up is positively pinned, this check is no longer required.
If you need to cold start any engine, warm it up first. Then use the primer ( two pumps for an O-200). Have a third pump ready to pump in as the engine begins to run. If there is no primer system, and you are confident with hand propping safety, mixture rich, throttle closed, and pull the engine through forward a dozen blades mags off. Then start it normally. If it won’t start normally, repeat. Then, have a proper primer installed.
Always treat the prop like it’s going to start. Many years back, I was pulling through my buddy’s 150M. I hollered “Mags off?”, “Mags off” was the reply. I pulled the first blade, and it started, and ran nicely. I was quite alarmed. I looked at my buddy through the running prop, and he, understanding my upset with the situation, held up the keys for me to see – through the running prop. Maintenance required. If he’d been doing live mag checks (at idle speed only) prior to shut down, he would have known that plane was defective, and dangerous.
If a warmed up O-200 is not starting well, don’t fight it into submission, have it maintained. Check the carb for proper operation, and the mag timing. A well maintained O-200 is a very easy engine to start.
And, on the topic of starting O-200’s, if yours has a key start clutch (not pull start, not a new direct drive geared starter motor), at the very first slip of the starter, do not fly, have the starter maintained. You have not damaged the $1000 starter clutch by slipping it once, but you will if you crank it slipping until it finally surrenders, welds itself shut inside, and starts the engine.
Yes, I was referring to hand propping with no primer nor carburator pump, the first bit is just priming with mags OFF
Yes, as you described or view it anytime the prop speed is high the trottle should be closed then reopened as prop came to rest, that creates enough depression to pull fuel inside the engine…
You can put fuel pump ON just as pressure rise but not during priming or start, it does no good and may cause flooding quickly, that “extra fluid pressure” will not bypass the carburator as with no primer you need a sort of “air vacum” or low Manifold Prsure from the throttle to get fuel from carburator to the cyclindres
Ideally the throttle is opened/closed from fully closed to just to few mm open as you move from low MP to max MP with a slight forward mouvement, how far/quick/many? Depends how quickly you hand rotate that prop and if close throttle means sealed ;)
You can also just leave the throttle 1mm open or even closed (if it does not seal properly) then just do quick prop spins and you are left with guessing the magic number of prop spins you need for your aircraft
This does not work on the aircraft I fly (you are never sure how the previous guy shut it down?) but my rule of thumb, the quickest I move the throttle the few pumps I need and I should not need more than 9 (I start to see liquid fuel comming from the exhausts even with slow prop spins and slow throttle moves), also it is always better to underprime and leave the engine/carburator to do the rest rather than just replicating the cycle slowly !
Thanks, that makes sense.
And while hand priming, the electric fuel pump stays off (but is turned on for actual starting with electric starter)?
And if it doesn’t start with electric starter, would you repeat the hand priming yet another time, or keep going with pumping throttle?
Zorg wrote:
If I understand correctly, you describe turning the prop by hand, and when you come to a mag click (compression stroke), the guy in the cockpit opens the throttle. You pull the prop through the click, and the guy in the cockpit closes the throttle again?
That isn’t necessary. The engine is typically pulled through multiple cycles to prime, with the throttle fully closed. It can then be started, either with the electric starter or by hand propping on suitable engines. After priming, starting is best done with a small throttle opening to allow sufficient air intake.
Zorg wrote:
Why can’t you leave the throttle (partially) open during the cranking?
The throttle is closed during priming because doing so generates vacuum over the carb venturi and that pulls fuel into the inlet tract. If the carb has an accelerator pump, as the throttle is being opened the carb squirts fuel into the inlet tract too. Regardless of what method you’ve used to introduce fuel locally into the inlet track, the task is to distribute air through the cylinders, with that air having picked up evaporated fuel on its journey.
Ibra wrote:
To hand prime: close trottle, full mixture, open trottle each time the prop cranks a positive turn and the mags clicks, stop when you smell unburn fuel in the exhausts or at your lucky number, mine is 6
Are you talking about full-on hand propping here, or merely priming the engine?
If I understand correctly, you describe turning the prop by hand, and when you come to a mag click (compression stroke), the guy in the cockpit opens the throttle. You pull the prop through the click, and the guy in the cockpit closes the throttle again? Mags and electric fuel pump are off during this procedure? Why can’t you leave the throttle (partially) open during the cranking?
Silvaire wrote:
For the Continental O-200: pull through 3-4 blades (approximately) with fuel on, mags checked off and throttle fully closed. Then crack throttle 3 mm and start engine.
For clarification, that means manually turning the prop about 1.5-2 full revolutions in its regular direction? Do I understand correctly that the intention is to circulate the oil and get fuel in the cylinders via the mechanic fuel pump?
And by “fuel on” you mean “electric fuel pump on” or just “fuel selector on”? (Sorry if this should be obvious.)
Peter wrote:
Different vacuum pumps have different limitations. Some I have had are OK to run in reverse.
That’s correct, however how many pilots (even owners) know if the one they have is good to run in reverse? It’s simply a risk not worth running.
For the Continental O-200: pull through 3-4 blades (approximately) with fuel on, mags checked off and throttle fully closed. Then crack throttle 3 mm and start engine.
Later Continental carbs have an accelerator pump like a Lycoming which can prime the engine, earlier carbs do not. Try the method I’ve described and see how it works for you. If you end up needing additional prime from the accelerator pump (when applicable) make sure you still pull the engine through before starting. The helps the engine to start quicker and reduces the chance of backfire.
Would it make sense to rotate against rather than in regular rotation if plane doesn’t have vacuum pump?
The extent to which this is an issue is system dependent. Different vacuum pumps have different limitations. Some I have had are OK to run in reverse. I have also never heard of a pump ruined just by hand turning of the engine.
I don’t have a primer. Two pumps of the throttle is enough from cold for my Lycoming O-320-B3B – you might want to experiment by reducing the amount of throttle pumping you do.
