Only prime while cranking...?

I have the accelerator pump and left off the electric primer and plumbing at overhaul. I too have been doing it wrong by pumping once then cranking. Always starts when flown somewhat regularly. But twice last winter, after weeks between flights and in 45 degree days, it took for freaking ever to fire. Both times had to get a charging starting booster as the poor battery couldn't last long enough. Tried one pump, nothing. Two pumps, nothing, three.....Nothing.
Any suggestions on my error?
Thanks!
 
AKBill said:
How common is it that the orifice for the primer gets plugged or fouled?
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Very common. The fuel gets baked in there and the residue plugs it up. It's worse on the engines that have the nozzle in the cylinder heads.
 
My O320 is hard to start in the cold weather. I normally start to preheat when the temps are in the mid 30's. I have a high torque starter (Sky Tec) and love it. I also have a Concord battery seems to be much stronger than the Gill I used for 2 decades. I'm sure I will get bashed from folks here but here is what I do. I never prime or use accelerator pump unless engine is cranking. 2 shots of primer with engine cranking them about 3 to 4 pumps of throttle, engine still cranking throttle back to 1/4 open.
 
AKBill said:
My O320 is hard to start in the cold weather. I normally start to preheat when the temps are in the mid 30's. I have a high torque starter (Sky Tec) and love it. I also have a Concord battery seems to be much stronger than the Gill I used for 2 decades. I'm sure I will get bashed from folks here but here is what I do. I never prime or use accelerator pump unless engine is cranking. 2 shots of primer with engine cranking them about 3 to 4 pumps of throttle, engine still cranking throttle back to 1/4 open.
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I have the same issue (but I'm sure your definition of "cold" is different than mine!) I'm going to try your technique this winter.
 
Goofy said:
I have the accelerator pump and left off the electric primer and plumbing at overhaul. I too have been doing it wrong by pumping once then cranking. Always starts when flown somewhat regularly. But twice last winter, after weeks between flights and in 45 degree days, it took for freaking ever to fire. Both times had to get a charging starting booster as the poor battery couldn't last long enough. Tried one pump, nothing. Two pumps, nothing, three.....Nothing.
Any suggestions on my error?
Thanks!
Click to expand...

I have an O-320 in my RV. I always use the electric fuel pump to get positive pressure to help ensure the primer has fuel to pick up easily. Without it, you are depending solely on residual fuel in the carburetor bowl from last flight for the accelerator pump to pick up. If it has set for a long time, that fuel level may be quite low to gone.
 
Here is the Lycoming starting procedures, they don't say to crank during priming. I think it is a good practice to crank while priming for reasons others have mentioned.
 

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One trick I was recently taught, if a particular airplane doesn't start easily, is to prime and then wait a minute before engaging the starter, to give the fuel a chance to vaporize. It seems to work well.
 
N3368K said:
[snip]

The R-985 on the BT-13 needed lots of pumping to get it going.
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Boy that made me cringe. I fly a BT-13 and it is specifically placarded on the panel (and on the original check list) "CAUTION: DO NOT PUMP THROTTLE". It doesn't say why, but I've since found out it is a spectacular way to get a stack fire.

If it sort-of starts but seems like it is dying I was instructed to leave the throttle barely cracked and use the primer to keep it alive until the other cylinders wake up. It works well.

Other than that, I'm in full agreement: there are lots of different techniques for various equipment.
 
Palmpilot said:
One trick I was recently taught, if a particular airplane doesn't start easily, is to prime and then wait a minute before engaging the starter, to give the fuel a chance to vaporize. It seems to work well.
Click to expand...

That delay is almost mandatory in freezing weather when vaporization is slow...
 
aftCG said:
Boy that made me cringe. I fly a BT-13 and it is specifically placarded on the panel (and on the original check list) "CAUTION: DO NOT PUMP THROTTLE". It doesn't say why, but I've since found out it is a spectacular way to get a stack fire.
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Yeah, but what kind of primer does it have? Is it a primer switch, or plunger type?

I doubt he is referring to pumping the throttle.
 
Skip Miller said:
That delay is almost mandatory in freezing weather when vaporization is slow...
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That delay is what often causes engine fires. The priming fuel hits the walls of the induction system and runs down into the carb and airbox and cowling, where a lean backfire ignites it. That's what priming while cranking, or priming and cranking immediately, is intended to prevent.

Sucking the fuel mist into the cylinder will often vaporize it by the heat of compression. Or the engine should be preheated.
 
aftCG said:
Boy that made me cringe. I fly a BT-13 and it is specifically placarded on the panel (and on the original check list) "CAUTION: DO NOT PUMP THROTTLE". It doesn't say why, but I've since found out it is a spectacular way to get a stack fire.

If it sort-of starts but seems like it is dying I was instructed to leave the throttle barely cracked and use the primer to keep it alive until the other cylinders wake up. It works well.

Other than that, I'm in full agreement: there are lots of different techniques for various equipment.
Click to expand...

I don't doubt what you said. There just was no placard you mention in the one I flew and no original USAAF checklist. I don't recall there was a checklist at all for it. Maybe I was lucky, but I never had an issue with a fire in it. This was in the mid-80s, before where finding about anything is a couple of minutes with Google. The old plane's engine was pretty tired and was hard to start, even more so if it was cold. Regular primer was good before trying to start, but when turning and it hit and sputtered, pumping the throttle a couple of stroke worked well to get it to fire up. You certainly didn't want to pump it without the engine turning over.

The method I was using was how I was shown by two WWII veteran/USAF retired guys. One was owner (retired O-6), the other the OPS Officer (O-5) who performed check-outs for museum pilots. Both had flown BT-13s in training. Don't know what they were taught, but that is what they taught me. It went pretty much what I had learned worked in Stearmans from my dad. He had several thousand hours dusting in them in the 40s-50s. He was also a WWII pilot (BT-13s in Basic) and 50s USAF IP in T-6Gs. Many of the guys I flew with were WWII pilots and pumping the throttle on most the trainers was common practice. Of course, it wasn't needed on planes with boost pump/primers like the B-25.
 
N3368K said:
I don't doubt what you said. There just was no placard you mention in the one I flew and no original USAAF checklist. I don't recall there was a checklist at all for it. Maybe I was lucky, but I never had an issue with a fire in it. This was in the mid-80s, before where finding about anything is a couple of minutes with Google. The old plane's engine was pretty tired and was hard to start, even more so if it was cold. Regular primer was good before trying to start, but when turning and it hit and sputtered, pumping the throttle a couple of stroke worked well to get it to fire up. You certainly didn't want to pump it without the engine turning over.

The method I was using was how I was shown by two WWII veteran/USAF retired guys. One was owner (retired O-6), the other the OPS Officer (O-5) who performed check-outs for museum pilots. Both had flown BT-13s in training. Don't know what they were taught, but that is what they taught me. It went pretty much what I had learned worked in Stearmans from my dad. He had several thousand hours dusting in them in the 40s-50s. He was also a WWII pilot (BT-13s in Basic) and 50s USAF IP in T-6Gs. Many of the guys I flew with were WWII pilots and pumping the throttle on most the trainers was common practice. Of course, it wasn't needed on planes with boost pump/primers like the B-25.
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Great response. I didn't mean to sound like an authority on the subject, as I'm still pretty new to the plane. Any time I see an uncowled R985 I examine it closely. It's a pretty cool piece of machinery.

To answer the other poster, on the BT there is manual wobble pump to supply initial fuel pressure (no electric boost of any kind). While wobbling to keep 3-4 psi you use the fat plunger type primer. It's like patting your head and rubbing your belly.

In my case I was getting some loud cough/backfire on a cold start. I wasn't really even pumping, all I was doing was milking/feathering the throttle like you do an old car just to keep it alive until it ran on all cylinders.

My uncle was standing next to the plane and kept gesturing to use the primer and not move the throttle. I had never heard of that technique so we had to talk about it later. He said he's had to put out fires before from people who pump the throttle.

I've tried the primer when it was stumbling after a cold start and it does seem to work without the drama.

I have to agree with another post that blames flight instructors for passing on bad information, often unknowingly, because the technique they learned for one engine type doesn't apply to every machine.
 
I was 23-24 yo when I started flying with a museum in Oklahoma. I'd only flown a few radials; W-670 220hp Continental on Stearman, R-680 300hp Lycoming on Stinson and R-755 Jacobs on Cessna T-50.

My checkouts were pretty basic. Couple of flights in backseat with Ops Officer and then once around the pattern and I was "Approved". I only flew the BT for one season and a handful of times afterward. I think I flew it about 40 hours total, so not exactly an expert myself...it was also over 30 years ago. I got moved up to copilot of a B-25 the following season.

I did fly a Beech 18 a little bit a couple of years later and was hired to run-up radial engined DEA seized a/c each month. It included a couple of Beech 18, a C-117 Super Gooney and two DC-6s. The R-2800 on those was interesting. Wish I could have flown one!


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I would trade one of my logbooks for that B-25 time. Awesome
 
It sounds to me like the CFIs are passing on incomplete information (probably in ignorance). There are many different configurations of carburetor, accelerator pump (or not) and primer type and placement. Not to mention fuel injected. You need to know what technique to use with the aircraft you are trying to start.
 
Dan Thomas said:
Tom, my 152 service manual says that the accelerator pump was only added halfway through the production run (from s/n 15283592 and on, in a run from s/n's 15279406 to 15286033). in 1980 they added it. SO you're half right.

But on the subject of nozzles? You've never taken one apart. Take an old one, grind off the rolled rim on the business end, take out the little brass disc with that tiny orifice in it, and see the channels milled into the body at a tangent to the orifice. They're to spin that fuel so that centrifugal force tears the stream apart into an atomized cone. Take a good nozzle and connect it to a primer line, get someone to pump the primer, and see the spray pattern. It's not a squirted stream.
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I'd wager there isn't a single one of those old carbs left in service because way back in the 60s that carb was superseded with the new ones from Marvel. Even when you order the old Part number you'll get a new MA3SPA.

The only carb that was in service in GA was the tillison. and anyone dumb enough to be still running one of those is dumb enough to deserve what they get.

OBTW I build about 3-4 of these carbs a year for folks who like to run auto fuel. I scrounge every salvage yard I can to find 1 piece ventures, and brass floats with metal float valves.
 
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AKBill said:
How common is it that the orifice for the primer gets plugged or fouled?
Click to expand...
The primer in a MA3SPA does not have an orifice, It does have a 150 mesh screen protecting the nozzle in the delivery tube.
 
Tom-D said:
The primer in a MA3SPA does not have an orifice, It does have a 150 mesh screen protecting the nozzle in the delivery tube.
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Primer in a carb? You must mean the accelerator pump nozzle, don't you?
 
Dan Thomas said:
Primer in a carb? You must mean the accelerator pump nozzle, don't you?
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The accelerator pump really doesn't have a nozzle. or a orifice. unless you call the fuel delivery tube an orifice/nozzle.
 
Tom-D said:
The accelerator pump really doesn't have a nozzle. or a orifice. unless you call the fuel delivery tube an orifice/nozzle.
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Carb primer= bad choice of words.
 
Fouled orifices aren't the usual primer failure, leaks are. There are one or two O-Rings there and these have a limited life. The better failure is they just fail to pump fuel when you push them. The worse failure is they leak fuel out or they let air into the system. This can have disastrous circumstances.
 
flyingron said:
Fouled orifices aren't the usual primer failure, leaks are. There are one or two O-Rings there and these have a limited life. The better failure is they just fail to pump fuel when you push them. The worse failure is they leak fuel out or they let air into the system. This can have disastrous circumstances.
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Fixed a leaker yesterday. First time in a long time. The usual failure is sticking O-rings. If you Google "sticking primer" you will find hundreds of complaints of it. I once measured the primer bore and piston land diameters, looked up the recommended sizes for those for the -012 O-ring, and found that the piston's land is too big. Couple that with MS29513 O-rings, which are 70 Durometer hardness, and you have friction. Lots of it. The primer relies on the fuel for lubrication, and it uses two O-rings, so the aft one--farthest from the fuel--runs dry and sticks. Some guys use Fuel Lube (EZ-Turn) to try to make it work, but that only makes it much worse in a few hours. DC-4? Same thing. A guy might be smart to use a worn O-ring on the fuel end and a new one behind it.

In the air brake remanufacturing industry we used to run into some control valves that were imprecise and jerky in their pressure control. A bunch of research and experimentation determined that it was the 70 Durometer O-rings, sticking no matter what fancy, expensive greases we used. We switched to 50 Durometer rings and the precision and controllability were amazing. Customers loved it. Can't do that in certified airplanes; got to put up with "aircraft quality" hassles.
 
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