Fuel Primer Installation for O-320

[Saville]

Originally Posted by jbDC9

As Raymo stated above, pumping, waiting, then cranking is not a great idea due to the fire hazard; a perhaps better technique is to pump the throttle while cranking as the atomized fuel is sucked into the engine as opposed to pooling in the airbox.

A few years ago I watched a hangar neighbor start a cowl fire in a carbed 150hp Cardinal using the pump/wait technique. Oops!

I take this warning seriously so I've been trying to do this with my carbed O-360 A1A.

I start the engine and then pump the throttle at about the same speed as I do when I used to pump first and then crank.

It NEVER starts. Good thing I have a strong battery.

I don't know what I'm doing wrong:

I never bring the throttle all the way back to closed - but to the cracked position

I pump about once a second.

Yet if I then pump the throttle without cranking 2-3 times it starts right up.

If there's a bit of technique here I'd love to learn it - I'd much rather pump while cranking.

Thanks

[Bob Martin]

Originally Posted by Saville

I take this warning seriously so I've been trying to do this with my carbed O-360 A1A.

I start the engine and then pump the throttle at about the same speed as I do when I used to pump first and then crank.

It NEVER starts. Good thing I have a strong battery.

I don't know what I'm doing wrong:

I never bring the throttle all the way back to closed - but to the cracked position

I pump about once a second.

Yet if I then pump the throttle without cranking 2-3 times it starts right up.

If there's a bit of technique here I'd love to learn it - I'd much rather pump while cranking.

Thanks

No "technique".......but some thoughts.
Does your A1A setup use a standard carb?
If it does, is the accelorator pump working? This is easily checked by removing the airbox and looking up into the throat of the carb while someone is stroking the throttle , you will see a strong squirt of fuel shooting up the throat toward the engine, then it will fall back in your face! So be careful.
FYI the accelorator pump is there to help supply the engines fuel needs when rapid advancement of the throttle is done.
Understanding how this works will help you understand just what you are doing when your use the accelorator pump for a make-shift primer.

[JimWoo50]

My carbed 0-320 without primer would never start on first attempt of the day
even when I added a pmag, and even then after only what I felt was prolonged cranking. I tried pumping throttle dUring cranking and it got a little better but sometimes the engine would roar to life with way too many rpm's.
I considered adding a primer but first I tried before preflight turning on the electric fuel pump, waiting for fuel pressure and pumping the throttle once or twice. Then I turn everything off, do my preflight, 10 min or so, get in crack the throttle and she starts right up and I usually have to add throttle to keep it running. No more high rpm starts and blasting everything on the ramp.
My theory and wishful thinking perhaps is that I am minimizing the fire risk from pumping the throttle just before ignition by allowing the fuel in the carb to vaporize while I preflight. I think the important thing is to keep the fuel pumped in to a minimum, 2 full pumps seem to be working well along with preheat in temps below 40f.

[snopercod]

Originally Posted by JimWoo50

I think the important thing is to keep the fuel pumped in to a minimum, 2 full pumps seem to be working well...

Many years ago I did a test to see how many pumps it would take before fuel ran out of the carburetor into the airbox. It took six pumps. I only had a primer line to cylinder #1, so clearly your results might be different. It would be best to run this test for yourself to be sure.

[JimWoo50]

Thanks for the suggestion.

[Darin Watson]

Originally Posted by Dbro172

You don't need a primer and could consider removing it. Especially with impulse mags or dual impulse mags. My -9A O-320 has no primer and haven't had any issues starting it down to 0 degrees. Same with my 0-360 Mooney. Just a few pumps of the throttle, let it vaporize and crank. 2 pumps, with 4-5 seconds in between on the RV is usually sufficient. Now with a pmag one pump would probably suffice. The mooney needed 3-5 pumps. My 182 with an O-470 continental had a primer and it was definitely needed to start that beast. I'd bet that most carb RV's dont have a primer.

Running an O-320 with two impuls mags an have not had the engine go more than a cylinder or two to start. Even hand propped a couple time when in the NWT and Yukon when my battery was going.

[lr172]

Originally Posted by Saville

Yet if I then pump the throttle without cranking 2-3 times it starts right up.


Thanks

Nobody is saying it is an ineffective technique. Your procedure dumps a bunch of liquid fuel into the air cleaner assy. Evaporation starts quickly, creating a good ratio of air / fuel that you suck up when you crank and it gets the combustion going (dumping fuel in the intake with the primer does the same thing). The problem is that almost all of the liquid fuel remains in the air cleaner assy and the vacuum is not strong enough to suck up the liquid). If you have a backfire, you now have a good solid source of fuel to ignite and burn for some time. Possibly you have never seen a backfire during a botched start sequence. It produces a large and impressive plumb of flames.

When I had a carb, the second I hit the starter I started pumping rapidly. It usually took three full strokes. Then quickly pull the throttle back to 1/4" and give it a chance to catch. If it doesn't, give it another shot or two. I could get three strokes and start well within 10 seconds. If your starter can't do that, it's time to look at replacing your battery.

The reason that the primer is easier is that you can shoot in the fuel (without backfire risk) and let it evaporate, just like your current technique. This is a time tested mehod that works well, you just need a way to do it without the backfire risk. When I used my primer, I always waited 15 seconds before starting to let some fuel evaporate.

Larry
Larry

[lr172]

Originally Posted by snopercod

Many years ago I did a test to see how many pumps it would take before fuel ran out of the carburetor into the airbox. It took six pumps. I only had a primer line to cylinder #1, so clearly your results might be different. It would be best to run this test for yourself to be sure.

The previous poster is talking about pumping the throttle in an updraft carb. In that case, every drop ejected from the accel pump goes to the air box. In your case, you're using a proper primer circuit and the fuel is going straight to intake chamber of the cylinder head, so 6 pumps sounds about right before seeing fuel in the air box.

Larry

[Saville]

Originally Posted by lr172

Nobody is saying it is an ineffective technique. Your procedure dumps a bunch of liquid fuel into the air cleaner assy. Evaporation starts quickly, creating a good ratio of air / fuel that you suck up when you crank and it gets the combustion going (dumping fuel in the intake with the primer does the same thing). The problem is that almost all of the liquid fuel remains in the air cleaner assy and the vacuum is not strong enough to suck up the liquid). If you have a backfire, you now have a good solid source of fuel to ignite and burn for some time. Possibly you have never seen a backfire during a botched start sequence. It produces a large and impressive plumb of flames.

When I had a carb, the second I hit the starter I started pumping rapidly. It usually took three full strokes. Then quickly pull the throttle back to 1/4" and give it a chance to catch. If it doesn't, give it another shot or two. I could get three strokes and start well within 10 seconds. If your starter can't do that, it's time to look at replacing your battery.

The reason that the primer is easier is that you can shoot in the fuel (without backfire risk) and let it evaporate, just like your current technique. This is a time tested mehod that works well, you just need a way to do it without the backfire risk. When I used my primer, I always waited 15 seconds before starting to let some fuel evaporate.

Larry
Larry

Hi Larry,

No need for the full repetition of the reasons why you don't want to pump without cranking...I understood all of that.

My problem is that it's not starting when I pump while cranking and I need to understand why.

I have a great battery and it can crank all day.

Is it possible it's being flooded? You mention "...the second I hit the starter I started pumping rapidly"...Can you give me an idea how fast? As fast as you could?

[snopercod]

Originally Posted by Saville

...it's not starting when I pump while cranking and I need to understand why.

Your accelerator pump linkage has three settings: High, Medium, and Low. Which one is your set on?