I'm primed! and the Cessna rule.

Great week-end!!!

First Questions:
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For priming:
1. I understand when installing a primer, some folks just prime one cylinder, some two, some all four. What is the right number of Cylinders to hook up to the priming system?

2. Where do I get the nozzles for the cylinders?

Now what is going on...
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I decided to install a primer valve for my O-320. After all, Cessna did it. My builder rule #5 = "If Cessna would do it, it must be OK".

I consider this rule for all safety of flight issues, however I do NOT apply the rule in the opposite. If Cessna did not do it, that does NOT mean it is unsafe. I use other rules for these conditions.

Anyway, I have installed the primer valve and the primer switch this week-end. All looks good. Copper 1/8 line from Gascolator to primer control valve. Small short run, as shot as possible.



The primer switch is a momentary contact rocker switch right next to the ignition key. Hard to mistake as anything other than fuel. And, yes, it will be correctly labeled.

The primer is just to get the engine firing enough to draw fuel from the carb.
Most people prime 3 cylinders. The fourth is usually used for a manifold pressure gauge.

Yeah, what Mel says. And don't listen to those that say it is worthless. I still love my primer on my O320. If the engine hasn't been run, I use it. If I were to do it again (not, no more carbs for me), I would ensure a lockout on the switch though.

Story 1: about a week ago I went flying while the owner of 55BC watched me fire it up and go. After the flight he asked me how in the heck I get it started so easily. "I always prime and it seems like it cranks forever before it gets going." So I then tell him how I never use the primer and use the accelerator pump after the first blade. Been years since I used primer, in any airplane.

Story 2: 55BC had one of those blue gascolators and I just took it off the airplane, and installed inline filters in the wingroots. Why? 55BC would vapor lock on the ground on 100LL when hot. Gascolators work fine on high wing airplanes where they are full all the time but they are not good in airplanes where fuel is being drawn from the gascolator essentially by vacuum. If the fuel gets warm, vapor floats to the top of the gascolator and nothing but foam gets drawn in that condition. Since removing it there has been no more vapor locking.

Maybe you've made your mind up. That's fine. But the primer systems just add complexity and cost. I flew behind a carbureted RV for 13 years and may have used the primer once or twice before simply using the accel pump as Bob describes. After a couple of years of never using, I removed the primer system.

The only thing that might be a consideration (for the 4 cyl, metered electric pump primer system) is the event where the engine cannot get fuel from the carburetor for some reason. Then you could possibly limp to a safe field using primer to keep the engine running. But this is a whole other discussion topic.

Good luck, either way.

Some engine/carb combinations don't have accelerator pumps. So I would think that is the question that needs to be asked.

If you are equipped with a Throttle Body Injector, like an Ellison, you will have to have a priming system for cold starts. No accelerator pump.

JonJay said:

If you are equipped with a Throttle Body Injector, like an Ellison, you will have to have a priming system for cold starts. No accelerator pump.

Click to expand...

A buddy of mine put a Rotec TBI in his Onex, doesn't prime and it fires on the first blade. He found that shutting it off with the key works better than shutting down with the mixture and prime to start.

climate????

sounds like one needs to provide a mixture near ideal for ignition to occur.
If your accelerator pump on the carb is working well by throttle action....and it starts, then you've got it.
Mine seems to like the 2 cylinder prime, even when hot, but honestly I don't find pumping the throttle makes it fire, and I believe the accel pump is working, ( but haven't torn anything apart to prove it.)

...the other consideration, the electric pumped priming might be an asset when it's below freezing, ( maybe not an issue in your neighbourhood!?)

rocketbob said:

A buddy of mine put a Rotec TBI in his Onex, doesn't prime and it fires on the first blade. He found that shutting it off with the key works better than shutting down with the mixture and prime to start.

Click to expand...

Interesting. Quick search on Rotec's web site;
"Starting the engine from cold is simply a matter of depressing the diaphragm over ride button on the Rotec fuel supply regulator, this in effect gives the engine a squirt of fuel while cranking. Typical primer nozzles can also be utilized if the airframe has existing primer systems."

Like the Rotec, my Ellison is stubborn to shut down with mixture. It will absolutely not start without priming. You have to have ample air flowing through the body for it to pick up any fuel.

The O-320 on my RV-6 has no primer system, just the accel pump. When the weather is cold, (OK, California cold, like 30F...) 2-3 pumps and it normally starts by the 2nd or 3rd blade. With warm weather 1/2 pump or so is all it needs.

The above is for the first start of the day.

Clarification for those not familiar with the carb accel pump starting process. Don't do it unless you're cranking. That fuel will drain right down into the FAB or cowling, and you risk a fire.

Funny thing is I too had the same gascolator and removed it last week due to vapor lock issues , installed a nice inline micro filter , and been fine since