Piper Dakota needs 8 primes to start?

[Trogdor]

Plane is a 1977 Dakota PA28-236 and in order to start the O540 in it, the plane needs like 8 primes to get it going. I find this very strange. Yes, I wait for the bowl to fill and yes, it will start with three chugs of the manifold while cranking (and yes, I don’t want to start a fire).

Anyone know why a Dakota would need so many primes? I’ve asked the A&P to inspect the O-rings on the primer itself thinking that maybe I’m not getting a full spray when the bowl fills. But they say it looks fine. POH says 3 primes (and only when it doesn’t start) to get this baby going.

Any clue?

 

[bluerooster]

Check the nozzles in the heads. They can get stopped up.

 

[Magman]

Per Blue , you may have some nozzles coked up and supplying no fuel.

Others may be clear and supply all the priming fuel.

So some jugs get insufficient fuel and others get too much.

Removing nozzles and reconnecting outside of cylinder allows

you to see how well they work.

A broken / cracked primer line will drastically reduce the amount going to

all cylinders.

 

[MajorTurbulence]

Not addressing your problem, but commenting with regard to the POH indicating 3 shots of prime similarly on my Archer2. The reality for me is that that first pull on the prime has, what feels like, nothing but air. Those following pulls, when waiting for fill up, do indeed feel full of fuel. So when counting I use 4 total pulls as a result.

 

[Domenick]

Here was the state of my O-320-D3G priming nozzles when the engine was replaced at 2925 hours.

 

[Magman]

Hoppe’s #9 May clean those up.

btw- Most pilots have no idea of how many cylinders are being primed

or where the nozzles are located. It makes a difference at times.

 

[brcase]

Does it have the correct primer installed? Most pipers usually 140/150/160 and few 180's I have flown as I recall had a short primer pump (compared to most cessna's) as a result they usually require about twice as many pumps as a Cessna. I could easily see with the O-540, that a long pump might have been in order, otherwise 8 pumps sound pretty probable as normal to me.

Sorry I don't remember how many pumps the Cherokee 235 I was teaching is for a while used, but then it was the student/owner doing the priming most of the time.

brian

 

[Hang 4]

Does the primer feel stiff/tight when you use it? That can be bad o-rings or clogged primer nozzles. O-ring kits available from Spruce.

 

[Trogdor]

Funny you mention that @brcase. It does have a short primer pump and I am now wondering if it is the stock one or has been replaced.

Note: I'm 99% sure we had the o-rings checked and the nozzles cleaned but I'll ask.

 

[kayakgeek]

When cold, it takes 7 shots of primer for a reliable first crank start on my 79 Dakota.

 

[Trogdor]

When cold, it takes 7 shots of primer for a reliable first crank start on my 79 Dakota.

I guess that is par for the course. Really strange given the POH.

 

[kayakgeek]

I guess that is par for the course. Really strange.

It has been consistent through the 12 years I have owned the plane. It was initially trying 3 or 4 and failing to start, so I spoke to the A&P who had been caring for the plane for the prior owner. He said it needed about 7.

 

[Dan Thomas]

There are numerous things that go wrong with old primer systems. Magman already addressed the most common: clogged primer nozzles. These little things have tiny passages inside them that direct the fuel at a tangent to the discharge hole so as to spin the fuel and make it spray in a fine cone, similar to some simple lawn sprinklers. Those passages coke up with baked-in avgas, and if the nozzle works at all, it just dribbles or has a needle-like stream. Neither of those atomize and evaporate the fuel well and it ends up just running down the induction plumbing instead of getting into the cylinder where it can do some good. I have tried cleaning them. I even made a device to force carbon stripper through them under pressure. Nothing worked. They are not a straight-through device like an injector nozzle and won't clean up with Hoppes #9. The best and actually cheapest, time-wise remedy is to install new nozzles. Makes a huge difference. AN4022-1.

Worn or aged primer O-rings can seal under pressure, pushing out the fuel, but can retract and suck air into the primer on the intake pull. You don't move much fuel that way.

Debris under the tiny check balls in the primer will let the fuel sneak past the intake check and just go back to the fuel strainer. Under the outlet check, it lets the primer suck the fuel out of the primer lines and nozzles. Everything can feel good, but it ain't.

And some folks lubricate those primer O-rings. That makes the primer gum up and sticky and can tear up the o-rings. Getting that gum out of the primer barrel can be tough. If someone used Fuel Lube (Ez-Turn) on them, the only thing that will clean it up is acetone. Gasoline is the only lube needed in there. If it's sticky and gummy, most folks will add more lube and it just gets worse.

 

[Trogdor]

It has been consistent through the 12 years I have owned the plane. It was initially trying 3 or 4 and failing to start, so I spoke to the A&P who had been caring for the plane for the prior owner. He said it needed about 7.

Have you ever replaced the nozzle?