How much prime?

[201er]

I'm terrible at starting my '78 201. I can't tell if I'm giving too much prime or not enough. I just end up giving some prime, cranking, giving some more, etc until it fires. On one hand, not enough and it can't fire. On the other, too much and you're dealing with a lot of cranking for a flooded start. I can't seem to find the optimal prime (on a cold summer start) to get it to fire up within a couple turns.

A) How long should I prime?

How can I tell when it's the right amount?

C) Is there a better way than POH method (mixture rich, boost pump on, boost pump off, mixture off)?

I can't seem to tell if I'm over or under priming in the first place because either way it requires a lot of cranking and changing stuff. How can I improve my starting procedure?

[FlyDave]

For cold starts I prime as follows:

• 
  
• If the airplane has been in a warm environment (75+ dF) I prime for 3 seconds
  
• If the airplane has been in a cool environment (55-70 dF) I prime for 4 seconds
  
• If the airplane has been in a cold environment (less than 55 dF) I prime for 5 seconds
  

My airplane (1982 201 with IO-360-A3B6) usualy starts within 2 blades priming according to these specs.

[201er]

Thanks Dave. Can you just clarify if X number of seconds is from the moment you turn the pump on till off or if it's the amount of time since registering fuel flow in the event it takes a couple seconds for fuel flow to register?

[The-sky-captain]

I shoot for 5 seconds, from the time I turn on the boost pump, and it works great for me. Any less and it seems take a few more blades. I do have a fairly new Skytec on so I'm sure that helps as well.

[Shadrach]

Quote: 201er

C) Is there a better way than POH method (mixture rich, boost pump on, boost pump off, mixture off)?

[FlyDave]

Quote: 201er

Thanks Dave. Can you just clarify if X number of seconds is from the moment you turn the pump on till off or if it's the amount of time since registering fuel flow in the event it takes a couple seconds for fuel flow to register?

[Magnum]

With the original starter (cold engine) between 2 and 4 seconds depending on OAT, but I keep the mixture full rich. With the skytec I need 1-2 seconds longer. Timing starts when the fuel pressure goes up.

Hot start (up to 30 minutes since shut down): Don't touch anything, just crank.

If it stood longer, 1-2 seconds then mixture cut off and start. The skytec really helps with a hot engine.

[steelerp]

Hot weather...hot hanger...plane hasn't flown today.  Bump throttle to a position that will give you 1000 - 1500 rpm.  Position mixture off the stop, somewhere you'd have it while taxing.  Finger on the boost pump.  Start cranking.  Soon as a few blades go by,  turn on the boost pump and watch the fuel pressure gauge.  As soon as you have pressure....let go of the boost pump,  continue to crank.    Engine starts every time. 

Cold or hot weather...engine recently shutdown.  Touch nothing.  Hand on the mixture.  Crank engine.  As soon as the engine cough's/sputter's/attempts to start.....move mixture up...move throttle up.  Just part way.  Engine starts every time.

77 M20J - 1 flooded start in two years...

Hope this helps. 

Rick

[jetdriven]

prime volume depends on throttle lever position more than anything.  Is is idle?  1000 RPM  full? 

[201er]

I read that a good technique for having the throttle in the right position for start is by leaving it at 1000RPM prior to shut down using the mixture. I don't touch the throttle since and try to start with it there. However, it seems like it needs more throttle than that to start but then I have to quickly come back on it cause it fires up toward 1500RPM.

Today I tried starting (oat 90df) using 3 seconds of prime but it didn't and I had to add prime and try again a few times till I got it going. Hot starts after getting dinner and again after fuel were a breaze. Nothing compared to starting Arrow hot! Second hot start it was about to start but didn't so I just primed for a second and that was enough to get it going on the next one. It's the (hot oat) cold starts that I still can't seem to get right. How do I tell if it's the result of priming too much or not enough at first?

[Shadrach]

Quote: jetdriven

prime volume depends on throttle lever position more than anything.  Is is idle?  1000 RPM  full? 

[Shadrach]

Quote: 201er

 How do I tell if it's the result of priming too much or not enough at first?

[alex]

201er, check your PM for some info I sent you...

[N33GG]

OK, I will throw some more fuel on the fire... no pun intended.

I used to work at a job where I had to fly many different makes and models of aircraft every day, singles and twins, big and small, hot and cold.  I never had the benefit of owners giving me tips and tricks on how to start each plane.  Some big engines could be particularly difficult on hot starts.  An old pilot gave me the formula that I have used for over 30 years, and it has yet to fail me.  I still use this technique to this date.  Here is the story he shared:

"Georgee (he always called me Georgee - ARRGH!  On the other hand, I was pretty young and inexperienced back then, although I didn't seem to know it.), the problem with trying to prime an aircraft engine just right, is that it is impossible to know when it is just right!  (Unless you fly the same aircraft every day and learn some number of seconds that seems to work...now back to the story.)  Sometimes you get lucky, and the engine starts.  But if it doesn't, you don't really know if it is under primed, or over primed.  The only way you can be sure what you've got is to over prime the engine.  That's right, prime it good, and then prime it some more.  Then, push in the throttle to full open, and pull the mixture all the way back to fuel cutoff.  Now, with one hand ready on the throttle, start cranking the engine.  As lots of air comes into the engine without any additional fuel, the fuel/air mixture with go from too rich to more lean, until the engine gets to the exact right fuel/air mixture and begins to fire.  When it starts to fire, let off of the starter, pull back the throttle to idle, and quickly push the mixture in to full rich."

I usually try whatever technique is recommended for each aircraft I fly.  But if it doesn't start immediately, I revert to old reliable.  Many of the aircraft I have owned over the years just seemed difficult and unpredictable, and I just stuck with the above technique.  My 68C seems to work pretty much OK with the published procedure.  My J model used to receive the over prime technique on a regular basis.

I am not saying it is right, just sharing what has worked for many years on many different aircraft, including many Mooneys.

Good luck!

[201er]

Hmm, makes sense but isn't that a bit hard on the starter (particularly when you're the guy paying to get it replaced)?

[Ned Gravel]

Quote: N6719N

....The only way you can be sure what you've got is to over prime the engine.  That's right, prime it good, and then prime it some more.  Then, push in the throttle to full open, and pull the mixture all the way back to fuel cutoff.  Now, with one hand ready on the throttle, start cranking the engine.  As lots of air comes into the engine without any additional fuel, the fuel/air mixture with go from too rich to more lean, until the engine gets to the exact right fuel/air mixture and begins to fire.  When it starts to fire, let off of the starter, pull back the throttle to idle, and quickly push the mixture in to full rich....

.....

I am not saying it is right, just sharing what has worked for many years on many different aircraft, including many Mooneys.

Good luck!

[N33GG]

Yes, by design it is a flooded engine start every time.  And no, it is not hard on the starter.  With the throttle full open and the mixture cut off, it only takes a second to clear the cylinders.  Be sure you are ready to reverse the throttle and mixture settings quickly when it fires.

[jetdriven]

it also can wash oil from the cylinder wall. Then it cranks and runs, sometimes at 1700 RPM immediately, like that.   Flooding you engine every time is going to affect its life.

[Jeff_S]

I have followed the "1100 RPM at shut-down, then just crank it" approach to hot starts with mostly good success here in the South. But traveling out to Colorado last week, with temps as high as the South (90's) but the super-thin air, I found the flooded-start technique to be the most reliable. I would always try the hot-start approach once, but if it didn't catch, I had no idea where the engine was so I just used the flooded technique to good effect.  Seems like it's the best approach when both the engine, and the outside air, are super hot.

[gsengle]

Has anyone noticed (if you have a fuel computer) that waiting for the flow from the prime aux pump to get to 2-3 gph (regardless of how long that takes) seems to be the key to an easy start?  I have a 550 in an Ovation.  Anyone else use this approach?  Am I doing the right thing?  It does seem to take a variable amount of time, and it will never start before hitting 2 gph minimum, regardless of hot or cold.

[201er]

So are you saying I'm not priming enough? My fuel flow still reads practically 0 after 3-7 seconds of priming cold.

[jlunseth]

My mechanic (Willmar) gave me a method of starting my aircraft (231) that appears to be foolproof.  At least, having used it for the last year I have had zero starting problems, hot or cold, low or high altitude.

The issue I was having was that the engine would fire, but would not want to continue running.  I would hold the primer button down until the engine "caught," (i.e. engine driven pump was delivering fuel).  Sometimes that worked.  Often I would have to try starting the engine two or three times before it would.  This method (holding the prime button down once the engine fires), is prescribed in my manual for cold weather starting.

What I do now is really simple, and aided by my 930, which shows fuel pressure.  With the throttle closed, mixture rich, and master on, I engage the high boost pump.  I let it run until the fuel pressure is somewhere around 19.5 - 21 psi (this is pressure not fuel flow in GPH).  Then I switch the boost pump off and push the prime button for 5 or 6 seconds.  Then crank and it will start and continue running every time, hot or cold.

What this is doing is making sure the fuel line is free of bubbles or air. 

I am not a mechanic, and don't know whether the fuel flow is the same on the Lyc's as it is on the Cont's, so check with your mechanic on this one, but it sure works for me.

[carusoam]

gsengle,

I have a similar experience.  There is no fuel pressure gauge, so I wait for the fuel flow gauge to finish it's initialization process first. 

Procedure:

Throttle in 2 complete rotations.

Low fuel pump switch 5 , 10 or 15 seconds based on outside temperature, then start switch...

Upon start unwind 1 rotation of throttle, turn off fuel pump.

Works well for temperatures over 40deg F.

at 1,000 rpm and leaned for ground ops, I get a fuel flow of about 2.7gph.  3.1 gph or so if not leaned.

Best regards,

-Engine=IO 550-

[jetdriven]

we lean ours for around 2 GPH on taxi, or as lean as it will run smoothly.  Io-360 Lycoming. Cant hurt it.

[201er]

I stumbled across the right amount of prime yesterday and she fired up immediately. Love that feeling. Now I just have to get a feel for where that point is. That was the first cold start where I didn't have to do a lot of cranking. I think it's because I gave it a little more throttle prior to priming. The flight before that I primed for 10 seconds, saw the fuel pressure come up but never registered fuel flow. Is that from not having enough throttle open?