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-   -   Unhappy Result on First Engine Start (https://vansairforce.net/community/showthread.php?t=114464)

erich weaver 06-24-2014 03:28 PM

Some of this discussion has gone off in the weeds with all the discussion of starting procedure and some incorrect assumptions. Check the original post. The purge valve is at ICO when the boost pump is run for 30-45 secs. That does nothing but circulate fuel through the distribution spider - nothing goes to the cylinders until the purge valve is pushed in. Similarly, the original post does NOT indicate starting at 1/2 throttle.

Rodney 06-25-2014 12:22 AM

Quote:

Originally Posted by N941WR (Post 891284)
The P-mags fire at TDC when below 200 RPM's. If your timing is off a little, you can get a kickback.

How many teeth is your starter ring? If it has 122 teeth then regarding it one tooth is equal to 2.9 degrees. (360 divided by 122)

One other thing, when my engine doesn't catch on the first try, I usually cycle the power to the P-mags. No real reason other than I am comfortable doing that and my P-mags are wired in a non-standard fashion so I can do that independent of the master switch.

Sounds as if your starter is incompatible with starter ring. Count the ring and compare that number with the part number and specs of the starter. This is the likely cause.

jpharrell 06-25-2014 08:30 AM

Ring Gear
 
The ring gear has (or had) 149 teeth and the starter is a 149 NL so there is no issue there.

But to explore a different direction, I am using a momentary contact push button switch rated at 16 amps for my starter. It is ACS part number 15-131, just like 15-132 but momentary contact style. Is it possible that the switch bounced or stuttered causing the starter to try to re-engage while the ring gear was turning? Anyone hear of a starter switch problem that caused ring gear damage?

jpharrell 06-25-2014 01:33 PM

Timing
 
As several have suggested, I have concluded my problem is at least partially related to my lightweight prop. I found the following quote on the Emagair website under "Tips and Tricks".
Low Mass Props: Kick-Back Precaution

Lightweight low mass props, by their nature, have less flywheel effect on the engine. This allows the engine to decelerate much quicker as the starter motor pulls against each compression stroke, and as the engine approaches TDC - where the plugs fire for startup. Under the right conditions, the combination of low mass prop, weak or undersized battery, corroded terminals, cold engine, long cable runs, tired starter (or any combination) can cause the engine to come to a (virtual) stop by the time it reaches TDC. At this point there is a 50/50 chance the engine will move forward or backward (a so-called kick-back) when the plugs fire. While the ignition may be performing as designed, the results are the same. Fortunately, there is an easy defense.

Simply re-time the ignition with the engine parked two or three degrees "AFTER" TDC (instead of "AT" TDC as you normally would). AFTER TDC means (from the TDC position) rotate the prop in the normal direction of travel to your revised target timing mark. Note: This will also slightly retard the operating advance range, but this small amount is not likely to be noticeable for most flyers. If desired, the shift can be nulled with the EICAD interface.
I will certainly try that when I reset the timing.

judoka5051 06-26-2014 05:33 AM

That's a better explanation than mine!
 
Good write up John. That's what I did, but I used the more aggressive curve so I didn't lose anything on the top end. In fact I still gained a little since I split the extra 4(?) degrees that the "B" curve provides.

Lance

jpharrell 06-27-2014 08:47 AM

Kudos to Aero Sport Power
 
I just wanted to report on the excellent support I have received from Aero Sport Power regarding this incident. I called them Monday morning to ask for advice and Doug immediately arranged to have a new flywheel painted to match my engine and shipped to me. The new part, smelling like fresh paint, was on my front doorstep on Wednesday night when I got home from work. That is all the way from Canada to SoCal. I just have to send my damaged flywheel back to them and my cost is basically the price of just the ring gear alone plus shipping. It would have cost me more to have a local engine shop remove and replace the ring gear on my flywheel. Thanks to Doug and Margaret and Aero Sport Power for helping me get this repaired.

Now I just have to install the new flywheel and reset/double check the timing. My plan is to re-time with a bias of 2-3 degrees after TDC and try again.

rightrudder 06-27-2014 09:15 AM

That's fantastic customer support. Kudos to Aero Sport, and good luck on your next start!

jpharrell 06-28-2014 04:50 PM

Update and More Questions
 
This is an update to my original post. I need some help understanding these P-Mags.

I installed the new flywheel this morning and got the prop re-installed. As a check, I performed another ?pull thru? test described by the Emagair installation manual to see if the timing was still set at TDC. As a reminder, in this test the spark plugs are out but grounded to the engine and the prop is rotated by hand while observing the spark timing relative to the timing marks. However, this time a watched very carefully as the TDC mark slowly approached the hole in the starter. To my surprise the #1 spark plug fired about 1.5 teeth (about 3.5 degrees) ahead of TDC. I repeated this several times with the same results except the two P-Mags would sometimes fire at different times by as much as half a tooth (1.2 degrees) in spite of being set exactly the same. At this point I was thinking the P-Mags were just set advanced by 1.5 teeth and this was the probable cause of my kickback.
I decided to reset the timing to 1 tooth after TDC which is about 2.4 degrees retarded. So I rotated the prop to 1 tooth after the TDC timing mark and followed the P-Mag instructions to re-time them. After doing so, I performed the pull-thru test again and found that the #1 spark plug fired ? tooth before TDC. What? There is that same 1.5 tooth offset advanced from the set point again.

So then I rotated the prop to 2.5 teeth after the TDC timing mark and retimed the P-Mags. This time I found that the #1 spark plug fired 1 tooth after TDC. Again, there is that 1.5 tooth offset.

I haven?t found anything in the P-Mag documentation about an offset between the set point and the spark during the ?pull thru? test. Maybe this is an intentional timing offset to account for latency in the P-Mag electronics, or maybe there is a problem with my P-Mags. I don?t know. I plan to call Emagair on Monday but I thought maybe some of you P-Mag owners may have encountered this before and can provide an explanation. I was just not ready to try another restart today and risk another ring gear until I can explain this behavior.

Anybody out there seen this before?

jpharrell 06-29-2014 02:36 PM

Still looking for P-Mag help
 
Anybody out there with P-Mag experience that can give me inputs on the post above?

krw5927 06-29-2014 03:46 PM

Quote:

Originally Posted by jpharrell (Post 892907)
Anybody out there with P-Mag experience that can give me inputs on the post above?

I haven't seen this behavior with Pmags before. Are you sure that you're ONLY turning the prop in the direction of in-flight rotation when setting the timing (counterclockwise, if you're standing in front of the plane looking at the prop)? Bumping the prop back and forth to get the timing mark aligned just right can cause this problem because the gears don't (and can't) mesh that tightly together.

I'm guessing you're doing it the correct way but just wanted to throw it out there.


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