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Unhappy Result on First Engine Start
I?ll get right to the sad punch line: I had a kickback and broke six teeth on the flywheel ring gear on my engine last weekend during the first start attempt.
Here are the details:
I used the cold start procedure recommended by AFP for the fuel injection.On the first try the engine fired after about two revolutions and ran for about 3 seconds then died before I could get the purge valve to the run position. I primed again and it did the same thing. I tried several more times with the same result. It really seemed like it was trying to start but was starved of fuel. On the sixth try I heard a clunk and the prop stuttered about a second after I pressed the starter button. Six teeth missing from the ring gear. The starter looks fine with no visible damage and the shear pin seems to be intact. Obviously I need a new ring gear but the most important thing is trying to understand why this happened and prevent it from happening again. I thought the Skytec NL starter shear pin was supposed to protect the ring gear from damage. I would like to hear comments on possible causes of this incident. The tooth damage was in several different areas. The photo shows it pretty well (yes, I already pulled the flywheel off). What can I do to make sure this is the last time this happens? ![]() |
Kickback
This sounds like it's not timed correctly, possibly you have the ignition leads plumbed to the wrong cylinders? The two wires closest to the vacuum fitting go the the cylinders closest to the prop, see the manual. It's a wasted spark system so it doesn't matter if you time it to the wrong stroke, as long as you had TDC.
Cheer up though, the ring gear is probably cheaper than the starter, but I would have hoped the shear pin in the starter would let go first. Good luck. Tim |
ring gear
I agree timing probably off. Ring gears are pretty easy to replace.
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It is certainly possible to suffer a kickback with pmags even if correctly timed, especially when using a lightweight prop. Ask me how i know...
Assuming you have the leads attached correctly and were able to identify TDC correctly, there are a few additional things that will minimize the chances for reoccurrence. First, retard the timing slightly by rotating the ring gear teeth past TDC by several teeth. Second, if the engine doesn't start on first attempt, on subsequent attempts, (master still on) set your switches to ground the p-leads, hit and hold the start button to let the prop gain momentum for a few seconds, then unground the p leads while continuing to hold the start button. Harder to write than do. Info from brad at emagair. Works for me. |
Ensure your Ring Gear marks are designed to align with the starter mark and not the top of the crankcase centreline. Ask me how I know ;) albeit got away without any damage.
Diagnosed by someone with better knowledge than me reverting to first principles and establishing TDC with a plug out and checking piston #1 TDC. I have also had both P-Mags re-time themselves once preventing start - I can only think some unusual voltage patterns? So check timing again. No idea about the AFP procedure, but seems somewhat complex? I had an RV-8 with AFP, albeit purge valve not fitted. Started fine using standard cold FI procedures i.e. Mix Rich 1/2 throttle, prime for 3-5s (until you get FP), then ICO + 1/8 throttle and start. |
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. |
With a lightweight prop, I engage the starter for a couple of prop revolutions and turn on the ignition system while the engine is already spinning.
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What Andy said......been there, done that. On my ring gear there are timing marks both on the front *and* the back of the ring gear. It's very easy to use the wrong mark when aligning the TDC mark with the case seam.
It is also a good idea to confirm that the ring gear has been installed correctly. A good reference: Lycoming documentation on Engine Timing Marks |
149 teeth
[quote=N941WR;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) John did mention he is using a Skytech 149 NL starter hence it is 149 teeth = 2.4 degrees |
I'm generally critical of 'recommended' start procedures on FI Lycomings, and this is no different. Cranking at 1/2 throttle seems like a very bad idea, no way you can move all the controls quick enough to follow that procedure without having the engine >1000rpm with essentially no oil pressure.
My general flow: Mixture rich (In your case, purge valve RUN as well) Fuel pump 3-7 seconds, looking for fuel pressure to show on the gauge generally. throttle cracked, just off the idle stop Crank until it fires, and then release immediately, let it either run or die. As soon as it fires, the starter is just weighing it down. That cold start procedure works in every airplane I've tried it in. Leaving the throttle up halfway seems a recipe for disaster. |
I totally agree with Steven, your cold start sequence is way too complicated. I have over a 1000 hours with the AFP system with NO purge valve and I have never had a hot or cold start issue.
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A little more information
I will add that I performed the "pull thru" test recommended by E-Mag Ignitions before the first start where you rotate the prop with sparks plugs out of the cylinders but grounded to the case to verify that the 1 and 2 cylinders fire at TDC and the 3 and 4 fire 180 degrees later. But I admit that I was looking mainly for gross timing issues like miss wired spark plug leads or spark well off the timing mark. I am sure the sparks were in sequence and approximately at the TDC mark but it is possible the plugs could have fired a little either side of the mark by a degree or two. If the P-Mags begin to advance at a low as 200 rpm as stated, is it possible that the timing could have advanced before TDC while cranking causing the kickback? I'd like to hear more from those with lightweight props and P-Mags.
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What they said, And...
For what it's worth I used the "B" curve and retarded the engine to split the difference between the A and B curve. So the engine is a little more retarded than the A curve during start and slightly more aggressive during full power. I can tell you that since I did this my engine is very easy to start.
Lance |
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I agree with Tom Martin and I would cut the boost pump time to about 3-4 seconds instead of 30 or more seconds.
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Concerning boost pump use - I've found that some move more fuel than others, but 45 seconds is a LOT. My injected Hiperbipe needed a good 10 seconds, while the RV will flood after about 2 seconds. The Rocket is somewhere in between.
EDIT - Completely missed the purge valve part - never used one. |
Yes it does.
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Lance |
Hydraulic lock
Everyone is right here on having you look at the timing, but with 45 seconds of priming there is also a possiblity of a hydraulic lock happening in one of the cylinders.
Vic |
I've seen it
I've had my engine kickback during start with 2 P-Mags but never had any damage like that. As a precaution, I set my base timing about one tooth retarded. That way on start it will fire after TDC but once running you can make up the advance with the timing curve. There is also a start delay as mentioned that will allow a programmed number of revolutions before firing the plugs. I wonder if there is something else going on here.
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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.
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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? |
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 PrecautionI will certainly try that when I reset the timing. |
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 |
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. |
That's fantastic customer support. Kudos to Aero Sport, and good luck on your next start!
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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? |
Still looking for P-Mag help
Anybody out there with P-Mag experience that can give me inputs on the post above?
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I'm guessing you're doing it the correct way but just wanted to throw it out there. |
Prop direction
Yes, I'm only rotating the prop in the direction of engine rotation for this test.
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What I would consider at this point is to forget timing by the ring gear. Pull the #1 plug and find TDC by using something to find the top of the piston stroke. Then compare true TDC with the ring gear/starter pin. Set your PMag timing using the TDC piston stroke. Then check grounded spark plugs.
Hope this helps. |
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TDC
You can quickly tell if the engine is at TDC, on the 4 cylinders all 4 pistons stop briefly at TDC, no piston movement occurs for 1-2 degrees of crank travel and you can easily feel the crank go "loose" at that position. In other words the crank will rock easily back and forth 1-2 degrees because the pistons are not moving. Before or after that your dragging 4 pistons around also.
To find TDC with a piston stop takes a procedure and a protractor, not just a stop or a dowel. And it's not likely you'll find the ring gear support mis marked anyway. Locate it at TDC via the index marks, check to see if the crank goes "loose" there. If it does move on. For now, maybe set both PMAGs to 5 after TDC, see if it starts and runs ok. Then you can use an automotive timing light on it using the marks on the back of the flywheel and the case seem to see where it's actually firing. A timing light with a variable advance knob will make this easy to see exactly where it's firing. Tim |
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Jim RV9a 45 hrs |
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TDC with the piston stop and flower pot (protractor) is a very accurate way of determining TDC and the "procedure" is just that - 5 minutes. Once verifying the ring gear support timing marks via piston location you never have to do it again and can use the marks on either side of the ring gear support; I prefer the backside and crankcase CL. |
First let me say I checked the accuracy of the TDC mark on the flywheel relative to the actual position of the #1 piston and the mark lines right up with the TDC position. I could see the top of the piston looking through the upper spark plug hole with a light shining in the lower spark plug hole. So I am confident that the TDC mark is an accurate representation of piston TDC.
I spoke to Brad at Emagair this morning and he confirmed what jimbo said about the spark occurring early in the "pull thru" test. He said the "pull thru" test is not very accurate at very low prop RPMs. It is just intended to be a confirmation of correct wiring, not exact timing. He assured me that the P-Mags will fire at the correct crank angle when the prop is spinning faster. However, he reiterated the problem with lightweight props and recommended setting the timing 3-4 degrees after TDC and removing the jumpers to go with the "B" curve operation. That way the engine will start with 3-4 degrees of retard and run with up to 35 degrees of advance relative to TDC. So that is what I plan to do and will try to start the engine again in that configuration. |
John,
I cannot imagine the frustration of having to go through this on a first engine start. It seems to me that chasing a few degrees of timing here are futile. I have the IO-320 with dual P-Mags and Catto prop. When I first started flying I had mine timed too far advanced and I have never had a kickback much less a broken tooth on a ring gear. I was B curve and two teeth advanced. I would exhaust all other possibilities to prevent a re-occurrence. |
[quote=MarkW;893227]John,
It seems to me that chasing a few degrees of timing here are futile./QUOTE] Its not at all futile. Not sure why you would think that. He is planning on following the manufacturer's advice that makes perfect sense, and its advice that has worked for others in the same situation (pmags with light weight prop), including myself. |
Maybe I could have worded that different. I don't believe retarding the timing is wrong, just not what caused the problem. I would hate to see him retard the timing a few degrees as Brad recommends and then give up on looking for other possible causes. Major timing issue maybe but not 2-3 or 4 degrees.
Erich, did you strip teeth on the ring gear? |
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