647jc

647jc

Well Known Member
I have a certified Lyc O-320, 160hrs since new with two Slick mags. The engine used to start fine, after a couple blades, no problems. About 9 months ago I noticed the engine did not want to fire when cranking until I released the key switch, I have a basic ACS switch.

So, my start scenario is, I prime the engine as needed, turn the keyswitch to start and the engine begins cranking over (I have a new battery also). I can crank the engine over for 4 or 5 seconds without any noticeable firing. Then, just when I release the key back to normal, the engine typically fires and starts. When it fires it hits pretty hard, probably because the right mag is firing and the timing is not retarded at that point. Once started, the engine runs fine, idles OK, run-up and mag check is good, engine runs great under power.

During my last annual I had an A&P check compression and mags. We verified the mag timing and proper operation of the ACS switch grounding the correct mag according to switch position and also verified the impulse coupler on the left mag was tripping at the correct time. But I still have the problem and I?m afraid I may damage the starter case with a kick-back since the engine is really starting on the right mag with no retarded timing. I have a metal Sensenich FP prop which has enough mass to pull the engine through the initial non-retarded firing but I think eventually I?m going to get a kick-back and destroy the starter.

Anyway, not sure what to do next, any additional tests I can perform? I?m ready to pull the left mag and send it in for re-work even though it only has about 160hrs on it? Any suggestions as to what shop would be best, I live in the Midwest?
 
Disconnect the left P-lead at the magneto and try it. If no change, run a ground wire from the right mag P-lead terminal with the left opened/disconnected and try it again. This should take your switch wiring out of the equation telling you if it's a mag/ignition problem or a wiring problem.
 
I verified the wiring with the A&P. He had a device similar to a continuity tester that connected to the L&R mag P-leads. With the master switch off we verified the ACS switch positions.

Position L: right mag P lead grounded, left not grounded
Position R: left mag P lead grounded, right not grounded
Positions start: right mag P lead grounded, left not grounded

Wiring and ACS switch is OK.

I'm curious if anyone has seen a failure mode of this type in a Slick mag with an impulse coupler.
 
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I would try the above advice anyway, it's too easy not to. Disconnect left mag P lead, ground right P lead, see if it starts.

Next test:

Pull 4 plugs, one from each cylinder, so the engine won't run.
Check that plug gaps are correct, and clean.
Reconnect one high tension lead from the left mag to its corresponding plug. Lay that plug on a cylinder fin (unpainted) or other ground.
Disconnect the P lead from the left mag. Leave the key off, so the right mag is grounded.
Turn the prop in its normal direction. As you come up on TDC on the cylinder corresponding to that plug (make sure it's the compression stroke, not the exhaust stroke) you should hear the impulse let go with a definite clank, and you should see a spark jump across the plug (best done not in direct sunlight).
If you see the spark the mag is okay.
You can repeat this test with the left P lead reconnected, master off, and an assistant holding the key on start.
 
First step I would take is to pull the plugs one by one and measure the resistance between the main electrode and lead connection - discard and replace with new any the measure greater than 5000 ohms.
 
My Warrior did the same exact thing till I replaced the key switch. I will never have one of those switches in any airplane that I own. They are pure junk.

If it will continue to crank and not fire till you release the key......... the switch is the problem.

Disconnect the left P-lead at the magneto. Leave the key switch off. Connect a remote starter button to the small lug on the starter solenoid. Get in the airplane and do a normal start except leave the key off, and use the start button in your hand.
 
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It worked at one point

So, everything must be wired correctly.

Remove existing switch and replace with new.
 
Sounds like you are missing the jumper on the keyswitch required for a single impulse mag. They will start fine without it sometimes especially when warm.
 
I have 160 hours on mine and it has started to act the same way. Starts after I let go of the switch. Sometimes after it cranks for awhile and I wait for a few seconds and then try again it starts as soon as I hit the switch. This is with the vans key switch. I am interested to see what you find.
 
I think you need to focus on the key switch. They can seem to check out just fine but still not be working correctly.

The part about starting when you let off the key in the start position is most likely due to the ungrounding of the right mag. This isn't so good and there's a good chance of some kickback and damage to starter / engine case.

If I was experiencing this, I'd really want to get it solved. If I couldn't resolve the key switch, I'd be replacing the key switch with a push button for starter and toggles for mags.

Dan
 
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