What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Slick 6351 Magneto Mystery

Robb

Well Known Member
I have a really strange issue with a new to me IO540-D4A5 engine. It has a lightspeed on the right and a slick on the left. When we did the first conditional inspection it appeared to be timed at 20 degrees retarded. I dont see how it could have even ran at that setting. I pulled the mag and it looked fine but had a weak spring on the impulse mechanism. I sent the mag off to have the 500 hr inspection done. I bench tested it and it was fine and sparked on all cylinders. Someone had set it for right instead of left turn. We changed it to left hand and set it to 20 degrees advanced and it was not happy. There was no indication of an issue in the log books and no mention of a relocated timing mark. It was running fine before the inspection but I did notice the Tempest fine wire plugs electrodes were brittle and broke off when the gap was checked? I guess Ill see when we install the mag how it turns out. Never seen this before and there are multiple notes of the mag timing being checked. It has 575 hrs total time and according to the logs it was replaced at 212 hrs by the original builder but no mention of any problems
 
checked the flywheel it is correct. Maybe the impulse portion was stuck retarded? Crazy scenario and I have had multiple very experienced guys look at it.
 
When checking or setting the timing of magnetos you need to turn the prop past TDC to trip the impulse back to its normal state and then reverse to prop rotation slightly to get back to the timing mark. If this isn’t done you will induce a error by the retard/lag value of the impulse coupled.
When you do this you will hear the impulse coupler trip, confirming it is not stuck.
 
Last edited:
Note that the Lycoming IO-540 D4A5 engine should NOT be using an impulse coupled mag. It should be using a slick start unit instead. May not need that with a light speed ignition, as I don't know much about those. But if you check with Lycoming or Slick, they will both tell you not to install an impulse coupling on this engine.
 
There is no mystery here. It works or it doesn't.

Verify snap of impulse coupling on TDC #1 by looking thru top plug hole at piston with flashlight. Then check flywheel marks. Back off 35 deg or so advanced, and bump prop to 25 and check timing with buzz box. Timing is 25 degrees advanced, not 20, on a parallel valve 540.
 
I have never heard of someone changing rotation direction on a mag. I was always taught to read the data plate on the mag and simply use that info to time, etc .... and of course, having the correct mag for the engine. But this is all relative to certificated aircraft maintenance. Changing rotation changes firing order and I guess some of the impulse parts would have to be changed. And Im also guessing internal timing would have to be reset but idk. Interesting.

Ellis
 
Last edited:
Can the impulse mechanism be removed and the same mag put back on the engine and operated as a non impulse without further modifications? I have SDS on the right and can start with only it. Slick shows different p/n for the rotor and case of the impulse and non impulse mags.
 
If the mag was 20 ATDC, it is possible that the previous owner sgnificantly increased the advance on the light speed to make up for the essentially missing spark (20* retarded = might as well not be there). If that were the case, setting the mag back to 20 BTDC, would create a VERY advanced timing.

Total timing on a dual plug system is a composite of both of the advance settings.

Larry
 
I am going to get in contact with Lycoming and Slick and see if they have any input. I agree I may not need an impulsed coupling mag on this engine. Not sure why they originally installed it but will find out. I did send the mag out to be rebuilt.
 
Since you mentioned having a Lightspeed system, and regarding impulse coupling, according to Klaus (Lightspeed founder) “Timing curve extends from 15° to 42° degrees BTDC. Full retard for starting holds the timing back to TDC. After starting, the system advances according to RPM and manifold pressure”.
Klaus also advised me to set the mag (Slick 4371) to 22 degrees BTDC. Joe Logie (Champion/Slick tech) says "25 degree advance is normal, change to 20 would have little/no effect".
 
Last edited:
I will post on what we finally figure out whats wrong and I get the mag back from the 500 hr service. Still dont have a definitive answer. Hopefully it will help the next guy that has an issue.
 
The D4A5's were originally sent with impulse mags. There has been some concern that there are possibly harmonics on that engine that could cause the impulse mag to disintegrate and cause serious damage to the engine, up to and including catastrophic failure. That's why the change to non-impulse mags coupled with a slick start (or electronic ignition).

We see a large number of these engines come through the shop on RV-10's during the year, and some of the engine builders set the timing at 20 and some set it at 25. Haven't figured out a rhyme or reason for that, except that obviously they run cooler when set at 20.

I did some testing a few years back and determined that 23.5 degrees timing on this engine seemed to be the best, and Klaus had the same opinion.

Just sharing....

Vic
 
Well I received the rebuilt mag and installed it today. Set the timing to 20 degrees after using a dunce cap to make sure the starter ring was correct and it was. Long story short it fired up great with 100 RPM drop on the new Slick 6351 magneto. It is still a mystery to me why it was firing off at 20 degrees ATDC with no issues in flight or run up. It is way smoother now and maybe it was the impulse coupling? Who knows
 
Well I received the rebuilt mag and installed it today. Set the timing to 20 degrees after using a dunce cap to make sure the starter ring was correct and it was. Long story short it fired up great with 100 RPM drop on the new Slick 6351 magneto. It is still a mystery to me why it was firing off at 20 degrees ATDC with no issues in flight or run up. It is way smoother now and maybe it was the impulse coupling? Who knows

Timing should be set to 20 deg BEFORE top dead center, not after. E.g., turning the prop in its normal direction, the mag should fire before TDC.
 
Installed the new mag after having it rebuilt. Set the timing to 20 BTDC as listed and it works perfectly. Not sure how it even ran where it was. I guess Ill never know but its fixed now. Must have been something really messed up with the old mag. All new lower fine wire plugs also
 
Back
Top