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Mag
I will try to explain this a little more. The impulse coupling is purely mechanical. At low r/m pawls on the impulse coupling engage fixed pins on the mag flange. The impulse coupling spring "winds up" and at a fixed point the pawls disengage and the impulse coupling momentarily rotates the mag at a much faster speed to improve the spark. If the engine starts the impulse pawls stay "retracted". The relationship between magneto drive gear and the impulse never changes. (A key maintains this relationship and there is no adjustment) The relationship between the impulse and the internal magneto timing CAN change. Dirty or damaged breaker point or worn breaker point "cam followers" are among the possible causes. The breaker point cam follower is some type of plastic which does wear causing the timing to change.
This MAY be what is happening to the OP. Whatever the cause, the "firing point" of the impulse has been compromised. All of the above and much more is what led to the development of the shower of sparks system which eliminates the impulse. There is at least some history of failed impulses destroying engines. Failed impulse fragments fall into the accessory case and can make their way thru the entire engine rather quickly. The Lightspeed ignition with the crank sensor eliminates this possibility IF you eliminate the impulse mag. |
All good points to consider. No pun intended!
I asked a fellow Rv 8 builder who hung his Van's supplied lycoming IO 360 with the same mags installed from lycoming to see when his impulse mag clicked. Guess what? 7 teeth past top center! That's from a new magneto on new engine that has not been run yet, other than test runs at factory. Now I realy don't get what's going on here. Is slick covering up a problem? Or is it realy not a problem at all? |
Slick Magneto Issues
There are actually quite a few published issues with Slick magnetos. I found Service Bulletin SB1-12 which described the advancing problem with a range of magnetos (between two serial numbers, which actually is between two dates in 2006 to 2008). There was a cam issue that caused the advancing timing and issues with starting and also high CHT's.
Further investigation on the 'net also allowed me to find a series of magnetos from about May 2011 through to early 2012 that had a similar issue. I had an April 2011 mag that gave me trouble hot starting and I've now swapped it out at 115 hours, for a replacement. The engine has only been started twice on this new mag, once cold and once heat soaked for 15 minutes, hot! Both times starting was great. More data to collect on this. Pulled the mag apart, used the Slick Overhaul manual, and could find no issue with any of the components. The only thing that was a real give away was the really poor spark from the mag, but there was no "obvious" reason for that given the components were all in spec. However I knew the mag was advancing, as it had advanced 4 degrees in 4 months. Previously it was 7 degrees advanced. I'll fly a few more hours then recheck the timing. If you're having trouble with the left mag, check the service bulletin and see if yours is in the serial number range. Mine was not the only one that has given trouble in our neck of the woods. An RV8 driver came over to me at a recent fly in to say he had the same issues, replaced the mag and never had the same problem again. It was a frustrating exercise trying to track down why I had such problems hot starting, but finally think the issue is fixed. Do check the spark plugs too. Mine were champion brand from Lycoming via Vans. Most were around 4400- 4600 ohms, with one at 6600 ohms. Weak spark from the mag and high resistance plugs = poor starting. The impulse coupling on mine fires off at TDC as expected. Hopefully the above may assist you or others with starting issues. |
I'll be checking service bulletins as suggested. I think right now I'll see how starting and run checks go after the condition inspection is done. My impulse mag is firing 5 teeth past TC. But it most likely was from the beginning because a fellow builder indicates the same thing with brand new and prior to first start on airframe. I will see how it performs again and if engine starting becomes problematic again in 2016 I'll be pulling that mag ad sending out for inspection and adjustment.
Thank you to all for input. Bill |
Another issue
I have a rebuilt engine not yet started.
When I turn the prop I do not get a strong Pop or sound from my impulse mag. More like just a click Sound. Do I need to replace the impulse gear? Can it be rebuilt? I have experienced in the past a much stronger sound from the Mag. Maybe something is frozen? Been a while since it was run. |
{I have a rebuilt engine not yet started. Been a while since it was run}
Might just be the assembly lube on the flyweights. I wouldn't do anything unless you have starting problems. Don Broussard RV 9 Rebuild in Progress Canopy time N14DB |
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Late Impulse Coupling Snap?
I just noticed the same behavior on my magneto as described here and I am trying to determine if it is normal. My magneto is timed to 28 BTDC and the magneto has 25 deg of lag, so you would expect the impulse coupling to snap at 3 deg BTDC. But, it snaps well past TDC. Is this normal or a problem?
The reason I think it might be normal is that someone else reported this for a new engine in this thread, and when I am checking where it snaps, I am just bumping it a degree at a time. That's not exactly what happens during starting. Thanks! |
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If timing is retarded 2-3 degrees, and the impulse has a 25 deg lag, then it will snap at 2-3 past TDC. As far as I understand and have experienced, a couple degrees one way or the other won't adversely affect starting (maybe even as much as +/- 5 degrees). |
Mags
It would be very helpful if people would describe their components-in this case magneto manufacturer and engine model- in greater detail.
If you have Bendix mags you probably have a 100 hour inspection on the impulse coupling. Failure of the coupling will likely destroy the engine. There are some who claim that the latest and greatest coupling does not require the inspection. In any case wear on the coupling or the pins can cause the lag angle to increase. I agree with the previous post that 28 degree TDC timing is not a good idea. I ran my 10-1 O 320 for about 75 hours at 22 degrees and then reset the timing to 25 degrees. All this got me was a slight increase in CHT. NO measurable increase in performance. If you were racing-either Reno or SARL, there might be a slight benefit to the higher timing number, but that will be at the cost of lower TBO. The angle valve 360's specs called for the timing to be reset to 22 degrees. This occurred many years ago. |
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