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03-01-2016, 04:19 PM
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Join Date: Jan 2011
Location: Carl Junction, MO
Posts: 172
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Left slick mag with impulse coupling timing
Help, I'm confused.
I'm told that you should hear the impulse mag click at TDC. Mine occurs 5 ring gear teeth past TDC (approx 12.5 degrees on 142 tooth ring gear) should I ad just? I can move mag to click at that point but then the timing light buzzer indicates at about 37 degrees before TDC instead of 25. This is new factory engine from Van's and I've flown for 132 tach hours over the first year. Starting became problematic. I'm doing condition inspection now using E 50 mag timing buzz box. Plugs looked fine. Filter quite dirty. Plug gaps .022 (range .016-.021).
I'm hoping just cleaning the filter and plugs and adjusting gap will result in easier starting. Also mag sync was off some and I fixed that. Just wondering why that coupling clicks when it does and if I need to do something with it.
Thanks!!
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03-01-2016, 05:36 PM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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Mag
Pull the mag and send it to a magneto shop. Something is very wrong with the impulse if your calculations are correct. I will look it up to verify but most impulse couplings have a lag angle around 25 degrees which means if the mag is timed to 25 degrees before TDC the mag will fire at TDC at low r/m for starting. The impulse is then disengaged at relatively low r/m after start. Impulse couplings do fail and it is very important to verify that yours is ok. Failed impulse parts can trash all or most of your engine. It is possible that the impulse is just sticking but this is unlikely with a new engine.
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03-01-2016, 08:12 PM
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Join Date: Jul 2012
Location: MS
Posts: 310
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Check the red data plate on the mag - it should list the lag angle, which is the number of degrees after normal timing (probably 25* BTDC) that the impulse coupling will fire. Our IO540 came with mags that caused difficult starting even though the timing was on. Turned out that model of magneto had too large of a lag angle, and when we swapped to a 'correct' (better) one our starting woes disappeared.
==dave==
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03-01-2016, 09:45 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,298
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Sounds like your timing has drifted. Your numbers do not point to a bad impulse coupler. Assuming you have a 25* coupler, everything adds up. The only thing you'll notice in this state is hard starting. In fact, you were probably getting better performance at cruise due to the additional advance. However, you are at a much greater risk of detonation in High MP situations. You'll want to get this timed correctly right away and in the future occassionally check nuts on the hold downs. As the new gasket compresses, it may need a re-torque.
I thought the general consensus was .016-.018 on the plugs. Better to be at the smaller end, as they will wear .002 or more between the 100 hour gap checks. The mag coils aren't that strong and you add a lot of wear, via heat, by running the gaps too big.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 03-01-2016 at 09:49 PM.
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03-02-2016, 08:51 AM
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Join Date: Jan 2011
Location: Carl Junction, MO
Posts: 172
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Thanks for responding. I'm going to try to research a bit more before I remove and send in.. I may not understand fully but isn't the piston on the power stroke once past the top center? Because that would mean mine is getting that impulse 5-6 gear teeth past to on the power side or just firing late. My calculations are about 2.4 degrees per tooth on 149 tooth gear. That's about 14.5 degrees past it.
I'm also asked to check "lag" time on data plate. Question I have is what is the lag time ref from. I.e. Is it lag degrees from top center or is it from the normal timing marks of 25 degrees before TC?
If anyone who has the lycoming O-360 with 4300 series slick mags could check to see when their coupling impulse actually clicks unload. I'd be interested to know if this is common or not.
Thanks again
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03-02-2016, 09:25 AM
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Join Date: Dec 2011
Location: delta, co
Posts: 204
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MAG
I agree to send it out and have it checked. Champion was having problems with internal timing a year or so back and never published anything about it! Sometimes the problems would not start till 50 hours or more.
Jesse
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03-02-2016, 11:12 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,298
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Quote:
Originally Posted by Kato's 8
Thanks for responding. I'm going to try to research a bit more before I remove and send in.. I may not understand fully but isn't the piston on the power stroke once past the top center? Because that would mean mine is getting that impulse 5-6 gear teeth past to on the power side or just firing late. My calculations are about 2.4 degrees per tooth on 149 tooth gear. That's about 14.5 degrees past it.
I'm also asked to check "lag" time on data plate. Question I have is what is the lag time ref from. I.e. Is it lag degrees from top center or is it from the normal timing marks of 25 degrees before TC?
If anyone who has the lycoming O-360 with 4300 series slick mags could check to see when their coupling impulse actually clicks unload. I'd be interested to know if this is common or not.
Thanks again
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sorry about that. I assumed when you said past TDC you meant advanced. I just assumed you used the wrong terminology because the numbers added up and it seemed too coincidental. If you are 37 advanced for normal timing and 12.5 degrees retarded on the impulse, something is wrong with your coupler. However, something doesn't add up here. It's possible that something bad is happening inside the mag. But that is a big timing drift (25-37).
If it were me, I would reset the mag timing to 25 (assuming that is the spec) and see what happens to your impulse timing. It's probably worth a bit of effort to insure you have a problem before you send the mag back and bear the time and expense.
The lag reference is the number of degrees late that the impulse will fire. If you set the timing to 25 BTDC and have a 25 lag, your mag will fire at TDC when below approx 300 RPM and at 25 BTDC when above 300 (impulse coupler stops working at a specific RPM). If your set the mag timing (via your box) at 37 BTDC and have a 25 lag, your mag will fire at 12 BTDC when below 300 RPM. You can see how I jumped to my conclusion and why I think you may have made a measurement error.
When turning the prop by hand you will hear the impulse coupler snap at the point you should expect mag firing when your RPM is below approximately 300 RPM.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 03-02-2016 at 11:22 AM.
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03-02-2016, 11:28 AM
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Join Date: Nov 2014
Location: Richmond Hill, GA (KLHW)
Posts: 2,189
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Don't forget to check the spark plug resistance too. If it is over 5000 ohms, the plug is shot and will eventually burn out a mag coil. Ideally, < 1500 ohms is what you want, IMO.
__________________
Ray
RV-7A - Slider - N495KL - First flt 27 Jan 17
O-360-A4M w/ AFP FM-150 FI, Dual PMags, Vetterman Trombone Exh, SkyTech starter, BandC Alt (PP failed after 226 hrs)
Catto 3 blade NLE, FlightLines Interior, James cowl, plenum & intake, Anti-Splat -14 seat mod and nose gear support
All lines by TSFlightLines (aka Hoser)
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03-02-2016, 12:27 PM
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Join Date: Jan 2011
Location: Carl Junction, MO
Posts: 172
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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?
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03-02-2016, 12:59 PM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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Timing
The late firing with the impulse is probably the cause of your starting problems. My 0 320 with 10-1 pistons is set to 22 degrees so impulse fires 3 degrees past TDC. Cold starts consistent on first or second blade and almost as good for hot starts. Bendix mags.
You probably have an internal timing problem but you simply cannot afford the chance that the impulse is damaged.
Aircraft Magneto in Helena MT is a highly regarded shop. Just ask for a check of impulse and internal timing. $200 plus/minus against the cost of a new engine if the impulse comes apart.
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