Author Topic: Kick back with Magneto  (Read 14905 times)

Offline weasel

Kick back with Magneto
« on: November 17, 2013, 07:51:18 PM »
I have experienced kickback several times when hand propping small VW based power plants and due to the painful experiences caused me to look into the cause and see if I could make some improvements.

When the engine is running we need the peak pressure within the cyls to happen sometime after TDC..actually we need it about 12-18 deg ATDC to keep it from detonating and still produce power.  The reason that our engines fire BTDC is that it takes time for the fuel to burn and produce the pressure. So we have to start it ahead of TDC by about 26-28 deg so that it will peak at the right time for our engines running at ~ 3600 RPM.

Now think about it. Running at 3600 RPM it takes way less time to get past TDC than when we are trying to hand start the thing so if you just pull it though and it fires at 26-28 BTDC its gonna snach back.

So the impulse coupler is used to delay the spark until the right time when you crank it BUT!!!!! there is a catch.  depending on the start speed and the timing setting for the specific engine that the mag was designed for, the lag angle (amount of delay degrees for starting) is different. It may be 15, 20, 25, 30, or 35 deg or some other number.

First take a look at how the impulse coupling works in a magneto.


When these engines are put together the magneto is usually a donor from some random application that the engine builder thinks will work.


I got to looking at the slick mag on my 4 banger and it is labeled 15 deg. lag angle. so with the mag timed at 28 deg. BTDC if you do the math this means that by turning the prop slowly through by hand the plug will fire at about 13 deg. BTDC.  You can see why it would want to snach on ya.

I started looking at ways to delay the start timing without changing the run timing.  You can order a new drive housing for the impulse coupler that has a different lag angle but I figured if I do that I will end up just trashing the one I have so there is not much to loose if I modify it.

I wanted the mag to fire right at TDC.

By doing some calculating. I came up with the amount to grind off the drive coupling.

I measured the radius of the circle and then used 13 deg for my triangle (the amount of additional lag I needed) then solved for the opposite leg.  This gave me the amount to grind off.

But wait! once you start grinding it off you no longer have something to measure off of to gauge how far you have gone. So measure the entire length of the "trip ear" in its original state then subtract the amount you calculated to remove. This will then be the length that you want the "trip ear" to be when you are finished. Just carefully grind it off and try to retain the original contact angle when you grind so that it wears nice on the impulse dog.

Works great now. No kickbacks.  I checked the Fairbanks mag on the LE and it was the same way.  Went ahead and did the mod to it.

Good luck.
LE - 92mmx78mm 1/2 VW 
Fisher Classic - Cassler 94mmx86mm Full VW
RV-10 - Lycoming IO-540

Offline weasel

Re: Kick back with Magneto
« Reply #1 on: November 17, 2013, 07:52:35 PM »
A little back ground on what I have on my 1/2 Vw - it's the Leburg EI system that was designed just for what we are doing (hand cranking and running the engine hard) - it's pgm'd to fire a degree of two after TDC at cranking speeds and therefore highly unlikely to kick back - with some other pgm'n it watches the revs at low rpm and retards more if the engine starts to stumble - how sweet it is... Ok, so much for the Leburg... Now on the 1/3 Corvair Ultravair we had the OT-10 cam and wanted to run the 2220 Slick out to 32 degrees BTDC and so just like you are suggesting the impulse mechanism had to be modified and in the manor you suggest  a n d  no kick backs... As an aside the 1/3 Corvair does not benefit a bit from advanced timing - improvement stopped at 26 degrees BTDC... But, modifying the mag impulse a must... Now the 009 Vw distributor is and Leonard's VID shows you setting the timing at [TDC] and therefore not as snarfy as my Leburgs but none of the engines I've worked with showed any notion of kicking back...  B U T  I know of three 1/2 Vw people that tinkered with advanced timing and got bit for their troubles - it bites nasty...  One more thing on the 009, there are a lot of type 1 engines running here in the desert - most of the community knows the breed well now and they all witness mark the distributor timing setting - that's an excellent idea for the mags too... Most have switched to one of the 009 electronic triggers and many carry a spare one to the desert with them... The type 1 definitely runs better with the electronic trigger...  Leonard has mentioned his theory that a mag spark in the type 1 leads to early valve trouble - [maybe not so much with the stainless valves and better valve seats]... I do know that my Dual EI adds 50 rpm at 3000 on my hemi'd heads... Good ignition matters and when your power is low every little bit helps (a lot)...
Post your article... You can include my forward if you wish...
LE - 92mmx78mm 1/2 VW 
Fisher Classic - Cassler 94mmx86mm Full VW
RV-10 - Lycoming IO-540


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