Sorry I couldn't reply because I don't have much notes about the Splitdorf mag. I was waiting for others to give an answer. I am sure that some of the reputable mag shops could tell the specified resistance both in the primary and secondary coil.
A fat blue spark at the spark plug is not a sure evidence the coil is allright. A spark plug has a too small gap to be a good testing tool out in the open air, ideally it should be a jig with 2 pointed electrodes where the distance is adjustable. A good coil should spark steady with a gap of 5-6mm (1/4") or more in open air. To put real stress on the coil, the gap should be adjusted as wide it can go with a stable consistent spark and the coil heated to 50-60C or 120-140F.
The magneto should give reliable sparks at low speeds also. If you do that kind of test it is important to test the magneto at fully retarded because the rotor is in a unfavourable position at that. Of course it should give reliable sparks at all positions btw. fully advance and retarded, but most important at fully retarded. With at least 4-5 mm spark gap (lower compression pressure at start) it should give reliable sparks down to some 300 rpm engine speed, 150rpm on the magneto. I am not entirely sure, it can be several causes but I think poor low speed performance is mainly caused by poor magnetisation of the armature. It is most important to do a proper professional by-the-book remagnetisation of the magnet.
Here is a clip about condensers from Classic Harley Tech, Restoring rotating armature magnetos. (Short story - The capacitors Dr.Falco recommend as replacements for use in Lucas, BTH, and other rotating armature magnetos are a pair Vishay 0.082 uF "AC and Pulse Double Metallized Polypropylene Film Capacitors," manufacturer's number BFC238320823. They are available from Digi-Key for $1.35 each ($2.70 for the pair required to be soldered in parallel) under part number BC1883-ND.) I am sure they are good enough for the Splitdorf rotating magneto type also if you accept a pair of condensers.
———– Sidebar About Replacement Magneto Condensers ———–
For at least 30 years there have been recommendations for a variety of replacement condensers and, figuratively speaking, for just as long the sides of roads have been littered with motorcycles whose magnetos have failed because of using them. Part of the reason for this is that manufacturers typically do not design capacitors for applications where they will be subjected to high current, high voltage pulses while being repeatedly cycled over a wide temperature range in the presence of moisture, organic vapors, ozone, and vibration, so they do not test them under these severe conditions.
Unfortunately, relying on recommendations — from motorcyclists, suppliers, or professional restorers — in selecting a replacement is quite risky. For example, several club magazines have recommended completely inappropriate capacitors from stores like Radio Shack that would fail within the first few moments of operation. For a number of years one well-known supplier sold replacement condensers with the claim “Modern substitute, very high specification, zero failure.” Despite this claim, they failed in service. Still, it took a number of years before enough motorcyclists complained about failures for the supplier to cease selling them. Currently another supplier is advertising replacement condensers that are of a type not rated for pulsed high current applications, and whose dielectric layers are made of a porous oxide. Even if they survive the current pulses of a stressed magneto, no extended environmental testing has been done of them, so there is no way to even guess how long they might last in the ozone-rich atmosphere near the points before delamination or breakdown of the oxide might set in. For quite a while one well-known magneto rebuilder sold condensers to other well-known rebuilders at a high price that he claimed were custom manufactured to his rigorous specifications. Not only were they just inexpensive capacitors from which he had removed the markings, they also failed in service. A number of magnetos used in the 2010 cross-country Cannonball Motorcycle Run, all rebuilt by the same well-known restorer, failed due to bad condensers.
Despite the examples given above, and many others, condensers that later failed in service always were claimed at the time to function quite well. To paraphrase Bruce Springsteen’s ‘Magic’, as far as replacement condensers are concerned, you are well advised to ‘trust none of what you hear, and less of what is claimed.’
Condensers are not mysterious components that are doomed to failure, but a proper replacement definitely does have to have the right combination of electrical, mechanical, and materials properties in order to survive in a magneto. The Fall and Winter 2011 issues of The Antique Motorcycle magazine contain a two-part article by physicist Dr. Charles Falco explaining the science of why the wax paper condensers that Lucas used in their post-WWII magnetos have a limited lifetime, (A snip of it further down) and detailing extensive electrical and environmental tests he made of particular Panasonic polypropylene film/foil capacitors, concluding from those tests they would last under the harsh operating conditions of a magneto for the equivalent of at least 40 years or 140,000 miles. As well as Dr. Falco having no connection with any supplier that possibly might affect his recommendation, to the best of my knowledge these are the only replacement capacitors that have been subjected to any such battery of tests designed specifically to mimic actual operation. Because they have the proper electrical specifications and survived a year-long set of accelerated electrical and environmental tests conducted by an independent party, these are the ones I use. Luckily, I have a large stock of them, because they are now no longer in production. Until someone like him conducts tests to identify another, in-production replacement, if you use any capacitor other than these Panasonics it is at your peril. Unfortunately, the Panasonic condensers I tested so extensively are now out of production
Here's a snip of Dr Falcos paper about common wax paper condensers: The following photograph and magnified inset shows the paper and wax layers in a Lucas condenser.
There are ~125 layer pairs of area ~1"x1-1/2", with a separation between metal foils of ~0.001". Waxes have dielectic constants in the range 2.1-3.1. Using a value of 3 for an estimate, capacitance = dielectric constant x permittivity of vacuum x Area/separation x 125 pairs = 0.11 uF. The actual capacitance from Lucas literature is 0.15-0.18 uF, agreeing very well with this estimate based on my measurements of the internal structure of the condenser.
Even with stabilizers, research showed it also was essential to hermetically seal the capacitors because the wax is somewhat hydroscopic, and moisture accelerates the breakdown. When the wax breaks down it releases hydrochloric acid which then attacks the aluminum sheets of the capacitor, releasing aluminum chloride. Unfortunately, aluminum chloride accelerates the breakdown of the wax further, in turn releasing even more HCl. While breakdown of the wax happens no matter what, the process rapidly accelerates in the presence of moisture.
The replacement condensers I (used to) recommend are a pair of Panasonic 0.082 uF polypropylene film capacitors (part no. ECQ-P4823JU). When soldered in parallel they produce a 0.16 uF condenser that fits into the available space in the end caps of Bosch, Lucas and BTH single and twin rotating armature magnetos. Unfortunately, the Panasonic condensers I tested so extensively are now out of production.
The capacitors I recommend as replacements for use in Lucas, BTH, and other rotating armature magnetos are a pair Vishay 0.082 uF "AC and Pulse Double Metallized Polypropylene Film Capacitors," manufacturer's number BFC238320823. These capacitors have pulsed current and voltage ratings of 1400 V/us and 630 VDC with a maximum operating temperature of 105 oC. These specifications comfortably exceed those needed to survive for years in the hostile electrical environment of a magneto. They are available from Digi-Key for $1.35 each ($2.70 for the pair required to be soldered in parallel) under part number BC1883-ND.)
