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I have a lawnmower with a Briggs and Stratton motor, model 128T05-5268-B1. This motor was equipped with an electric starter, but the starter, battery and all the wiring have been removed. I start it with the pull cord. The mower will start, but it is reluctant. I have checked the choking mechanism - it seems to be working properly When hot the machine sometimes starts right up, other times it fails to start at all. When cold, if it is not cooperating, starting fluid does not help. The carburetor has been checked and cleaned. All seems well in there. Once started, the motor runs strongly.

While I was doing some other work I happened to check the spark of the spark plug. I've watched more than a few videos showing how to test the spark on a similar motor and they get a good spark when just turning the motor over by hand, slowly. I get no spark when I turn the motor over by hand. In order to get a visible spark, I need to use the pull cord and get the motor turning quickly and for more than one compression cycle. Then I do get a spark, but to my eye, it looks weak. Here is what I've checked:
- The spark plug is new
- The magneto gap was set to 0.008, 0.012 and 0.business card without any difference (Presently it set to 0.business card.)
- I inspected the grounding lead to the coil for cuts or burns - it had a very minor cut and I re-covered it with a full length piece of shrink tubing.
- I checked the cut off switch to make sure it was not grounding in the operational position.
- I tested the magneto. Using an ohm meter on the 20K scale, I got 5.80 ohms on both magnets and the grounding lead. Spec is 6.00 ohms.

I've read a number of reviews on replacement magnetron coils where the buyers exclaim that the new coil fixed their starting woes. I'm suspecting that I may need a new coil, but the ohm readings on the coil indicate that it is OK. Any suggestions on where else to look for the root of this poor starting?

You have a solid state coil, which cannot be tested using an Ohm meter, even though they give you specs for it. You need a special tester. In your case, if you feel you have a weak spark, I would replace the coil, but remember that coils are electrical parts and 99% of the time they cannot be returned.

I don't where you are getting your info but it is off the wall to say the least.

First Briggs Magnetrons require 350 rpms to spark correctly. And Briggs does not publish any specs for testing these Magnetrons other than they must be able to jump a 1.66" air gap using their testers.

I do however know the latest Magnetrons test around 6 K ohms on the secondary to coil metal frame and the kill tab to metal frame 1.5-1.7 ohms but this does not test the trigger circuitry. This was in house testing. But as said this only test the windings and not the electronics which can not be test due the configuration of the kill circuit.

There is no magnets in the Magnetron coils.; just the flywheel.

I tested the magnetron coil again using better test information. From the metal laminations to the grounding lead measures 1.6 ohms on the 200 ohm scale. The spark plug lead to the metal laminations reads 6.0 ohms on the 20K ohm scale. This matches StarTech's specification. Perhaps my issue is not electrical. I'll have to look around further.

have you tried to start it with the kill wire un hooked?

I've not tried unhooking the kill lead, but I did check the lead for cuts and burns, and i ohmed out the kill lead pad and the grounding mechanism and all is well with them. I am getting some spark, so I don't have much concern about the kill switch.

Put the meter away. Unhook the kill wire from the coil. Remove the spark plug. Use a gap tester or pull the boot back and position the plug terminal approx 1/4" from ground and pull the cord like you are starting it. If it will jump a bue spark across a 1/4" gap the magetron is good. It will run the engine. If it won't jump a blue spark across the 1/4" gap the coil is bad. Meter readings are useless testing a briggs magnetron contrary to what some utubers say.

The energy in a spark has to come from some where.
In a magneto it comes from the rotational energy in the flywheel via the flux in the magnet .
All the rest is fixed and just modifies the energy into a high voltage low current spark.

So the crutial things are the speed the magnet is traveling at , the strength of the magnet and the distance the magnet is from the coil .
You can only adjust 2 of them, speed & distance .
BEcause there is no mechanism for varying the time of the spark in relation to the position of the piston at different engine speeds it is always a compromise between right for starting ( full retard ) and right for running at full speed ( full advance ) .
Of these two, the former is not as important as the latter .

Whoa here, although, it true that most Briggs are fixed timed CDI systems there are Magneto systems that have variable timing setups in the trigger circuits. And I have seen these systems to fail in such that the ignition timing goes to full advance instead of the normal delay spark advance. I had one case where the ignition fired so early that it was setting the paper air filter on fire.

If I remember correctly Briggs uses a 20 degree BDTC setup. Even that varies as the rate of magnetic field builds to trigger the trigger circuit. The voltage on the base of the primary NPN transistor must exceed at least 0.7 volts to switch the transistor on, And trigger use a Darlington transistor setup so the actual trigger voltage is even higher.

The latest Magnetron Coils have 5 SMD transistors and one D13003 transistor on the trigger circuit board along with 8 SMDs (inductors, resistors, and capacitors). And the transformer itself has has a turns ratio of about 1 to 18.

It also can noted that Briggs system uses the wasted spark setup; therefore, it is possible for the spark plug to not fire under compression due weak voltage but still fire during the exhaust stroke.

FWIW Angelo Notaras ( who was the founder of Atom Industries ) was one of my courier customers and I spent may an afternoon in his workshop.
You might know them as Atom Computerised Ignition Modules .
Once again the USA government banned the sales of them in the USA but I think it must have gotten overturned because nearly all of the NOS ones seem to be in the USA .
So I know how they work and it is still a compromise between being right for starting and right for full power under load .
And you of all people will know that a magneto will produce a weaker spark at cranking speeds than it does at running speeds.
Ask any person who rides pre WWII motorcycles .

Mr-Mom is having problems getting a spark at cranking speeds and is worried that he has a problem with his magneto .
And I for one would agree with him that a replacement will probably fix his no start problem.

Atoms for sale in the US
The original Magnitron was an exact copy of the brown atom which hit the market 2 days after the Atom patient expired.
Interestingly , and it says a lot about the contempt that US companies had for their customers , none of them took up the offer of fitting them under license at 50¢ a unit. A set of points was about 4 times the price , let alone grinding a points cam and fitting a points plate .
Italian, German & UK engine makes all took them on but the USA just let their customers suffer till they could use the technology for free .

A lot of those Atom modules have made their way into older Stihl chainsaws with electronic ignitions that failed and ones with points too.

Atom were Husqvarna chain saw distributors and it was the short comings of the old points system fitted that led to the invention of the Atom units.
They never intended to sell them as a stand alone unit until they got the requests for them
Husqvarna sent a team out to investigate the new system and introduced it for the next season and Stihl did the same shortly after although they persisted with the CD points ignition for quite a while longer.