Cast Iron Briggs - No spark when hot

I have a 1974 or 1975 model 16 HP single cylinder B&S cast iron engine Model 326431 Type 0139-01 Code 7409261 on a 1975 model MTD 990 (145-990.026) with the Delco starter/generator and voltage regulator.

The engine will crank and run fine when cold. I can run it about 2 hours and cut it off to put some more gas in it and it will be several hours or the next day before it will have spark to crank and run again. I had bad points and/or condenser last fall and replaced them this spring along with a new spark plug. This problem started after about 10 hours of use. About a month ago, I cut it off to put some more gas in it and I never got it to run again until I purchased one of the later style coils with its on pickup (Oregon version (I think it was part #33-334)of the Briggs part#398811)and replaced the old coil (and disconnected the points and condenser) with the new coil (gapped with a "business" card provided with another Briggs coil I had purchased for another mower) and put in another new spark plug (gapped at 0.030). I have been told that on these old Briggs engines, the polarity on the flywheel is supposed to be reversed and Briggs wants you to send it to them to re-polarize it to work with the later style coils, but I found out that all you have to do is flip over the coil to reverse the polarity. I installed the new coil one way and it would not run, so I flipped it over and it cranked up. After I installed the new coil it cranked and ran fine when cold and I am still having the same problem trying to crank it when it is hot on several occasions. So far it has cranked again after cooling completely down to ice cold. The spark tester showed about the same amount of spark when cold with both the old and new setups, but the spark appears to be a little weak, more yellow than blue colored. I have tried it with all the wires unplugged from the ignition switch, so I do not think it has anything to do with the switch. The engine has several hours on it and is using some oil, but not smoking, but I do not think it is a compression issue or fuel issue, since it will not fire off of starting fluid. I have not put a tach on it, but I think the starter is turning the engine fast enough when it is hot, since I think I have heard that the new style coils need about 700 rpm to produce a spark.

Any thoughts on what the problem might be?

Coils fail when hot! I should know the mechanism but I don't. I know the permeability of the iron is reduced with temp and this could be the problem. Or if you have some shorted laminations in the core but how much and how much for no spark-weak spark I don't know.

Other problem could be old coil, varnish worn off wires, when hot copper expands and bare wire to bare wire shorts. Only takes one turn to kill a coil. Reason is volts per turn. If you don't have any volts across a turn, the coil pumps all it's energy into that turn trying to bring things back to equilibrium so you have no energy left to dump into the plug.

Thanks, Texasmark1, I was also thinking about a coil before I changed to the new coil because of a weak spark and I was wanting to get rid of the points anyway. But the spark when cold was about the same with the new and old coils. I suppose the new coil could be bad, but I would assume that if heat was the problem, it would have died on its own before it ran for 2 hours straight. But every time I cut it off or it runs out of gas, it will not show any spark until it is ice cold if it has been off over a few minutes. The first time it happened, once I got it cranked, I cut it on and off several times in a row and it seemed fine, but once it gets some heat in the engine it will not fire up again. I guess just a few minutes difference in heating and cooling of the metal and/or copper windings could possibly cause enough of a gap between something or the touching together of something that would break a connection of create a ground that would affect the spark. I would not think that the flywheel gap would change enough due to heat expansion and contraction to create a problem. I have heard that sometimes the magnets on the flywheel (its 44 years old)can get weak and cause a problem, but I am not sure what affect it has on the spark, and I do not know if heat will affect the strength of a magnet?

How about the "kill switch" circuit... must be a terminal on the engine where a wire from the coil primary and breaker points connect, then the "kill switch" on the machine connects to that.

Have you tried running it with the kill"" wire from the machine disconnected?

Also, verify that the wire that leads to that terminal from the coil isn't being inched, or contacting a a cooling fin and shorting under certain conditions.

You can look up the properties of Alnico 5, the material of which most utility magnets are made. The magnetic curves vs temp should be posted. In short, if your magnet was weak hot, it was also weak cold. I'd look elsewhere.

How did you test spark? Spark plugs are said to have a tendency to be bad at high rpms and or high heat settings. Don't recall whether or not you included a new plug in your search for a solution. On the failure mechanism therein I have no idea.

These are good points that I didn't address as I wasn't aware of the type of ignition circuit used in that older machine. Surely, if the coil is actually a magneto and has a kill wire grounded when the ignition switch is turned off, the problem could very well be as you indicated and surely requires investigation.

The kill wire in current magneto type (so called Magnetron) ignition systems, do what I said happens with a shorted turn. Grounding the kill wire in the ignition switch is in fact a deliberate "shorted turn". Older switches that have been exposed to usage and the elements can develop metallic tracks from contact movement which can cause shorts. The other thing is that Phenolic resins are used in most switch circuit cards used on most rotary switches. Phenolic resin circuit cards absorb moisture and become conductive.

The"Magnetron" is a microwave power tube developed around the time of WWII primarily as part of a light weight, compact, high frequency RADAR transmitter.

The trigger for the coil , might be in
wrong place as you have flipped it, ?
If that?s not it , then it might be over heating
you have to adjust the mixture while it is
under load , too lean = too hot

I want to know what you find, same 16hp Briggs in my '73 Simplicity did similar, ran a battery ignition and found the problem was likely the exhaust seat coming out of the block once hot.