Hello again.
I took your advice and started with the ignition wiring. I tool the switch apart, dislodged every wire from it's port in the wiring harness, and cleaned the heck out of them all. Tested for continuity with as much as I could get to. Started up the engine and it seemed to run much better for a few seconds, then back to missing and popping when I released the cluth/brake pedal. I'm not done, with my testing/cleaning, but came across something else that baffled me.
There is one single black wire that goes pretty much from the main wiring harness, behind the starter, then becomes to separate wires that go to the two "Armature-magneto" #691060 (according to Sears Craftsman part names/numbers... thing that has the spark plug wire attached to it). When checking continuity on this wire, I get about 4 ohms (if I'm saying this correctly... the needle jumped about half way instead of pegging at zero) on the wire that went to the closest coil "Armature-magneto". I wasn't too concerned because I was at least getting something. When I checked continuity on the wire that went to the other sides coil, I didn't get a signal at all. I mean nothing! Needle didn't even jump a tiny hair. After looking into the splice area, I discovered that there was strong continuity on the single wire, but when it went into two, there were devices in each line (maybe resisters, or capacitors... of which I have no knowledge at all). From this point on, checking for continuity again, I still had nothing on the one wire. If I skipped over the inline device, continuity was again strong. Does any of this make sense? Was this designed that way? What are those little devices, and what do they do? Can't understand how I don't have continuity, yet engine would occasionally run a bit smoother. I look forward to your response if you have some time.
Model #917.272450, 21.5 HP B&S Intek Twin.
Thanks again.
Jim

{Couldn't find old post that you answered for me, but here's how it went for FYI and memory to my situation}:
I got an 02 Craftsman LTX1000 for free. Very happy. It ran great (really purred) after I found the bad fuse that got me this freebie. Seat sensor never killed engine when I lifted off, but would make engine run rough. Otherwise ran great. Now, it's running rough all the time, as if my butt is not on the seat. I can pull the choke a little to make it run slightly better, but still not even close to right. I heard stories about Craftsman having bad wiring from the factory, but this one was good for years. Gave it a general tune up... plugs, fuel & air filter, cleaned the carb & intake, blew out dirt and dust from everywhere, etc, also replaced cracked leaking fuel tank with brand new one. No different. Jumpered the seat sensor, no different. Engaged the mower deck sensor, no different. (blade brake doesn't engage when releasing blade engage lever. Maybe that's another question) It runs slightly better when depressing clutch/brake pedal, but still not purring. Also, I get more popping/backfiring when blades are spinning and pressing/releasing clutch/brake pedal. So, I guess my question is this: Has anybody heard of this before, and could all these problems be linked to a bad safety sensor either under the seat, mower engage lever, or clutch/brake??

Another interesting problem is ignition switch. I was told the old one was bad so it was replaced with brand new one. That may be true, but the engine continues to run for about 5 seconds after switch is turned off and key removed.

I've worked on this thing for a while and can't find a cure, and don't have money to try replacing things that might not be the main problem. Please help. My grass is getting out of control.

{And you answered}:
Hi,
I was mystified for quite a bit of that explanation, thanks by the way for being thorough - those one and two liners are not always a lot of help. It was one small thing you said that was the key!
Quote:
The engine continues to run for about 5 seconds after switch is turned off and key removed
Was the clue.
You definitely have electrical problems. They are somewhere in the ignition ground system (that is the circuit that grounds the ignition to shut off the engine.) Now that clue, that time delay means that the engine shuts off when the fuel shutoff solenoid shuts and then the fuel in the circuits runs out. That is what causes that five second delay. Cannot say where it is but I would look between the engine and switch. I would say this is why the safety circuits do not work properly since they use this circuit, also.
Peace,
Clarke

Hi,
Well, you have been busy, and done your due dilligence. I think I may help in that all you told me says that you have on of the hateful situations where fixing one problem leads you to another; in other words you have more than one thing giving you this situation.
To explain what I am saying; you have one functioning ignition ground and one not in that wire that has you puzzled. It needs to be fixed, but doing that will make the machine run badly all the time if at all. Why? Because you also have a bad safety switch! How? When it runs badly that is that switch intermittently tripping and killing the ignition to the cylinder that has the good side of the ground wire. When you cut the switch off, I would bet that the sound of the engine changes a bit (running on one cylider) before the fuel runs out because of the solenoid. That is because it only shuts off one cylinder when you turn the switch off.
That wire will have to be replaced; but first you will have to figure out which other switch is giving problems. I think your key switch is probably sound (no guarantee, but seems likely.) That will leave the seat, clutch/brake and PTO switches. Seat switch would be doubtful; I think it does not work because of the ignotion ground wire (probably on cylinder again.) The other two both appear to be the same switch (#176138) used twice and located on the right hand side of the machine. Cannot tell by the parts drawing it they are behind the engine and to the rear under the dash, or beneath the machine. Would not be surprised to find one beneath the dash (blade) and one beneath the running board (clutch.)
No matter which, I would look at them, maybe mark them, draw a note of wire locations (black is the one in question), unplug them, remove them and test the side that the black wire was attached to. I said mark them to know where each came from. Check the operation for the side with the black wire with switc in both in and out positions, check for too much sensitivity (wiggle the button), see if in too far or slightly causes problems. It is in one of them!
Wiring diagram attached as picture and pdf document (I like pdf better for printing.) I have the manual, but it is too large, is available from Home Improvement Projects, Product Manuals & Home Management Tools - Managemyhome.com and is the only one I have seen there twice.
Peace,
Clarke
PS To find your old post use the Search, but start with Advanced and the right hand box to Search by Poster; search for yourself.