auto mechanic tries fixing a lawn mower, it goes horribly wrong!

[13aceofspades13]

one of my friends from church calls me telling me her lawn mower (a scotts 20/45 with a 20hp air cooled gasoline brigs and straton horizontal V twin.) is running funny, i head over the next morning to asses the situation, she describes the issue, the engine runs as if one cylinder is missing and before the event she heard a pop, like a professional mechanic i verify the issue and study my engine carefully, i turn the engine on, it starts, but it runs poorly, chugging, smells like its running rich, as much as i apply the throttle engine speed increases marginally, a good indicator one cylinder is not firing. before i make assessments of what the issue could be, i go over the possible causes, starting with the several main necessities for engine life.

proper fuel air mixture

compression

ignition

i started with the air filter, knowing this is a carburetted engine in which both cylinders share the same carburetor, so i ruled out the carburetor being the reason for one cylinder miss, and started with the most plausible cause of the issue, ignition system, first i removed the spark plugs, left bank black and charred spark plugs improperly gapped, but dry, the other was brown but wet, but had a decent gap set, i adjusted the spark plugs best i could, i didn't know the gap specs so i estimated roughly .030 inch, cleaned them a little, and re installed, looked at the secondary wires, felt them for kinks and bends, looked for cracks buldging deteriorating splitting or stretched look, none, to test which cylinder was firing and which wasnt, i pulled one plug wire, and left the one plugged, the left bank ran the same reguardless if the wire was plugged in the right bank, the engine would not start with the left bank un plugged, i unbolted the right bank ignition coil and did a comparison in resistance between the two and both came bout with about the same resistance, i even tried swapping them, they where both the same exact coils, even with the coils switched it was the same result.

i quickly checked compression, as both cylinders reached compression stroke they seemed to have good resistance, and both where about equal, i did not have a compression tester at the farm, so it was the best i could do to compare compression.

i did a quick engine run again and with the gap set and all terminals and electrical connections for the ignition cleaned the engine ran better, almost like a cylinder wasn't missing, but there was a cylinder missing, the right bank plug wire wasn't even plugged in.

next i examined the primary wiring (wiring heading towards the ignition coil, the left bank had voltage when the engine was turned, but the right indicated no voltage, i checked the continuity of the wire, and it seemed to stop after a splice, i re wired and spliced the primary wires with what wires she had (granted they where a little larger gage, re spliced the best i could, and to my dismay, the engine would not fire at all! i was basing my trouble shooting off a automobile ignition system, but this Briggs and straton engine seemed different, i looked at the splice i removed that had the right bank primary wire reading infinite resistance after it, it had one big wire that spliced off to two smaller wires, i looked really closely at the splice and i noticed it looked like it had two little resistors or diodes in it, i did a simple little test with my DVOM set on ohms, one way on the working side, the resistance was infinite, the other way it read about 9 ohms, there are probably diodes in it, i tried replacing my splice with the original one, and still neither of the cylinders would fire, in chance the resistance was critical to the ignition coils, i checked there resistance, and it was the same as before i replaced the splice, no damage done too the ignition coils, my mind raced through the possibilities, i could not think of a answer why the engine would not fire, i have put the project aside for the night, to think on it and ask for advice

could anyone here tell me what may be wrong based on what i have said?

 

[Gundamhead]

Most mower engine failures 101.

Bad fuel. Somebody goofed the oil/gas mix, or got water in the fuel. Dump it anyway, start fresh. Just do it.
Bad plugs. They need to be replaced every now and then, most folk never do. Just do it.
Bad wires. You got some of these, but they need to be replaced anyway, so it's on the 'just do it' list.
Bad operator. Somebody ran over a stump or rock and killed the engine. Any big nicks on the blade? Pull out the plugs and spin the blade. Anything 'feel' wrong? If so it's most likey the answer.

 

[Grendels]

Ace: Diodes are there to make sure the current in the wire only goes one direction. So if the driver for the coil is using an AC current, they probably just stuck in two different diodes, one allowing current in one direction, and then the other is in the opposite direction. There by allowing the current to flip back and forth between the two cylinders. That is an easy way to do the timing.

 

[TBadger]

In all my experience of fixing mowers, its just like Gundam said, Plugs/Wires/Filters/fuel mix.....If you were close I'd just give you teh extra push mower I have sitting in the garage since I bought a propelled one at Lowes. Same type, just all for parts now.

 

[13aceofspades13]

figured out the issue.

what looked like primary wiring was one of the causes of the issue, but actually i still have a bad igntion coil, talked too a small engine technitian in town and he said a bad coil on a lawn mower may not be indicated buy doing a resistance test.

what looked like primary wiring, is actually wires too the engines coils, which shut off the coils during shut down, the reason the engine was running so cruddy, was beacuse it was shorting out, causing the engine to want to shut down, i un plugged my wiring and the engines one cylinder ran, the engine would not shut down for a few seconds however, beacuse the engine would keep running untill the carb ran out of fuel when i shut it down, i am going to have to try to find a creative solution to my issue with this beacuse the engine should turn off emmediatly as i turn off the ignition, not 15-30 seconds after...