BTW - have I mentioned how much I love you guys?

Forgot to mention - when the mower sits for a week between uses - when I fire it up, theres a short puff of black smoke that comes out of the exhaust - almost exactly one puff - The oil is never low though, so it can't be burning much of it. I had just cleaned the plugs back in April, the one on the running cyl was a little blacker today than normal - cleaned it again - the one on the non-running cyl was much cleaner (hmmm, that's one way to make em last longer!)

"I'm assuming I can't just swap the wires from left to right side"

You are right.

There are a number of things it could be. From your description, I think blown head gasket, which is common on these engines, is not the problem.

This engine uses an ignition coil for each cylinder. These coils are connected with a weird "Kill" wire system with a couple of diodes in it to keep the coils from shorting each other out but provide common kill wire for both. When one of these diodes fail, it causes several puzzling symptoms. Since you say it does not have fire at one plug wire PART of the time, I would suspect this harness.

The only way I know to tell for sure is remove the cooling shroud, disconnect the Kill Wire Harness from the spade lug on BOTH coils. Re-install the cooling shroud and see what happens. The normal way of turning the engine off will now not be available to you so you will have to turn off gas, pull plug wires or something that way.

One of the ignition coils could be failing but this is not common.

Walt Conner

Nice description.

So far with all that youve mentioned - i suspect your engine is in good shape ... Mostly a spark problem.

That puff of black smoke is you probably choking the carb while starting the mower and your seeing too much fuel in the system - due to the misfiring cylinder. Remember black smoke - too much fuel / white smoke means your burning oil.

Once you get this ignition problem resolved - you might consider changing the oil ... This will ensure you get any raw gas out of your crank case - which might have been pushed past the piston rings.

Electrically im pretty weak - but I am thinking its 1/2 half of your system - perhaps one of the 2 coils? I'm not sure if coils can be swapped from left to right positions.

I think I would start by pulling off the engine shroud and looking for ground shorts, mice nests, chewed up wires, toe nail clippings, and the eternally missing - left sock ... Then perhaps i would try and swap the coils from right to left.

On my B&S twin - I can start & run the motor with the shroud off... I wouldn't do it for long as you need the shroud for cooling. but you can certainly run your quick diagnostics with the shroud off.

Once your all fixed up - dont forget to gunk down the motor and clean the cylinders real good... Your already there - might as well do it.

Good luck

I've got a huge fan from a chicken house I can put on the mower if I need to for cooling (it will nearly knock you off your feet if you cross in front of it) - I'll go ahead and pull off the shrouds as suggest by both of you, rejiggle the wires in there I can't get to now - and if it has two coils, both the same part #, I'll try swapping them left/right to see if it makes a difference. I'll also try Walt's suggestion of bypassing the kill to see if that's the issue. So far, it seems like something I should be able to do without accidentally welding on a piece of farm fencing (dont ask)

Thanks for the replies. My wife has started calling me a lawnmower nerd since I spend an hour a day for the last month just reading here...

The 400707 is the old opposed twin which I believe was rated at 14 hp. This engine had one coil with two sparkplug leads. The sparkplug lead that is dead - may be leaking to ground. Look for damaged insulation.

My mistake, I was thinking of the 407707 which is a V twin OHV engine. Forget all that diode in the Kill Wire Harness business. Post above is correct, 400707 is an opposed flat head engine. It does fire both plugs at the same time from a single CDI Unit. Either wire would work IF it was long enough to reach which one won't be.

Are you sure it is ignition? The valve seats have a habit of coming loose in these engines especially if the cooling fins are plugged as suggested above.

By the way, B&S cautions against running this CDI with on wire disconnected.

Walt Conner

Walt/Larry - Noted, thx for the additional info. Tomorrow is designated troubleshooting day.

Other good news - I found a 10 acre lawnmower junkyard within an hour of my house...woot!

Suggested troublshooting not yet fully completed, but here's some updates. With the new knowledge that I CAN swap the left/right plug wires, I did. Exact same problems I had before, but as you probably know, exactly reversed. Which means:

The cyl/engine is still basically 'good' (humor me there, I like the mower!) - it WILL run on both cyls, just not at the same time. Doh! I swapped the wires back, and the original non-firing cyl was back where it was previously. Glad to know I'm not crazy. I pulled the plug from the good side, grounded it against the block and hit the starter after turning out all the lights but one in the corner. Good looking spark!

I replaced the plug/wire, and pulled the other side to try the same thing. This time, I used a pair of insulated pliers to hold the plug against the block. I had a flashback of working my my stepdad's garage when I was 11. The wife and the new baby were VERY happy to hear less cursing this time, but I still cursed a little - because it appeared to have similar spark to the other side. Did both sides again, making the wife watch and she agreed, appeared to be similar spark.

I checked all the wiring, no obvious chew marks/rips/tears/breaks. Amazing how well it's held up over the years, why cant my cars do that? Oh yeah, they get used more than once or twice per week.

I do have this odd little tool that everyone I mention to thinks I'm crazy - something my father-in-law gave me to check for bad wires on a car. Looks liek the top half of a clicky ballpoint pen - you rub it up and down the wire while the vehicle is running - if it lights, you have a crack/etc. It didnt light at all on either wire - even though I now new both are getting spark. And appearantly gas.

so - what the heck?? I came home during lunch today and threw the battery charger on at 2amps for about 6 hours, if anything it has a slightly over-charged battery (charger came off an hour before my testing)

So - spark appears good. Must be getting gas or wouldnt run at all on that side regardless of wire. Plugs are good for same reason (but I did switch them and clean them again.) So, the coil has two leads coming out of it - do they both get the magic spark of life from the same place, or does it have two sets of internals, and one side is going bad? I can find 100 sets of instructions on how to test coils but appearantly the wife's rabbit DID eat the leads to my multimeter - so other than the wire to the tongue test, I guess I'm done for the night (note - rabbit was never in the same room as the mower - ever.. I know they love to eat wiring and the 3rd time it ate a computer cord belonging to said Wife, she agreed that bunny is perma-banned from the garage and basement under penalty of stew.)

I'd still try a new spark plug or 2.

FYI - 2 new plugs didn't correct the issue.

I did find someone else with an acre or two of old mowers a few miles down the road - Stated he's sell me a used coil to try for $15 - and to feel free to keep bringing it back and swapping for others until I was satisfied I had a working one, lol. I'll take him up on that this weekend, after I spec out my existing one with a shiny new multimeter from RatShack. Thx again for all the suggestions, and keep em coming if I've missed something.

yaaaaaaaay! Problem seems to be resolved. Or at least until I cause another one by the 'fix'. Stopped by one of the junk yards on the way home from work, gave the model # of mower/engine, asked for a used coil. Gentleman (who is down to around 3 fingers btw) stated that they were 'all the same' in that line, and handed me a two lead single coil off of an 18hp Rally mower, also made by AYP in rought the same year - mine's 14hp. Shrug. Took it home, removed cooling fins, removed old coil, could NOT get the dang funny looking clip holding the ground wire on, and it came apart in pieces. Checked 'new' coil, it had a simple lead that I could squeeze a cheater onto for the wire, so I did that for testing.

Started mounting screws, turned flywheel until magnetic part made me nearly crap my pants as it grabbed the coil. Peeled it back a hair, centered it on the magnetic part, stuck a piece of matchbook cover between the two edges to keep it from rubbing (close enough for government work?) - tightened everything, put on one plug lead and hit the starter. It fired! Tried the other wire, same plug, cyl, it fired! Tried both on the other side, same results. Tried both, and holy he:: I must have flooded it cuz there was white smoke everywhere for a few seconds. BUT, it was running, and on both cyls! I put it back together, and went and mowed my side yard. Perfect! Now I just need to tear it back down, pull the coil back off, and solder on that ground wire to the lead. Otherwise 2 months from now it's either going to shut down, or not shut off (I'm not sure which will happen), or cause the Earth to explode, and I won't remember why.

Thx for the advice and direction - I'm a nerd by trade, but not afraid to tear something down (as long as I think someone else that does it for a living can manage to put it back together...)