Do you have a link to the electrical schematic?

My first guess would have also been the key switch.

You should be able to better date it from the engine CODE, if original.
YYMMDDzz
Assume the engine is a "few" months older than the tractor.

I went out today and looked at a few things. It started back up and is still running poorly. Both cylinders have spark. The right hand side (looking from the rear) plug was jet black. It did have spark. I replaced that plug and it still ran the same. I checked the plug on the left side and it was not as black but it did have spark. I am not sure if it was my imagination or not but the right plug seemed hot when I took it out and the left plug was not. There was a few minutes between as I changed the first one. I am taking it to a shop in the morning. If anybody has any ideas I am still interested as I would like to be able to point the shop in the right direction to help keep the labor cost as low as possible.

You might post the Model & Type of the engine so one can better identify it.

The "behavior" when running wants me to think carb or valve problem.
The behavior when turning the key off points to ignition.
This should be a challenge, to say the least!

A link to a schematic???????
At this point were kind of at "my V8 Chevy is acting up, what's wrong"?
We need more details

Locating engine numbers:

Here is a link that might be useful: Model #

and/or

Here is a link that might be useful: B&S #

I found the following numbers on the engine.
Model 446777
Type 0115 E1
Code 031 028YG
ELS 656 OHV.
I am going to take it to a shop in the morning. I was hoping that I could/can pinpoint the problem as something common that people have seen before, helping to troubleshoot it before I take it to the shop. That way I could save a little on the labor bill in case they have to dig around for a while trying to figure it out.

Some of the experts (like mownie for one) seem to only check the posts in the mornings. If they don't chime in before you take it in to the shop, they may be able to give you some ideas before you ok the repairs.

That wasn't meant to imply that Bill isn't one of the experts. :)

I'm NOT an expert-
I'll defer to Mownie, Tom Plum & Walt Connor anytime.
They ACTUALLY work on these.

The only thing that I can think doesn't apply to this engine.
Apparently, some older opposed twin Briggs had some kind of diode(s) in the kill circuit. Even then, I'm not sure I could make a "connection" to this problem.

About the only thing I would do is see how it acts with the fuel solenoid wire getting disconnected as a "kill connection". Maybe the engine reaction or not might point somewhere. Just musing tho-

Isn't there one "switch" (between the ignition switch and the afterfire solenoid) that controls both the current to the spark plug and the current to the afterfire solenoid? Is it possible that that "switch"'s malfunction could cause both a delay in ignition/spark cut off and improper main jet/pilot jet balance during operation? Really, really musing here.

This post was edited by grass1950 on Tue, Jul 30, 13 at 0:38

Carb solenoid takes 12V to "activate".
Coil/armature takes 0 volts. It has a grounding tab to kill spark.
It works just like your walk behind lawn mower.

There could conceivably be an Operator presence Relay that could open the circuit to the fuel solenoid and ground the coil, but the key switch does both anyway..

The first scenario could be used with a seat switch however.

The magneto kill circuit functions by grounding the primary circuit of the magneto when you turn the key switch off, or if a switch in the machine safety interlock system calls for engine shut down.
The Briggs Vee twins have 2 magnetos because the cylinders are arranged in an uneven division of 360 degrees, so each cylinder must have its own magneto in order to operate.
And.....because each magneto has to function independently of the other, the kill wires going to each magneto must be electrically separated from each other also.
To accomplish this separation of the magnetos and the kill wires, Briggs incorporates 2 diodes into the kill wires. The diodes permit the magnetos to function as separate entities for engine operation, but still allow for killing the magnetos with the key switch or an interlock switch when it is necessary to shut off the engine.
Here is why this engine is acting goofy.
Briggs magnetos will continue to make spark as long as the engine is turning.....unless the kill wire circuit becomes grounded......(by turning off the key or interlock activation).
Normally, when you turn the key off, 2 separate things happen that shut down the engine rapidly.
1...the magneto kill circuit is grounded by the switch so the mags instantly stop making spark.
2...the fuel solenoid valve (anti-afterfire valve) is deprived of its 12v power from the ignition switch, but is still being fed by the output power of the alternator, so at first.......the solenoid valve stays open.
But because the engine ignition is no longer making spark, the RPM is dropping rapidly.
As the RPM drops toward ZERO RPM, a point will be reached where the alternator is no longer making enough current to keep the fuel solenoid open, and the fuel solenoid finally closes when the engine speed falls to around 500 RPM or less.
Any fuel remaining in the engine and muffler after the fuel solenoid closes will be swept out through the muffler and a loud bang is averted (by the purging of residual fuel).

Now, this is what is happening in the issue taking place with the subject engine.
One of the diodes in the kill circuit is defective. Don't know if it is defective OPEN or if it is defective SHORTED.
This is causing one of the cylinders to be dead during operation, but because the kill characteristics have been affected, the cylinder that is dead when the key is in the run mode, starts firing when the key switch is turned off (because the kill circuit is unable to ground that magneto.
So the engine is now running on the opposite cylinder because it now has spark, and the carb continues to feed fuel to the engine so long as engine speed is above 500 or so RPM...........so it takes a lot longer for the engine to actually shut itself down.
For this engine (according to the Type number of 0115) you need to replace the Briggs harness, part number 691220.
That harness has both kill circuit diodes in it and will be a lot easier than trying to test out and replace individual diodes.
Replacing diodes is of course possible, but you would need to obtain diodes at an electronics type store ( e.g. Radio Shack) and the diodes would necessarily have to be installed with the correct bias (polarity).
I think you will be more comfortable just replacing the available Briggs harness.

By the way, Briggs Opposed Twins (boxer type engine) DO NOT have these diodes because by virtue of the 180 degree opposed layout of the cylinders, BOTH cylinders are fired by a single magneto, and in fact.......BOTH CYLINDERS receive a spark at the same time. The reason this works is that one cylinder is on the compression stroke while the opposite cylinder is on the exhaust stroke. This is known as "wasted spark ignition system".
Actually, ALL magneto ignition systems that are powered by the flywheel magnets throw a spark 2 times in a complete 4 stroke cycle (2 revolutions of the crankshaft, or 720 degrees of rotation).
The sparks occur near TDC of compression stroke and TDC of exhaust stroke.

This post was edited by mownie on Tue, Jul 30, 13 at 1:07

That's exactly what I said. Except I like to be more precise and not so "wordy." Yeah, that's the ticket. Yep, what mownie said, Ok, so I had no clue. My head hurts. G'nite.

Couldn't sleep. I've got to ask. Why does the dead cylinder start getting spark and start firing only when the ignition is off when it wouldn't fire when the ignition was on? I understand that it is able to fire because the kill characteristic has been diabled, but why does it generate spark at low speed and what happens that finally causes the engine to stop?
Thanks mownie.

Mownie- Thanks for clarifying the diode issue.
That makes sense.
I just had it backwards.

This is going to be my discourse for the week, it's long and "winding".
***" Why does the dead cylinder start getting spark and start firing only when the ignition is off when it wouldn't fire when the ignition was on?"***
The cylinder that is essentially dead, may actually BE FIRING.......but because it is CONNECTED somewhat to the OPPOSITE magneto by the defective diode, it may not be firing a full spark.....and/or the spark it is firing is likely so far out of range (regarding spark timing for that cylinder) that it is only lighting a "candle" instead of a "firecracker". The cylinder that is being affected by the defective diode might even be firing at a time that causes it to "fight" the opposite cylinder.............and that right there is probably why the engine seems to "smooth out" when the key is turned off.

As to the effect of having a defective diode that has CONNECTED the 2 magnetos together at a time when they should be separated.....picture this.
There is just ONE magnet on the flywheel that is responsible for powering BOTH magnetos as it passes by them (but not simultaneously, because the mags are located at different places around the flywheel).
Let's call them magnetos 1 & 2.
A magneto has 2 SEPARATE coils of wire inside, the primary and the secondary. The primary coil is comprised of relatively large diameter wire while the secondary coil is made of extremely small diameter wire.
Normal operation: Flywheel magnet flies past magneto 1. Magneto is saturated from the magnetic field and current is induced in the magneto coil windings. The primary coil (being made of larger wire) makes the best use of the passing magnetic field and thus generates a "higher amp" current than the smaller wire in the secondary coil.
The "trick" or magic to a magneto making a spark is due to interrupting the primary coil circuit while the coil is at its moment of "maximum saturation" by the passing magnetic field (flywheel magnet).
Older mag systems used breaker points to interrupt the circuit, modern mag systems rely on electronic means to interrupt the primary circuit.
Anyhow.....it is when the primary circuit gets interrupted (circuit becomes OPEN) that the spark for the plug is created.
The spark is created in the secondary coil due to the collapse of the ELECTROMAGNETIC field that arose from the primary coil being "washed" by the passing flywheel magnet.
The electromagnetic field that sprang up to surround the primary coil........also grew (expanded) to encompass the secondary coil windings. When the primary coil circuit gets interrupted, the electromagnetic field collapses and the electromagnetic force (EMF) literally falls back toward the primary coil center..............as it falls, this EMF passes through the windings of the secondary coil..........and it is the passage of the EMF through the secondary coil that causes a HIGH VOLTAGE current to seek a path to ground in order to dissipate, hopefully.....that path to ground will be across the spark plug gap....... and your engine can run.

To stop a spark in a magneto ignition system when you want to kill the engine......you provide a SHORT to ground in the PRIMARY coil circuit. When you short out the primary coil, it is unable to utilize the passing magnetic field of the flywheel magnet, and so there can be no spark created either.

How does all THAT apply to the Briggs Vee twin dilemma in this thread?
Take another look at how the magneto ignition is laid out on this engine.
As the flywheel magnet passes magneto 1, all that stuff I described above happens, and a spark occurs in cylinder 1.
The flywheel magnet next travels on to magneto 2 and repeats the entire process, with a spark in cylinder 2.
Now, while all that EMFing was taking place, consider that the kill wire circuit that is required to STOP sparking, was not affecting anything, because the kill wire circuit function had not been invoked by the key switch or interlock, and EACH magneto was permitted to just do its magic without any interference from anything, because the diodes in the kill wire circuit keep each magneto blissfully unaware of each other.
That is how a Briggs Vee twin mag system in good order operates.
Now, lets toss in a defective diode in the kill wire circuit that connects............but does not really CONNECT......the 2 magnetos together.
One of the diodes that separates the magnetos ELECTRICALLY....suddenly develops a problem.
This might mean a lot of different things because a diode can "go open", become shorted", or its value and bias might even change, or some combination of those..........which can cause "crazy things" to happen in the ignition.
Let's suppose that one of these defects has caused the primary circuit of magneto 1 to become at least partially connected to the primary circuit of magneto 2.
The upshot of this is that ANY current induced in magneto 1 will be dispersed into the primary windings of magneto 2.
The resultant EMF will be present in BOTH magnetos, but will be only about half as powerful as it should be, and a weak spark MIGHT be delivered to each cylinder, but cylinder 2 is "too many degrees" out of phase to make use of any spark at this time.
Now the flywheel moves on to magneto 2 and the EMFing occurs there. But this time, everything happens as it is supposed to....................because the defective diode that connected magneto 1 to magneto 2 earlier, IS NOT able to connect magneto 2 to magneto 1...................because the SECOND diode in the kill wire circuit prevents the defective condition to be passed backward to the other magneto.
This "separation of powers" that must be present in the kill wire circuit requires 2 diodes to work properly. and both diodes must work properly in order for the system to work properly.
And finally..........the dead cylinder BEGINS firing when the key switch is turned off because that magneto is no longer influenced by the opposite magneto at this point, because the opposite mag primary circuit has been grounded by the key switch. The formerly dead cylinder now comes to life because it IS NOT GROUNDED (due to the defective diode) and will continue making spark until the engine speed drops below about 350 RPM (the minimum RPM for Briggs Magnetron to function).
I apologize for causing anybody to fall asleep or if you got a headache.
Just think how my head is spinning after writing this.
If the forum ever implements a "word limit", I'll be in trouble. :^)

This post was edited by mownie on Tue, Jul 30, 13 at 12:58

That's where experience AND education comes in-
You've seen the problem and are smart/educated enough to understand it.

I kind of thought that was what was going on but.....
Honestly, I am clueless but have a basic understanding of what you are saying. I printed your first respons of and took it to the shop that will be looking at my mower. The guy agreed that you are on track and commented that it was nice that I found somebody who knows what they are talking about. Thanks for the help and once I get the mower going again I will do an update.

Thanks mownie,

Your expenditure of so much time and sharing of knowledge is much apprciated.

Mownie, I read every word you posted above. I have a headache. I'm gonna lay in the hammock and drink beer until I get up the courage to go look at my Briggs twin and see if it's one of those weird-o's. On second thought, I think I'll just stick with the hammock routine.

Take two diodes and call me in the morning. :^)

Trying not to leave you guys hanging. However the wiring harness is on backorder from one place. I have contacted another to see if they can get it any quicker. As soon as I have it up and running again I will let you know.

Thanks for the update. Looking forward to hearing if that cures what ailes you.

My mower guy called me today and said he had the mower done. The diagnosis above was 99% correct. Turns out a mouse had a nest built and had chewed threw one of the wires near were the diode was. So Mownie was dead on with his diagnosis,obviously no way to predict that over the diode being bad. He also found a few little minor things, nothing of which were causing the problems. There is a square block were the large wire goes into the starter. That was loose and about to come out. He said the only thing he cannot figure out is that it is running a little rough at idle, but as soon as you give it some power it smooths out and runs fine. He said that there was no adjustment on the carburetor to try to smooth the idle out.

Non adjustable carbs are very much limited to being "theoretical" devices.
Meaning that without any means to compensate for lean or rich mix, it works nearly right most of the time (keep fingers crossed).
Carbs are actually "multi stage" devices having specific fuel circuits that come into play or become active at different points through the operating range of RPM and engine loading.
At idle speed RPM, the carb is using only the idle (aka "pilot") jet circuits for fuel and air because the throttle plate is nearly shut and no venturi vacuum is offered to the main fuel jet circuit, so that big ole engine is basically just stumbling along on the meager amount of fuel and air available through the pilot circuits.
Open up the throttle just a little bit and you begin to allow the engine to start sipping fuel through the main jet circuit and the RPM will increase some. It is still running off the pilot circuit but now it is also getting fed by a portion of the main fuel circuit.
Now, jack that throttle plate wide open allow and the engine to draw in the maximum amount of air it can and the venturi vacuum will allow the maximum amount of fuel to flow through both the pilot jet and the main jet, and RPM will rise to the governed RPM. If the engine is loaded lightly, the governor will close the throttle plate to keep the RPM at the set point with the least amount of fuel needed to maintain that RPM.
If the engine load is increased, the governor will sense a drop in RPM and in response will open the throttle plate to increase the RPM.
If the engine is under its theoretical maximum load, the governor will hold the throttle plate wide open and the carb will be delivering its maximum possible fuel flow, and engine RPM will be steady.
Of course, a host of variables can change things (and do) in actual operation.

Thanks for the info. I got to use the mower today and it was running smooth, starts good, and shuts off like expected. So the major problems are fixed. The thing I noticed is that the max RPM's are not where they should be. I think that might also be why it is running a little rough at idle. I jumped off the mower at one point and reached up and pushed a little on the throttle cable were it attaches to the carb, and it sped up a little. This problem could have started before the day it first really acted up on me. I noticed it was not quite running as fast as it should for about 15 minutes before things went crazy. Any ideas to troubleshoot the lower then normal engine speed when the throttle is all the way up?

Make sure that the cable clamp which anchors the throttle cable casing to the governor swivel panel has not slip forward a little.
If it has done so recently, you may be able to discern a "cleaner looking" spot on the casing just ahead of the clamp.
Having the cable casing slip is equal to moving the throttle lever down to a slower speed position.
The throttle lever on the dash panel can be all the way up to highest speed position but if the casing has slipped under the clamp at the other end, well.......you may just THINK you are at full speed position.

This post was edited by mownie on Sun, Aug 11, 13 at 21:08

Thanks, I was/am hoping it is just something a little out of whack. Once I manually raised the RPM's some I figured it was not an internal issue with the engine and likely something in the cable. I noticed the throttle lever seemed a little stiffer then normal when I used the mower. Could that mean anything?

I hate to hazard a guess on that stiff feeling, but it might simply mean your throttle cable is in need of lubing.
The pivot hinge for the actual the throttle control lever in the dash might also need some lube.
Some machines have an adjustable tension feature on the throttle lever to help keep the throttle from backing down due to vibration, others simply have a notched or serrated disc that acts kinda like little teeth or steps to keep the throttle from moving.
I would try to oil the cable and pivots all the way from dash to carb.

I found the clamp and made an adjustment and I think it is running at proper speed again. Thanks to all who helped. No doubt it saved me a few dollars in labor hours being able to give my mechanic something to look for. He is a friend of mine today when I went to pay him he tried to only charge me for the parts. I gave him an extra $30.00 to get a couple cases of beer. Still a bargain but he wouldn't take any more.

Glad to hear your back in the sadle.
That's always been a conundrum for me. Friends doing things for me at no charge for their time. Only fair that I return the favor. But how do you calculate friendship barter? A friend who is an auto mechanic will spend four hours fixing my car. He probably makes $18-25 an hour, but I know the dealer he works for charges $70. Low end charge per hour in my profession is $150. When he is in need of my service do I give him a $100 dollar discount or a $240 discount?
Forget the monetary value, It was 4 hours of his life, why should four hours of my life be more valuable? it's not. Life is finite. If necessary, even spending up to an hour or two more than 4 hours of my time in return makes me comfortable in calling it even.
Anyone have a take on this?

I think you are right "trying" to keep the see saw balanced using an hourly exchange instead of "$ per hour".
I would opine that if he spends 4 hours working on your car (for example) you spend 4 hours of your time working on his problem (should that ever happen).
The rates of pay, or fees, or charges, or whatever you want to call it.....are largely subjective (to all kinds of marketing pressures/factors) and are really just somebody's opinion of what the traffic should pay.
But as you stated, 4 hours (of someone's finite lifetime) spent for your benefit is 4 hours they could have spent doing something for themselves instead.
So, if the scales would ever be balanced, it would have to be in "raw time", not some arbitrary $ amount.
The tougher (for me) aspect of bartering time is trying to categorize persons that I might be "bartering with".
Some persons are simply enjoyable to be around, no matter what I'm doing for them (or they are doing for me) and I place a higher value on the mutual friendship than could ever be expressed in monetary terms.
Realistically speaking, this subject is much deeper and complex than what can be addressed in brief.
The subject almost always has some factors rooted in benevolence, and even "familial altruism" (when it comes to relatives and in-laws/outlaws.

well, yeah.... Mownie, me friend... but thet don't mean we is gonna start no kissin stuff, no ! :)

Roger that...me no kissy kissy!

Do you guys know what the normal RPM range should be at full throttle on this engine? I adjusted the cable last week and just ran it for a few minutes and it seemed fine. I mowed my yard with it yesterday and it still seems slow. I notice the sound is not quite what it used to be and it doesn't seem to throw the grass very far and it is bogging a little easier then it should. My friend that worked on it for me has a Vibra Tach which I was a little skeptical of but it seems to have some what decent reviews. Anyway I checked it with the Vibra Tach and it was reading 3200 RPM. I used the Vibra Tach above each cylinder on the engine just to see if it was consistent and it read 3200 on each side of the engine. I looked again and confirmed that the cable is in the right position and the throttle linkage is all the way against the stop as the manual says it should be.

When I was shopping for repacements, I don't remember seeing any with a maximum RPM greater than 3600 with many at 3300. The governor is going to determine the max RPM. Some owner's manuals for the mower will have the governed RPM listed in the carb adjustment section.

Briggs usually defers to the equipment manufacturer as to governed RPM, but typically, as has already been mentioned, 3,600 RPM is the max recommended speed.
Most gen sets have to run at 3,600 to provide 60 hertz output power, so 3,600 certainly ain't gonna harm it.
In lieu of a real spec to go by, I usually set them to around 3,400 or 3,500.

flyin-lowe,
I was able to improve the throtle response on my mower by using a rattle can carb cleaner on the linkage and down the carb throat. The new filter and pre-filter probably didn't hurt either.

I will try that. I could be wrong but I feel like if it was something like a bad plug, fuel filter, air filter, etc. I would see other symptoms other then low RMS at WOT. I feel like it is more of an adjustment issue somewhere. The mower is starting and running smooth, and I feel like if it were a plug or filter issue it would miss out or surge or something else other then just running very good but not reaching full RPM's. Am I off by thinking this way?

You can adjust the "governed" maximum engine speed be reducing or increasing the governor spring tension. This can be done by either adjusting the governor spring anchor tab, or on engines that have adjustment holes in the governor arm, by moving the governor spring into a hole further away from the governor shaft to increase max. governed speed. This will have no effect if the engine isn't reving high enough to activate the governor. If you don't see an increase in max. governed engine speed, then check for a deficeincy in air and/or fuel intake which can prevent an engine from reaching full throttle. This is why I suggested that you clean the carb. external linkages and throttle plate/throat to make sure they are not sticking/binding and preventing full speed.

Hopefully this is my final update. My mechanic friend stopped by the house today with a tool to adjust the governor. He made small adjustments and we gradually increased the RPMS from 3200 to 3600. It sounds like it used to now. I haven't mowed with it yet but it is running smooth again. Is there something that typically happens that causes the governor to get out of whack or did I just put a band aid on another problem?