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D140 - DOM 042413 - Briggs & Stratton 40 V-Twin engine - 48" deck.

Runs normally for first 1/2 hour, then engine slows down. Runs slow (maybe 80% to 90% of normal RPM with throttle lever staying at same setting) for about 10 minutes or so and then goes back to normal RPM. Then alternates between running at normal and slow RPM in 10 to 15 minute cycles.

Fuel delivery and governor checked out okay. Friend was using mower and I heard it running from a distance and notice exhaust sounds got ragged when running slow. Suspect it was losing a cylinder. Tried starting with one plug wire pulled while it was in its running slow phase. Found it would start & run when the left cylinder plug wire was off, but not vise versa. So the left cylinder is intermittently dead and it appears to be heat related since it heals itself and the goes bad again.

Magnetos and diodes tend to stay dead when they die, so has anybody seen this intermittent problem before?

My guess would be that you have a coil or plug wire breaking do with either a hot short or hot open. I would check the wire for any visible breaks or cracks. Either way I would probably replace the coil. Please post the engine numbers, as I’m guessing because I can’t see which type of ignition system you have.

Check the valve lash.
They could be just far enough out of specs to give grief when hot .
A valve guide could be loose
OT the coil may have an internal break in the wire which just touches when cold
You could also have a problem with the kill wire
So take the blower housing off, remove the kill wire, check that the carb solenoid will shut down the engine then do a test mow to see if the problem goes away.

Thanks for the replies.

First I'll try disconnecting the kill wire on the cylinder that dies and then lash the valves later. (They've never been been adjusted since I bought the mower in 2013 - so they definitely might be the problem. That'll get done whether it turns out to be a spark issue or not.)

Also going to clean the magneto mounting post tops and coil frame areas where the make contact to make sure there is a good ground for both high voltage circuits. Plus brush of any loose rust on flywheel and make sure the air gaps are good on both.

And get the numbers off the motor so I'll have them hand for the next time I've got engine questions.

Swap the plugs, see if the problem goes with the plug.

Sorry about not getting back sooner, but other stuff keeps popping up.

The engine is a 40777-0170G5.

The air gap on left magneto was only about 0.003", but adjusting didn't make any difference.
Swapping plugs didn't make any difference.
Pulling the kill wire didn't make any difference, other than running until fuel in carburetor ran out when key was turned off.
That leaves valve clearance or a bad magneto.

Lashing the valves this afternoon. Is that done with engine cold or at normal operating temperature?

Also, what should the magneto air gap be? 0.012" to 0.014" is what the owners manual says, but they were both tight and didn't look as if they had moved. One was found at 0.003" and the other was 0.008".

Thanks.

Set the magneto gap to what the owners manual says, and do the valve lash when the engine is cold.

I always set air gap at .010”.

Slobcat said:

D140 - DOM 042413 - Briggs & Stratton 40 V-Twin engine - 48"
So the left cylinder is intermittently dead and it appears to be heat related since it heals itself and the goes bad again.

Magnetos and diodes tend to stay dead when they die, so has anybody seen this intermittent problem before?

Click to expand...

Actually, coils failing when hot then working when cool is a common problem. A coil will probably fix it.

Checked the rocker arm clearances. Found all of them at about 0.007", so they were a whisper loose, but my T-40 Torx wrench seems to have grown legs and deferred adjusting until it finds its way back home, or I buy another one.

Had both magnetos tested at a local small engine shop. Both were dropping out, one quite often, the other not as much.

Not in stock, so I ordered a pair. Half of lawn still needed mowing, so put the old magnetos in, but swapped side with them.

Mowed a bit over an acre without a hiccup!

My John Deere D140 must have acquired some resistentialism (Spiteful behavior manifested by inanimate objects.)

I'll install the new magnetos when the arrive and adjust the valves at that time.

Thanks for the help.

Magneto coils came in. Installed and engine running like new, or better.

I say "or better" because the air gap was way off originally (about 0.003" instead of 0.010") and it sounds like it is running faster than it was right before replacing the coils.

What is the RPM supposed to be with throttle lever in the detent before going into the choke position?

Looks like a lost spark system (plugs spark on exhaust stroke as well as compression stroke), so there will be a spark on every revolution for both cylinders.

I think most small engines have this, ignition trigger is on the crank flywheel, not sure on an opposed flathead, might be using every spark in one cylinder or the other? Glad to hear it's running as it should.

tom3 said:

I think most small engines have this, ignition trigger is on the crank flywheel, not sure on an opposed flathead, might be using every spark in one cylinder or the other? Glad to hear it's running as it should.

Click to expand...

It is an OHV V-twin and has two magnetos, so I'm pretty sure each plug sparks every time the magnet on the flywheel goes by.

I'll be triggering the tachometer function on my DMM with a coil of wire around the spark plug cable of one of the cylinders, so I'll probably have to select the 4 Cylinder scale to get an accurate RPM reading. I suspect the engine turns at about 3600RPM at normal operating speed and was wondering what it was dropping to with one cylinder dead.