Briggs 16HP Vanguard carb / fuel problem

[Wurzel]

Progress at last thanks to your pointers, it's only running on one cylinder, #1 is down.

The plugs are in a very different condition, the one on #1 cylinder having done little to no work. Swapped the plugs with each other and #1 still not firing, tested and confirmed no spark on #1.

Also I would confirm by using a Ohmmeter that both of the diodes in the kill wire harness going to the magnetos are Ok. (one not shorted) You can check on line how to ohm the diodes.
Those are isolation diodes and if one is shorted the ignition will be erratic at different rpms. While checking the diodes check the air gap on both magnetos as at .010-.012.

Checked #1 magneto and the gap was too big, set correctly but still no spark. Have now run out of light, but initial tests with multimeter suggest the kill diode on #1 magneto is open. Will test properly tomorrow to confirm - thanks again for all your help!

 

[TobyU]

The coil Gap is pretty forgiving on those. Just use a business card or whatever but most of them will fire just fine even at the widest position and it's more important to make sure it doesn't rub on the flywheel but a few thousands here or there is not going to cause a no spark condition.

If you actually have no spark jumping from The wire to the engine block or on a good known plug where it won't jump the plug gap then you certainly need to figure out the no spark condition for that one.

 

[Forest#2]

Just couple tips:

A OPEN diode will not keep the magneto from firing. A shorted or low resistance diode will result in erratic/misfire operation. The diodes purpose is as isolation diodes, isolating one mag from the other when the engine is running. I've replaced those diodes with a 1N4007, 1 amp, 1000PIV (peak inverse voltage) buying them dirt cheap on fleece bay. I just cut the wire leads on the old ones out of the wire loom, solder or crimp in the replacements and use heat shrink tubing over the replacements instead of buying the Briggs diode harness. If that diode is burned open or charred it's due to voltage getting onto the kill wire and on mowers that have been setting long time outside water gets into the ignition switch and rust inside will allow cross tracking of voltage. If you loose the/a magneto again later you will need to replace the ign switch. Be sure you get the polarity correct on the replacement diode when installing (the white line ring or cathode will go towards the magneto)

Those magnetos can be found for less than $20. I've had good luck with the ones from Amazon and fleece bay. I try to keep a spare magneto around. I've have previously bought the $70 Napa mag replacements and have them fail within a years time.
I would at the very least replace both magnetos, set the air gap at .010-.012 (most plastic cards are too thick at .015) Air gap is not real critical just make sure you have clearance to the magnet after setting by turning the flywheel by hand. Save the one that's firing as a spare maybe.
Then replace the one diode or wire harness. Be careful BEFORE trying to turn out the 5/16 machine screws holding the mags. That is steel screws into aluminum and they will twist off easily, FIRST slightly/lightly peck on their head with a small hammer and the vibration will loosen the corrosion. Make sure you get them just good snug when going back. I sometimes use little bit of Anti-seize on their threads.
You cannot make any logical ohmmeter tests on those mags because their is electronic sealed inside. The real test is to use a old spark plug and set the gap at approx .060 and look at the spark in shade or a darkened room.

I would also pull both valve covers and check the valve lash before ordering parts. Might need valve cover gaskets after doing so. You can find specs for such on-line.

BUT before jumping in and buying mag's and diodes even though you do not see spark. I would compare compression on each cylinder. Most likely ok but now is the time to test such. You will be mainly checking to see if valves and valve seats are Ok. The heads on some of those Briggs OHV's will bite the dust in a surprise way and no or weak compression.

It appears you had two issues going in, clogged carb and a weak or dead cylinder, maybe.
But it was running good when stored. (Murphy's law strikes again)

 

[bertsmobile1]

Just to clarify things
The magnetos have a Hall Effect Trigger chip in them which is what makes them work
They have a kill wire attached to stop them working when you shut down .
On twin cylinder engines you get a ripple down the kill wire that can interfear with the other magneto so they put diodes in there to clamp this signal.
On B & S engines the diode is usually in the kill wires and quite obvious.
Thses can be replaced for a few cents or the sub loom can be replaced for a lot of dollars .
The chip inside the magneto coil is neither repairable nor replaceable ( I have pulled a few apart ) .
So you test the coils by removing the kill wire
No spark with the kill wire removed = dead coil that has to be replaced

 

[Wurzel]

Just to clarify things
The magnetos have a Hall Effect Trigger chip in them which is what makes them work
They have a kill wire attached to stop them working when you shut down .
On twin cylinder engines you get a ripple down the kill wire that can interfear with the other magneto so they put diodes in there to clamp this signal.
On B & S engines the diode is usually in the kill wires and quite obvious.
Thses can be replaced for a few cents or the sub loom can be replaced for a lot of dollars .
The chip inside the magneto coil is neither repairable nor replaceable ( I have pulled a few apart ) .
So you test the coils by removing the kill wire
No spark with the kill wire removed = dead coil that has to be replaced

Removed the kill wire to that cylinder and it's still not firing, so am assuming a dead coil - happily they're fairly cheap to replace.

 

[bertsmobile1]

Removed the kill wire to that cylinder and it's still not firing, so am assuming a dead coil - happily they're fairly cheap to replace.

Yep stone dead
Now if the kill wires have diodes in them then test them as well lest they lead to a very short service life for the replacement coil.
And make sure you get the correct one or the engine will never run properly
Also test the kill wire for voltage
It is a ground wire so should only ever be ground ( engine off ) or open ( engine on )
A problem that had me going mad ended up being corrosion in the socket of a parking brake switch that allowed a tiny voltage to pass down the kill wire if you had your foot flat on the brake when cranking the engine, but not if the brake was on the latch.
That mower came back more times than a black fellas boomerang .

 

[slomo]

Removed the kill wire to that cylinder and it's still not firing, so am assuming a dead coil - happily they're fairly cheap to replace.

TIP

Make sure you LOOK at the new coil. Some say UP and some other mounting tips on them. Summation, it is possible to mount one back to front or front to back depending on your coil.

She sure idles down nice with one dead cylinder (from your videos). You should of verified spark several pages ago. Get a cheap inline spark tester. Or just ground a plug out......

 

[TobyU]

From all of this I think we've learned one thing and it goes back to what I always tell everyone when troubleshooting a mower...
Always start by making sure the machine is getting enough fuel especially on the no starts because well over 85% of the time that is the case..
After you have confirmed it is getting fuel or it's running, if a twin engine, always cancel out each cylinder to make sure it will run on both.
This is most easily done with a towel or a glove and popping the spark plug wires off one at a time with it running at a medium speed. No reason to have it going full speed but don't have it idling either.

In this situation this would have quickly determined that it was only running on one cylinder and we could have moved away from even talking about fuel for the time being.

Then, since you know it's a cylinder running issue, the next thing you do is check for spark and since you already have the plug out there's no reason not to put your finger in the hole and crank it over a few times just to see if it goes puff puff puff puff and blows your finger out of the hole with a regular pattern. It should never double puff or try to suck your finger in the hole forcefully.
If it does that you have some valve train issues.

So this would have quickly told us it's a spark related issue which leads us straight to the coil..

Then, as others have stated the way you check a coil before you ever take it off the machine is to pop the kill wire off it and then check for spark again..

Don't even bother with the ohm meter or other basic static bench tests because in the real world those are not accurate enough to spot anything but the most blatantly fried coil.

You need to see a good strong spark spark spark spark spark spark spark as you crank the engine over with no missing Sparks or no sitting there just barely moving around the tip of the spark plug or the insulator.

 

[slomo]

 

[Wurzel]

She sure idles down nice with one dead cylinder (from your videos). You should of verified spark several pages ago. Get a cheap inline spark tester. Or just ground a plug out......

Yep - I certainly should have, I got distracted chasing down a fuel issue, I even have that tester...

Not being familiar with these engines I'd totally missed it was only running on one as well, had it been a four pot it would have been obvious. In any case, coil ordered, fingers crossed it works. Noted re orientation, the one I've taken off was clearly marked as such, will check on install of new one.