briggs 825 series issue - burns out spark plugs

[nyquil junkie 225]

I have a Briggs engine, an 825 series (190cc) that eats spark plugs. It will run like new for about 5 minutes then, start missing and running rough eventually, it won't start at all.

Put a new plug in and it runs great, for about 5 minutes. Then you need a new plug. The plugs go from a new healthy plug with a strong blue spark to a nearly dead one with z weak thready yellow spark.

The engine doesñt smoke, the plugs are like new and clean when you take it out o replace it.

I'm stumped. These engines have a simple magneto coil on the flywheel, no points or condenser that I see. Would a wonky magneto cook plugs like this if not, what would?

The plugs are definitely ruined, I put every one in another engine and it won't run with these new but ruined plugs either.

I have a wide assortment of brands in a can ere since I thought it was a bank problem but I have nkg champion denso AC ..... it burns out all plug brands the same.

While it runs it runs like new. For the 5 minutes it takes to kill a new plug.

Any input you guys can offer, I really appreciate it. I'd hate to junk a good engine over something simple.

I've never see this problem before. Ugh!

[MarkPalmer 80]

You describe to a tee the condition of the Briggs Magnetron coil being bad, the spark gets weak as the engine warms up to the point it will run rough or die, then not re-start until cool again.  However the part if the spark plug is removed when this happens and it's still clean but won't spark again sounds very strange to me.  If nothing fouled it, and it didn't get physically cracked or damaged, the power to it from the coil becoming weak should do nothing to it at all. 

 

-Mark-

[nyquil junkie 225]

Well that's the odd thing, once the plug is screwed, letting the engine cool off doesn't help, it won't start after that hot or cold with that plug. At first I thought I had a cracked intake tube and when it got warm it started sucking air, but that was ruled out by simply putting anew plug in the engine and it running like new for a little while longer. A sucking crack would still be making it run bad with a new plug swapped out while the engine is hot.

"Until it cools" isn't part of the problem, once the plug is ruined, its ruined, in theory you could run the engine all day if you had a case of brand new plugs to waste.

LOL

I'll buy a new coil/mag for it an se if that cures it. But I've never seen a mag/coil do this before.

But what else is connected to the plug that could kill it but the coil/mag thing? A bad air gap wouldn't do that, a bad wire wouldn't run at all.

Freaky.

[MarkPalmer 80]

Definitely a weird one.  I've ran this one by some fellows here at work, and they have never heard of a plug itself getting damaged by power from a coil.  A basic spark plug is just an air gap between two pieces of metal held in an insulator.  The only thing I can remotely think of is something in the coil is making the voltage go WAY up for a very short period of time, well beyond that of what the Magnetron can normally produce, and if they are resistor spark plugs its damaging the internal resistor.   

 

-Mark-

[Fordiesel69 214]

Try an autolite small engine plug, or an old champion from the 1990's.  The newer champion small engine plugs are garbage. 

[nyquil junkie 225]

I bought a new coil for it, put it on and it ran for all of 3 min before it quit.  This time a new plug was no help. Thready weak spark. So

I removed the neural safety, no help there.

 

I give up, this makes no sense, there is nothing in the spark making but the magnet on the flywheel, and the coil.  New coil and new plug. no kill switches hooked up, it should just work.

 

It should at least spark. the ignition is all new. Ugh.

 

I'm off to harbor freight.  I spent half the cost of  a new engine so far.  99 bucks will buy a new engine.

 

Still this problem will bug me until I figutre it out. 

[MarkPalmer 80]

It's one of those seemingly impossible things that drives you insane. Breaks-and-Scrap-It engines are notorious for many a man losing his mind

-Mark-

[posifour11 723]

Don't give up yet. Did the magneto come with a new plug wire? Could the wire have a break in it that separates when it gets hot like the old fusible links?

[Save Old Iron 1,563]

Interesting. Some property of the plug must be changing. Maybe electrical characteristics of the plug are changing.

 

I have never done this myself - really never had a reason to - maybe measure the electrical resistance of a new plug from the spark wire terminal to the inner electrode. Note the reading and then measure the resistance from the spark wire terminal to the ground electrode.

 

Make the same two measurements on a "bad" plug and see if there are any significant changes in either reading.

[Save Old Iron 1,563]

Don't give up yet. Did the magneto come with a new plug wire? Could the wire have a break in it that separates when it gets hot like the old fusible links?

Good point

 

You could do a resistance check on the mag secondary by measuring resistance from the spark plug terminal to ground. Resistance probably should be somewhere between 5 - 20 K ohms. Look for any significant change in resistance between when it runs well and when it will not start.

 

that's it - that's all I got on this one.

[MarkPalmer 80]

Disconnect the small wire from the magneto coil all together and see what happens.  It is just the kill wire.  Any small current leakage developing to ground between the wire and the ignition switch will lower the coil voltage. 

 

-Mark-

[dsholler 54]

This was a thought based on no information or experience, so take it for what it is worth.. but is it possible that the spark plugs are becoming physically damaged? either when they are installed, or by some kind of action when running the engine? Have you run them in the tractor when it was being used the whole time?

It is odd that they work for a while and then fail, but what if the contact within the plug were somehow compromised in a similar way to the suggestion Posifour11 made about fusable links...  I cannot think of how this could happen, or at least how it could happen consistently, but it was just a thought

[WH nut 553]

Do you have another motor you can try the plugs in when its fails?

[nyquil junkie 225]

Yep, I have several good reliable engines, I've tested the plugs in them. The plugs are somehow damaged, because you get hardly any spark out of them once they have been killed in the BnS.

I'll have to test the old coil, and see what reading I get. I'm a sucker for punishment so I'll probably tinker with this engine till I figure it out.

When I find time I'll mess with it. Tomorrow it gets a shiny new predator 173cc engine with a nerve soothing 2 yr replacement warranty.

LoL

[Callen 64]

I would try a different brand spark plug or getting them from a different store.

[WH nut 553]

How hot is the motor getting before it quits. If you arent seeing any damage to the plug heat is the only thing I can think of

[nyquil junkie 225]

So much for harbor freight engines. It ran like a dream for 3 minutes and stalled never to be started again.

They gave me all my money back as if they expected me to bring it back the next day.

LoL

[MarkPalmer 80]

I just helped a friend out with a similar 12.5 HP Briggs Intek yesterday that had a no spark issue.  The trouble was traced to the ignition switch being shorted inside, keeping the small kill wire the the magneto grounded.  I saw nothing unique with this magneto setup that could cause it to damage spark plugs, it's just the standard every-day Briggs Magnetron.   

 

-Mark-

[nyquil junkie 225]

I guess I never updated this problem.

After some discussion with the DR company, I said I want to buy a new engine.

So they said here is he engine for that model.... a 6.5 b&s.

I said but it had an 8.75.

They said they don't carry that engine no more.

Really....

After a while of chasing them around the bush for an answer to why that engine was replaced, the lady more or less said the B&S 8.75 that was on it originally was "not the best quality" so they changed it.

It took 45 minutes to get them to admit that POS model was indeed a POS with issues from the factory they dint want to deal with on new DR wagons.

They have me a nice discount on a new engine that they use on their wagons now.... seems to run just fine.

Maybe they just told me that and gave me a discount (and a new clutch unit!) To shut me up.

Whatever. I won.

lol

Edited April 20, 2014 by nyquil junkie