REELAV8R

My neighbor of many years was moving and needed to unload some of his stuff quickly. So he offered to sell me several things at one price. I took him up on the deal and one of the things I got was a nearly new push lawn mower.
He said he could not get it to run and suspected a fuel delivery issue of some sort.
I finally got around to looking at it and sure enough if I squirted a little starting fluid into the carb it would run until that was exhausted then quit.
I tore into the carb, float bowl and fuel delivery which was very basic gravity feed system.
Fuel seemed to be getting delivered just fine.
So I took off the pull starter which exposed the coil that fires the plug via a magnet on the flywheel as it passes inducing a voltage that is then amplified to jump the gap on the plug.
I've worked on plenty of lawnmower type motors so I immediately noticed that the gap between the flywheel and the coil pickup was too large on one side to effectively induce a good voltage. I reset it and tested the spark. Nothing. Hmm. So I did it old school and held onto the wire with my hand while turning the flywheel back and forth past the magnet on the flywheel. No shock from the coil. Bad coil?

So I decided to spin the flywheel via the flywheel nut and a cordless drill with the plug out of the head and observe the spark in the gap on the plug.
First thing I noticed is that it didn't spark at all until it was spinning fairly fast. Maybe 100 rpm or more.
I changed the gap from .040 to .030. similar results. So I tried .050 figuring it would have to build a larger charge to jump the gap giving a better spark. No spark at all.
Coil not capable of jumping that large of a gap apparently. Set gap at .025 and still weak orange colored spark, better at higher RPMs but not great. I had to close the garage door to even observe the spark due to it's weakness.
The plug was a brand new, still shiny, briggs and stratton brand plug old school style copper electrode. Plug construction/brand maybe?

So I got a NGK fine wire electrode plug out from a dirt bike application. These are made for weak electrical systems and spark easily. Sure enough it was a nice hot blue spark even at .040". Not the right reach for this engine though, too long, might get hit by the piston.

So I pulled out a plug for my corvette. An NGK BKR6E V-power plug. Set it to a .030" gap and the thing threw a great big spark. Hmmm...So I put the briggs plug back on at .030" and again weak spark and it took quite a bit of RPM to get that.
So the NGK went back on and I notice that not only was the spark better it sparked earlier, less RPM than the Briggs plug could.

It was a really good illustration of just how a plug difference can make a big difference is spark performance.
On an 8 cylinder engine it probably goes unnoticed. But certainly poor spark quality is going to cause poorer fuel ignition leading to less power and partial misfires vs a good spark quality.
A single cylinder pull start engine will really highlight that effect.

After reassembly the mower started easily with little pull effort, first pull every time.

Gale Banks 80'

So did You end up using the NGK Plug in the Mower ? Yes, those of us who Race Vintage MX are always interested in how to make a Open Class Bike start !

REELAV8R

Yes, using the NGK V-power plug. I used the fine wire NGK plugs on my two strokes, both 125's and 250's. 9 heat range, 8 in the winter. BR9EG and BR8EG.
Idle circuit lean enough was the secret to keeping a two stroke from carbon fouling the plug I found in mine. didn't have any 70-'s vintage stuff though. All late 80's, 90's and newer.

centralcalvette

As a former Auto Tech, I have several spark testers. When working on my lawn mower I found that the spark is much weaker on a lawn mower. I now own a special "lawn mower spark plug tester" that tests the weaker output of laen equipment.

Another thing to keep in mind is that even though you might see a spark when you test it, fuel and compression make it harder for the spark to occur. I was taught that the spark literally travels across the fuel molecules.

mrvette

Age 71 here, I get tired of pulling the rip cord.....so removed all that crap, cut a hole in the top plastic, got a welded on 3/4" socket on a cut off 3/8 extension that goes into my 1/2 inch drill.....

GOD I"m LAZY!!!!!


stupid kill switch gave me grief, I stuck an old .mil surplus switch on the side of the plastic.....built by Microswitch, they NEVER FAIL.....

vette_jim

Great write up...I have a practically unused Ryobi blower that is about to go to the trash. Does not have a good spark but plug works in a chainsaw. I've tried everything, maybe NGK will do the trick . Ditto on the comment above about pressure effecting spark delivery....as pressure increases the ability of a spark to travel decreases. I worked on avionics in the Navy, some UHF radios and radar equipment had to be pressurized at 15 psi else you would get arc-over at high altitudes.

REELAV8R

So that begs the question, how are you doing this without the engine grabbing your drill once started and spinning it or hucking it across the yard. You just pull it off of the nut real quick like?

All you need for a kill switch is a bent piece of metal bolted to the head somewhere that you can push down onto the top of the plug. The spark will take the easiest path to ground which is through the head, not you.

hotrodnick

I worked with a crew chief of a alcohol pulling tractor. They found Autolite worked best for them. I like to use NGK or Autolite, my 79 has Autolite Platinum in it(they were on sale). We have a lot of customers come in who won't put Champions in their lawn mowers.

REELAV8R

I've had decent results with autolite's when I used them. I stopped using champion in the 80's.
I've never had any issue with NGK's. I use them almost exclusively now if the one I need is available.

BLUE1972

Discovered the same thing with Champion plugs, lawn mower ran poorly all summer and had trouble with the snow blower. Changed the Snow blower to Autolite prep for winter and all the trouble went away.
Since it was Nov. I changed the lawn mower also and again all the trouble went away.

My 1980 Chevy truck will not run on Champion plugs, no power , hard to start and poor gas mileage. Loves NGK and AC plugs...