What would cause a new spark plug to die so quickly?

I'm working on a '72 CB450. I replaced a dead plug last week and now
the new plug is dead. I'm using NGK B8ES. Other than being defective
(which is highly unlikely) what would cause a spark plug to die very
quickly?

 

Be more specific about what "dead" means, please. Other information
(e.g. Does it run fine just after a plug change? What's the plug look
like when you pull it? Can you get the "dead" plug to switch cylinders
by switching plug wires (if applicable on this bike)?) would also be
helpful. The more (relevant) information that you can give, the easier
it is for us to give useful advise.

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

Sorry about that. Thanks for letting me know. The new plug (that
apparently is dead) has no spark (checked by removing and grounding
plug body). When I removed it it was black so I need to lean out the
mixture more. It was firing on both cylinders yesterday but today when
I started it the left exhaust pipe felt cold. When I put in another new
plug the left cylinder started firing again & now the pipe is hot. I'm
just learning to work on bikes so it can be very frustrating. I'm
seriously going to look around for a one-carb bike to simplify
maintennence. I just did a rebuild on the carbs and they are both
double-checked squeaky clean. I just want something to get around town
in to save on gas but I don't want a scooter.

 

It happens. I had a nearly-new plug fail on my Ducati once. Just put
another one in and move on. And maybe carry a spare ;-)

 

No problem, we get people of all experience levels here, so it's
difficult to know how to address an issue without good information.
What's obvious to some is quite difficult and subtle to others.


Another thing, it's _much_ easier to follow a thread if you post new
material to the _bottom_ of the message. That way, the reader can
proceed top to bottom and have things make sense.

On to the bike...

If the problem truly is a bad plug, leaning out the mixture won't help.
The plug (again, if it was a bad plug only) was black _because_ it
wasn't firing. All that gas goes somewhere, after all.

And now, a week later, the new plug has failed? I'd either get a new
plug (as ToG suggested) or clean off the old one and swap plugs from one
side to the other. If the "bad" plug starts working, your problem is
somewhere else. If the new, new plug fails quickly, your problem is
somewhere else.

Have you double-checked your carbs to make sure that they are adjusted
properly? For example, double-checked float levels?

Frankly, I'd not worry about finding a one-carb bike. You'll spend more
effort on that than you would on tracking down this problem, and your
frustration is, as far as I can tell from here, likely the "normal"
frustration of someone new to the field.

If you tackled a carb rebuild while still new, and did it right, then
you've done about as hard a thing as anyone needs to do on a motorcycle
(not that there aren't harder jobs, but most people don't attempt them
fresh off the street, as it were). It only gets easier from there. HTH

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

1970's Japanese motorcycles used lower inductance, smaller ignition
coils because they produce a spark quickly at higher RPM. The
disadvantage of those smaller coils was that they didn't produce much
voltage. It was something like 9000 to 12000 volts.

The engineers were stuck with a compromise. How could they get a coil
to discharge its electrical energy quickly enough to fire a mixture?

Answer: Make the coil smaller, with less turns of copper wire and
accept the disadvantage of lower voltage output. A motorcycle coil
might put out 12000 volts, while a car coil would put out 35,000 volts.
We used to put K-Mart coils on our motorcycles. They cost $6.66 and
worked very well. But they burned up our ignition points rapidly.

The stock, low voltage Japanese coils had a problem with slow voltage
rise time, i.e., the amount of time it took the voltage at the center
electrode to rise from zero volts to enough voltage to jump the gap.

Inductance is a property of a coil that is based upon the size of the
coil and the number of turns in the windings. Coils actually induce a
counter-electromotive force,
i.e., a voltage that *opposes* what you are trying to accomplish by
sending current through the coil. You're trying to get voltage out of
the coil and it's actually working to slow down the voltage going out.

That phenomenon is called "inductive reactance"

If the inductive reactance of the coil was too high, the voltage would
take longer than about 45 to 50 microseconds to rise to a level at
which the spark would jump the gap.

Hot rodders and speed tuners tended to believe that they had created an
engine so powerful that it would take God's Own Lightning to fire the
spark plug, so they went after high voltage coils that put out as much
as 50,000 volts. I had such a coil on my Triumph sportscar.

But, actually, even the low voltage Japanese coils would fire a spark
when the voltage was high enough to jump the spark plug gap at some
pressure inside the chamber
on the compression stroke.

The problem was that slow voltage rise time. If the voltage rise time
was more than 45 to 50 microseconds, the voltage would leak away across
the carbon on the center insulator of the spark plug. The voltage would
leak away faster than it would rise, so the plug wouldn't fire at all.

We would constantly be cleaning our spark plugs on those old Japanese
motorcycles. I threw away a box containing about 200 old NGK and Nippon
Denso spark plugs when I cleaned out my garage.

Part of the problem was the condition of the ignition points. I was
forever filing the pits of the ignition contacts and burnishing the
points surface smooth with a points stone.

That was a long narrow abrasive piece of material that would fit
between the points contacts.

The low voltage coil compromise made by the engineers means that the
fuel/air mixture was to be kept richer than that mixture which would
produce the best fuel economy.

You can have a situation where you have leaned out your idle mixture to
get the highest idle possible with the smallest throttle opening. And,
if you managed to set the idle mixture perfectly while the engine was
hot, it might not start very well when it was cold and it would take a
long time to warm up and be very "cold blooded" until it got hot.

"Cold blooded" means that an engine is unresponsive to the throttle
until it's warmed up.

Your "dead plug" problem seems to have *something* to do with mixture
control. Your carburetor could be set too rich in the idle mixture
circuit, or the jet needle might be raised up too high (if there are
slots on the needle that allow you to raise the needle). The needle jet
(brass tube that the needle goes into) might be worn oval
by the needle banging against it when the engine vibrates. The main jet
might be too large. The idle jet (if replaceable) might be too large.
The idle mixture screw may be set wrong. The float level might be too
high.

All of those carburetor settings might contribute to the spark plug
fouling due to dry soot. If the spark plug insulator is wet and black,
that can be from oil fouling (in which case the center insulator would
be black and oily), or it could be wet fouling from gasoline (in which
case the wet blackness wouldn't be oily). That might be caused by
a float that is stuck in the lower position and the fuel level has
risen too high.

To complicate things further, you can still have wet fouling when the
idle fuel air mixture is too lean. The engine fires every other
compression stroke and unburnt fuel can deposit on the spark plug
center insulator.

Then there is spark plug heat range. The length of the metal body of
the spark plug determines how quickly it can get rid of heat. Slow
riding around town may require you to use a spark plug which is one
heat range hotter in order to avoid fouling.

If your NGK plug is a B8ES, a B7ES is one heat range hotter and will
help avoid fouling the plug. If you use Nippon Denso's, a W24 is
equivalent to a B8ES and a W21 would be equivalent to a B7ES.

If you decide to use a hotter spark plug to "solve" your fouling
problem, that's a sort of bandaid fix for whatever is really wrong. You
have to remember to use the B8ES plug if you go for a longer high speed
ride because a hotter spark plug might burn a hole in your piston.

When a spark plug is too "hot" for the application, you will hear a
"tinkling" sound inside your engine when it's hot. It sounds like a
loose valve, but the engine is actually pre-igniting from heat inside
the combustion chamber before the actual spark fires across the plug
gap. That's called "pinging" and it can destroy a piston in about
10 or 15 minutes if you keep riding while the pinging is happening.

Don't ask how I know that...

 

I had a brand new plug fail on me recently. One of the plug caps was
arcing to ground and it ruined a brand new plug in just a few hours of
run time.

Ken in Albuquerque

 

Thanks for sharing that info. I just went out and fired it up (at
night) but could see no arc to ground through the cap or wires so that
can be ruled out as a suspect.

 

I'll carry 2 spares from now on just in case one of them dies a rapid
death or is even DOA. I didn't realize they could fail so prematurely.

 

Thanks for your input. I'm unsure how to adjust the float level. I
followed the Intertec manual which advised bending the tang until 20mm
was measured from the gasket surface to the bottom of the float bowl
with the fuel cut-off needle all the way in but when I did that I
obviously did something wrong because then NO gas went into the float
bowl. So I bent the tangs back to the approximate original level they
were before and gas once again filled the bowls. If you would please
explain to me how to adjust the float level I'd be greatly
appreciative. Is there a motorcycle maintenence manual for dummies ?!

 

Thanks again for your help. Google has been acting up. I just wrote you
a response but it was erased when I clicked 'post message'. Very
frustrating. I'll reply again tomorrow (if I remeber!).