possible outcomes of this scenario?

Hi,


Right after I bought my first Honda XR250, I decided to pull the plug
because it wasn't running like I thought it should. It was jetted too lean.
Soon afterward, I learned that one should ALWAYS blow out the spark plug
area BEFORE removing the plug due to the liklihood of there being dirt in
the plug hole that could fall into the engine.

I didn't do that the first time.

IF dirt did fall into the cylinder, what would be the possible outcome in
terms of the way the bike runs or how the engine sounds?

I pulled the plug out last night, AFTER cleaning out the plug hole with
compressed air, and I notices a little bit of brown sediment attached to the
tip of the spark plug. The plug is the color of Nestle Quick so I know my
jetting is right.

Could this be remnants of dirt that fell into the cylinder?

 

You'd need a few ounces of dirt in the cylinder to actually *hear*
anything, but a few large grains of *sand* could sure scratch up the
cylinder wall...

Says who? Chocolate colored plugs can also mean that the engine is
using a lot of oil and that carburized oil is being deposited on the
spark plugs...

Is that a 4-valve engine? What's the maximum ignition advance, around
32 degrees or around 45 degrees? Older engines needed lots of ignition
advance, in order to complete burning of the mixture in the cylinder by
45 degrees after top dead center. That gives the piston a good hard
push downwards without having the piston speed away from the decaying
pressure wave of expanding combustion gas in the cylinder...

As rule of thumb, the combustion process occurs in a time equal to
twice the maximum ignition advance. So, if an older design needs 45
degrees ignition advance to "get 'er done" (all combustion essentially
completed by 45 degrees ATDC, the whole process is going to take 90
degrees of crankshaft rotation, or 1/4 of a turn...

A more modern engine, with 32 degrees of ignition advance is going to
complete its combustion process in 64 degrees of revolution...

Degrees of rotation translate into time. An engine turning 10,000 RPM
completes one turn of the crankshaft in 0.006 seconds. If the engine
needs 32 degrees of advance, the combustion process is completed in
0.001 seconds. If the engine needs 45 degrees of advance, the burning
takes place
in 0.0015 seconds, so more of the heat in the combustion process is
lost to the metal surrounding that process. More fuel that should have
been burned to produce motion cools off and carbon is deposited in the
older style engine that needed all that ignition advance...

So we were able to "read" the spark plugs and tell well the engine was
burning its fuel. We were seeing lead deposits too, that gave the spark
plug "color"...

We thought that a nice tan color was a great indication of good
combustion, but we hadn't seen what a spark plug from a 4-valve per
cylinder engine looked like if it was running good...

First time I pulled a spark plug in my new 4-valve per cylinder GS-1100
and looked at it, I was scared by how white the porcelin was. There was
*nothing* on that spark plug. Conventional wisdom said that the engine
was running so lean it was going to burn valves pretty damned quick...

The GS-series had Suzuki's twin swirl combustion chambers. The twin
swirl technology swirl causes rotation on the fresh fuel/air charge
coming into the engine. Rotation mixes the fuel and air better, it
breaks up the fuel droplets into the finest particles you can imagine.
Gasoline is almost gaseous, not a liquid when it's swirled like this.
That allowed Suzuki to use the 32 degrees advance I mentioned above and
the engine ran incredibily clean, compared to the dirty-running older
engines...

I used to hang out with the motorcycle magazine writers and motorcycle
drag racers, so I asked one of the knowledgeable guys what was up with
the bone white porcelin...

He said that I should look deep down inside the spark plug to see if
there was a narrow ring of black carbon down where the porcelin
insulator touched the steel sheel of the plug. if I saw that, I was
safe. Another racer told me that using spark plug color to gauge
jetting had been obsolete since unleaded gas had been mandated in the
mid-1970's. He said that he would almost recommned jetting an engine so
lean that I saw metal throw-off on the
insulator. Shiny little aluminum balls on the tip of the spark plug
indicate that the engine is jetted so lean the piston is melting. I
didn't go with that guy's lean jetting recommnedation, but I do watch
for little black specks burned onto the insulator nose. That indicates
the engine is pinging, and I can hear pinging anyway...

What do you want to hear? Do you want somebody to opine that you are
right?
That dirt probably blew out of the cylinder long ago..

---------------------------------------------------------------------------
There's a story about a CEO of a large corporation. He was frustrated
by being surrounded with a bunch of "Yes Men" who would agree with
everything he said. It was an extreme case of "the boss is always
right". This defeated teamwork in the company and re-directed all
creativity and responsibility right back at the CEO...

One day, he called them all into a meeting and he showed them a small
ball of an unidentified material. He was rolling it between his
fingers, and he handed it to one of his "Yes Men", and said, "What do
you think of this material?"

The Yes Man examined it, rolled it between his fingers, and noticed
that it was sticky, and a little elastic. He said, "That's some really
neat stuff, Boss. I think it might have possibilities."

The Boss passed the unidentified blob of gooey material around the
room, and got the same response from each of his Yes Men, who began
praising the material lavishly for its elasticity and adhesive
qualities. They all handled it, rolled it around, and scrutinized for
whatever praiseworthy potential it might have, so they could please the
Boss...

The last Yes Man said, "I think we should develop this wonderful new
high tech rubber and market it commercially, Boss. Say, where did you
get this neat stuff anyway, what's it called?"

The Boss said, "I got it out of my nose. It's called a *booger*"

 

And I'm guessing that a scored cylinder wall would result in oil burning and
reduced compression?

The engine is a 4 valve RFVC engine with 8600 miles on it.

Other symptoms observed on this bike of a lean condition were: excessive
popping on deceleration, cold natured-ness, and a lean miss at every
throttle setting above idle. Increasing the main jet one size and the pilot
one size cured both problems. This bike has the stock exhaust baffle
removed and the intake snorkel removed, but the original owner had left the
jetting stock and a bike that came from the factory jetted to the leaner
side.

Not a condescending lecture on engine timing. Refer to the original
question if you're unsure as to what I want to hear.

My background is mostly with carburated V8 engines with 2V/cyl, like Chevy
smallblocks. I've spent a great deal of time working on them, modifying
them, and yes, even tuning Holley carbs for optimal performance.

But I don't recall ever seeing brown deposits on the tip of the electrode
(the part you bend to adjust the gap). And this engine has a knocking noise
that is driving me absolutely bonkers.

Yes, I know that tearing it down will be the ultimate solution. However, I
was hoping that someone here might have either seen or heard of a similar
issue. Preying that I wouldn't have to dump $400 in parts just to get a new
piston, rings, timing chain etc.

I hadn't considered that the residue to be burnt oil, but that makes sense.

The engine doesn't leak, smoke, or otherwise run poorly. It starts on the
first kick nearly every time. To me, that would indicate that compression
is ok. It just that GD noise, know what I mean??

And if it does burn oil the amount being consumed is minimal, or
undetectable on the dipstick as of the last 200 miles I've put on it since
the first oil change I did right after I got it. I could also swear that
the engine wasn't making that noise when I bought it.

I've been through several valve adjustments with no success in isolating the
noise. My XR 400 is tuned properly and doesn't make that noise either.

I was trying to find out from the experts whether a scored cylinder wall
(caused by dirt falling into the combustion chamber) could cause audible
piston slap.

Thanks,

Roy

 

<snip>

Generally speaking, getting some dirt into the cylinder
is not a big worry. It will blow out and there is a minute
risk of damage.
BUT there is a real concern that after doing this mistake
a few times you might have to repair the plug hole thread
because you have damaged it by screwing the plug into a
sandy thread.

 

Yes, if the cylinder wall is actually scored or scuffed up badly enough
to prevent the rings from seating, but the only way to tell if you have
low compression is to actually do a compression test. If you had a
borescope like inspectors use to look for cracks in jet engine turbine
blades, you could poke it down the spark plug hole and look around
without diassembling the engine, otherwise, the only way to know is to
take the engine apart...

When an air-cooled engine runs lean, it runs hotter than normal. This
may have collapsed the piston, A collapsed piston makes a
chuff-chuff-chuff sound when it slaps...

Removing the exhaust baffle and the snorkel obviously makes the engine
run leaner and hotter. If you have the snorkel, put it back in the
airbox. The snorkel is absolutely necessary to make the air in the
airbox resonate at the correct frequency and prevent reverse airflow
through the carb at certain RPM during the camshaft overlap period....

Oh. There's nothing wrong with your motorbike. That brown stuff on your
sparkplug was the sand, neatly packaged and deliverd to you for your
pleasure and enjoyment. Have fun riding your Honda, and if you do have
any more questions, we'll be happy to tell you exactly what you want to
hear ;-)

Go to www.partsfish.com and register. Then look at the partsfish for
your engine and see whether it has a counterbalancer or not. If that
thing skips a tooth on the sprocket it can really cause a lot of
noise...

You might look for a dirtbike forum dedicated to Honda XR's then. But,
you might find yourself waiting for an answer for a long time, or you
might get an answer immediately...

Overheating from running a lean mixture might have collapsed the piston
skirt, but the only way to prove that is to measure the piston, but you
don't want to take the engine apart... :-(