What should my piston and valves look like?

 

I am doing a rebuild on an ATV and wondering if anyone would mind
commenting on how the valves and piston look.

Pics are here:

www.stoneip.com/z400/athena.html

The top of the piston is black.  some deposits in the middle, then they
get less as you go out, and the very outter edge (maybe 1/8th to 1/16th)
is light brown/clean

If you'd like better pictures, I can get them.

I'm trying to read on if my jetting is too rich (which I assume).  But I
  can only find 2-stroke information that says this would be too lean
(no piston wash) but don't know if that applies to thumpers.

Thanks for any help..

 

Burning a little oil were ya? The brown you see at the outter edge
looks pretty good. You'd have better luck to read you plug once you get
her back together.

 

Bike Guy Joe wrote:

Oil?  Nope... not burning any oil that I know of..  why?

I do read the plug, but it's hard to know what method to use..
insulator, base ring, etc.. etc...  reading the timing and heat range is
no sweat.  And I normally stick to reading the base ring for jetting.

 


Bradley V. Stone wrote:

So. Your engine has been burning oil. Are you planning to replace the
valve guide oil seals as well as the rings? Maybe the valve guides are
worn out too.

Do your intake valve heads look like black carbon balls on the ends of
sticks? Your intake valve guides oil seals are worn out.

You can have an engine that's been running too lean and still see
burned-on oil deposits because an engine will suck oil past the valve
guide oil seals if it can't get enough gasoline to burn. It will try to
run on the oil, which is really low octane.

It's not even octane. Oil fumes will detonate like diesel fuel. The
engine will ping and detonate on oil fumes.

(My little pickup truck had a blown head gasket between two cylinders
and those cylinders were running really lean. The compression in one
cylinder was pushing the fresh intake charge backwards out of the
adjacent cylinder through the hole in the head gasket. But the tops of
the pistons had lots of oil deposits, as did the intake valves.)


So, you don't want to see any deposits on top of the piston at all, you
think deposits are a sign that something is wrong?

You could lean up the mixture, advance the ignition timing, or use a
hotter spark plug to make the engine run hotter and reduce deposits.
You can tell you're approaching a dangerously lean condition when there
is an area of bare aluminum on the piston directly under the spark plug

Naturally, if your fuel/air mixture isn't lean enough for the
combustion temperatures to reach the melting point of aluminum, you're
going to see a thin layer of black carbon soot on top of the piston.
The time to worry is when you see NO SOOT LAYER directly under the
spark plug.

The next step after the bare aluminum patch on top of the piston is
little shiny silver aluminum balls melted onto the spark plug
insulator. After that comes the spark plug that's filled with melted
aluminum, the piston with a hole in the top, and the broken connecting
rod caused by the expanded piston seizing in the bore.

I have seen all of those conditions. Pistons with holes in them make
depressing "trophies".

I wish I could describe the strange *groaning* sound the exhaust pipe
makes when an engine is running too lean.

Fuel/air mixtures in any internal combustion engine have to be rich
enough to avoid melting the piston top. If you had exactly correct
stoichometric fuel/air ratio where there was enough air to burn all the
fuel, the temperature inside the cylinder would reach 3000 degrees F
and you'd melt the valves.

But, the temperature would never get that hot. Aluminum will melt long
before that.

Pro racers who build engines for competition and run them really HARD
and tear them down after every few RACES don't see a lot of carbon
deposits. Guys who ride for fun and don't run that hard will tear their
engines down every few YEARS and they will read article written by the
pro racers and they will try to use the racer's comments as a guide to
what their fun machine should be like inside.

Does that make sense?


Your fuel air MIXTURE is really too rich if your engine 8-strokes.
8-stroking is when the piston fires every other compression stroke. The
exhaust sound is blubbery. Even slightly rich mixtures will cause a
dull, thudding sound in the exhaust pipe.

But amateur tuners are unable to separate the major sources of rich
mixture because they do not understand the OVERLAPPING nature of the
various fuel/air mixture circuits in a carburetor.

Amateur mechanics read all those short articles in the magazines and
they get the idea that the idle circuit only affects the mixture when
the engine is idling and that the main jet is only working when the
throttle is wide open.

Actually, the idle circuit works from closed throttle all the way to
7/8th throttle. The jet needle/needle jet works from 1/8th throttle to
7/8th throttle, but the idle circuit is STILL
adding fuel/air mixture to the total.

And, whatever comes through the jet needle/needle jet variable orifice
has to come through the main jet. If the main jet is too small, the
restriction of the variable orifice will
be larger than the restriction of the fixed orifice.

So the engine will come onto the main jet too soon and the part
throttle mixture will be too lean.

But, you will notice that I said fuel air MIXTURE (as opposed to
"jetting") in my earlier comments. Ass-uming that rich mixture *always*
comes from jets that are too large, home mechanics will run out and buy
a bunch of expen$ive brass jets and experiment with swapping main jets
because they don't understand how to adjust the damned idle mixture
screws!

Got CV (diaphragm) carbs? Most engines come with CV carbs to satisfy
the EPA requirement for cleaner air at part throttle running.

Home mechanics will turn the idle mixture screws on the CV carbs out
too many turns and the idle RPM will decrease, so the home tuner will
turn the idle rpm up and this uncovers the transition ports downstream
of the butterflies. Now the CV carb is running off of four idle ports
instead of one and the sound of the exhaust pipe is "toot-toot
toot-toot toot-toot", as the engine blubbers on rich IDLE mixture!

And, amateur tuners usually don't know how to tell what the relative
sizes of their jets are from the numbering on the jets. Hexagonal jets
have big numbers and those numbers are directly proportional. IOW if
you have a #200 hex jet and you install a #300 hex jet in its place,
the #300 hex jet will flow 50% more fuel.

But round jets have a different numbering system. Like, if your engine
has a #100 round main jet and you install a #150 round jet in its
place, that jet will flow about 250% of the fuel the #100 flowed.
That's because the round jets are identified in comparision to a hole
that is exactly 1 millimeter in diameter. The hole in a #100 main jet
is 1.0 millimeters in diameter, the #150 main jet has a hole that's 1.5
millimeters in diameter, and if you work out the area = pi times the
radius squared, you'll see that the
area increases exponentially.

In order to get your carb jetted correctly and the needle and idle
circuits adjusted correctly, go to www.factorypro.com/tech/carbtun.html

Also, how do you "read" a spark plug? If you look at thpse charts you
see in repair manuals, they show all the *abnormal* conditions of lead
fouling and carbon deposits and pinging and detonating. The charts
*never* show you what a spark plug should look like when it comes out
of an engine that's jetted just right for full throttle running.

Those magnifying flashlights that the tuners carry in their shirt
pockets are for looking deep down inside a spark plug. You want to see
a narrow band of black carbon soot on the insulator, way deep inside
the spark plug. It should be about 1/16th to 1/8th of an inch wide. The
rest of the insulator should be clean and white.

But you have to do a "plug chop" to see that indication. You have to
run your fully warmed up engine at full throttle with clean spark plugs
for a few miles, then shut off the engine and coast to a stop without
ever letting the engine idle in order to see how the spark plug is
"burning" at full throttle.

 

CK wrote:

How does one come to that conclusion?  That it is burning oil?