Plugs and Compression Testing
The engine seems to run rough. In the exhaust you can hear an
occasion 'poofing' sound like somethings not firing on all cylinders.
Under loaded conditions the tractor seems to lack power and it belches
black smoke out of the exhaust. For some reason it just doesn't want
to start up without cranking and cranking the starter. All these
conditions can be signals that your unit is in need of a tune up.
Ok, so what is involved in a tune up? You say, swap plugs and file
the points....now that's a tune up! If one wants a tune up that will
last there are certain procedures and steps that will breath new life
into your engine. It will also give you a very detailed summary for
the overall condition of your tractor engine.
First thing to do in a tune up is to warm the tractor up to operating
temperature. Then take a can of engine cleaner from the auto store
and apply it to the engine area. A small paintbrush will stir up
those extra greasy spots and help the solvent dissolve the tough areas
of build up. Let the solvent set the required time and then hose the
engine off. It's so much nicer to work on a clean engine. Start the
engine back up and let it run for around ten minutes. This will warm
things back up and evaporate any residual water.
While the engine is drying get a piece of paper and clip board out and
make a list of any super greasy spots you saw on the engine that might
require a gasket replacement. Look for other causes such as a broken
oil line or a leaky pressure sending unit. Walk around the tractor
and note any grease build up around rear wheel seals, power unit
pumps or hydraulics. Also make note of the condition of the fluid
levels: engine oil, radiator, tranny, and/or rear end. Any other
observations of the fluids themselves should also be written down, ie:
milky engine oil, rusty antifreeze, excessively low levels. Continue
with a new section covering the condition of the radiator and cap ( is
there any antifreeze leaking from the core, does the cap or core have
calcium deposits showing), the hoses (are they hard and brittle, soft
and spongy, grease soaked and leaky), and the fan belt (shiny and
glazed, cracked and split). Also take note of the general symptoms of
any engine problems you encountered (missing on load, hard starting,
overheating, just plain runs rotten). Also note the color of the
exhaust; blue, black, white or gray. Wipe your finger in the exhaust
pipe opening and take note of the residue on your finger; sootie,
wet/black, dry black/ gray or brown. These notes will come in very
handy.
By now the engine should be dry and you can start the tune up
procedure. It may save some frustration if you marked each plug wire
with a piece of masking tape and magic marker. Number 1 plug is
usually closest to the radiator end of the engine. When removing the
plug wires make sure not to jerk on the wire. The internal insulation
won't take bending or twisting motion. Instead, grasp the boot at the
plug itself and gently pull with a small back and forth twisting
motion. If possible take a compressed air gun and blow around each
plug base to remove accumulated debris. If no compressed air is
available loosen the plugs one or two turns then crank the engine over
a few times. The compression will blow away buildup that would
otherwise fall into the combustion chamber on plug removal. Next
remove the plugs and place them in order on the bench. By doing a
visual check on the plugs one can get a quick and simple overview of
the engine condition. Plugs that are operating correctly will be
clean in appearance. They might have a fine light gray or tan coating
on them. The electrode tip may be rounded from long use, and the gap
will be above specs. These are all indications of a properly matched
and operating plug. It also indicates that the upper portion of the
engine; the rings and valves, are all in a good state of wear. Now
let's say the plug has a lot of oily black carbon or goo on it. This
is an indication of a no fire condition or excess oil in the
combustion chamber. A no fire condition can be determined by taking a
good spark plug and connecting it to the corresponding plug wire. Use
a heavy glove and ground the plug electrode end to the block or other
grounded surface. Then crank the engine over for a few turns and see
if there is spark at the plug. A no spark condition will be reason to
investigate the electical circuitry from the plug on back. If there
is a spark on the replacement plug, then install the old plug in the
same postion and repeat the process. If the plug does not fire then
clean it thoroughly on a wire wheel, reinstall it and try again. If
it sparks then further internal engine checks are needed. If it does
not fire then the plug is shorted out. Further checks on the engine
intergity would also be recommended to isolate the oil buildup
problem. A blue exhaust indicates burning oil and is a good indicator
of internal engine problems. Further engine tests to determine the
oil penetration to the combusiton chamber will be covered under
compression testing. Let's continue our discussion on plug appearances.
If the plug is black but dry and fluffy it means there is too much
fuel being brought into the combustion chamber or that the installed
plugs are not the correct heat range. Perhaps there is too high a
fuel level in the carb, a stuck choke or plugged air filter. First
check with an auto store to make sure the proper heat range plug is
installed in the engine. Heat range is determined by the length of
the ceramic insulator from the tip of the electrode to the sealing
ring of the plug. This length forms a resistance to the electrical
impulse from the distributor and will determine whether or not it
fires hot or cold. The longer the insulator the hotter the spark.
Too hot a spark will cause detonation in the firing chamber and result
in piston and plug erosion. This condition is characterized by bright
tan or white glazing on the porcelain insulator. If the heat range is
correct and this condition exists that can also be an indication of
too lean of a burning mixture. Check for a low float level, too small
a carb jet, or a plugged fuel filter or restricted line. Too cold will
cause extensive carbon build up and, eventually, misfiring. If the
plugs are the correct heat range then check the air filter for excess
debris and plugging. The enriched fuel mixture will show up as black
exhaust under regular or loaded conditions of the engine. Refer to
your inspection chart to verify your engines exhaust color. A stuck
choke lever, a carb float set too high, or a stuck needle valve will
also cause too much fuel to enter the engine and cause plug fouling.
These conditions will be discussed under carburator problems and cures.
With respect to the above, other plug conditions are relatively rare
but myriad in number. Scavenger deposits appear as crusty yellowish
or whitish buildups on the electrode surface. These result from the
chemical makeups of various fuels. Clean the plugs, check the gap and
reinstall. Severe detonation, preignition, or over advance ignition
timing can leave aluminum deposits from the pistons around the plug
electrode. This indicates extensive internal engine damage.
Preignition can also severely erode the plug end giving it a melted
appearance. If the electrode appears to be dished then check the coil
polarity and make sure the wires are attached according to your
tractors specs. Squashed electrode ends result from a mismatched
plug. The plug is probably too long and the piston has actually hit
the end of it.
When installing used or new plugs it is important to check the plug
gap. This is the distance between the two electrodes at the base of
the plug. If putting in a used plug check the electrodes for cleaness
and squareness. A small ignition file should be used to break any
oxidation off the ends. Make sure and file the firing surfaces flat
with no protruding edges. Take a wire feeler gauge (flat feelers will
give an inaccurate reading on older plugs) and set the gap to specs.
If no specs are available a rule of thumb would be .035". Make sure
and bend the electrode from the side to attain this measurement.
All in all it is important to thoroughly examine the plugs on removal.
These little items can give a wealth of information about the
internal condition of an engine along with related componets like the
carb, ignition system, and air filtering system.
We talked earlier about investigating a high carbon oily goo residue
on the spark plug using a compression test. This will help determine
the exact cause of the oil buidup. Generally, oil will come in
through wore rings, wore valve guides, or worn valve seals (if so
equipped). This determination divides the leakage into two
catagories; the heads (valve guides), or the bottom part of the engine
(the rings). Using a good compression tester one can isolate this
location. The tester is, basically, a pressure gauge with a hose on
it that screws into the spark plug hole. They are available at many,
if not all, automotive parts stores. After warming up the engine,
remove the plugs and screw the tester into the first spark plug hole.
Disconnect the coil secondary wire at the distributor and ground it on
the engine somewhere. Then open the choke and the throttle all the
way to provide unrestricted air passage into the intake manifold.
Crank the motor over five or six revolutions or until the gauge needle
stops rising. Make note of the psi rating on your clipboard. Also
note how the needle rises; goes up in jerks, all at once, little at a
time. Go on and do the balance of the cylinders recording each
reading as you go. Now it's time to examine the readings. If you
have access to the pertinent engine manual you can find the psi
compression specs in the tune up section. If you can't find the
appropriate specs then you need to focus on the continuity between the
different cylinders. Generally, a reading between cylinders of no
more than 10 psi to 15 psi is permissable. Engine spec compressions
may vary anywhere from 80 psi to 150 psi so look for the average on
your unit. If three out of four cylinders are around 80 psi and the
fourth is real high at 120 psi then one could assume that the firing
chamber has an inordinate amount of carbon buildup. As you review
your notes observe how the test went for each cylinder. If the needle
action came up only a small amount on the first stroke and little more
on succeeding strokes, ending up with a very low reading, burned,
warped or sticky valves are indicated. A low buildup on the first
stroke with a gradual buildup on succeeding strokes, to a moderate
reading, can mean worn, stuck or scored rings. If two adjacent
cylinders are low, a blown head gasket or warped head to block surface
could be responsible. Add a little heavy weight motor oil to the
cylinders with low readings and recheck them. If the compression goes
up a noticeable amount, worn rings are indicated. If the addition of
the oil produces no significant change, valve trouble, a broken piston
or a blown gasket may be at fault. If the readings of all cylinders
are within reasonably close proximities then one can assume that the
upper end of the engine is in good condition.
Spark plug evaluation and compression testing are important parts of
an engine evaluation because it will determine if the engine is in
adequate condition to do a tune up on. It is a shame to invest in
assorted tune up parts only to have the motor still miss and run lousy
because a burned valve or broken ring was not diagnosed.
Contributing Author - Curtis Von Fange
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