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Contributed Article

Tuning Up Your Tractor
Plugs and Compression Testing
by Curtis Von Fange

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.

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