After checking out the spark plugs, it took more time to explain this than it really did to do the inspection. At any rate it was decided that a compression test was in order to verify the internal engine status. That way one doesn’t have to buy any tune up parts until you are sure that the engine is in reasonable mechanical condition. If the tests show a fictional compression reading of 85, 80, 82 and 86 psi, this is well within accepted parameters of a 15 psi spread. If the spark plugs had a light gray tinge to them and the gap was not excessive, this is normal. If each one also fired with a sharp bluish white spark when you tested it with each separate tractor plug installed the…
secondary wire was properly grounded against the block. It was decided to clean, regap, and reinstall the plugs as the wear on them was very little.
The next step is to examine the secondary ignition wires; commonly called spark plug wires. These items transmit a high voltage pulse from the distributor to each spark plug. Contrary to popular belief these wires are quite sensitive. Most secondaries have a soft copper core wire surrounded by stainless steel or carbon impregnated thread mixed with an elastomer type conductor. The outside covering of a heavy layer of insulation prevents the 12,000 to 25,000 volts from bleeding out when the wire is carrying current. The resistance type wires do not handle sharp bending or jerking and can break internally thereby ruining the wire. Routing of wires near exhaust systems and high temperature areas can cause the exterior coating to dry out and crack. Excessive exposure to oil and antifreeze can chemically break down the coating causing it to swell and get slippery or slimy. Pulling on the wires to release them from the spark plugs can separate the conducting material causing internal arcing and/or can pull the steel plug connectors off the wire ends leaving them dangling off the plug. Any of these misuses can cause the wires to leak voltage and either misfire to another plug wire, cause carbon tracking in the distributor, cause an intermittent fire at the plug, or no fire at all. Keep them clean and separated from each other with the proper wire clips available at auto stores. When removing the wires from the plugs make sure and grasp the rubber boot, not the wires. Secondary ignition wires have the habit of looking good, but not necessarily being good. They can be the cause of irritating engine ‘poofs’ that simply can’t be located or can make an engine run erratic and backfire. There is a real quick way of identifying bad secondary wires on an engine. Run the engine on a moonless night away from all forms of light. When your eyes get adjusted to the darkness watch the plug wires while on the high idle setting. Bad wires will send eerie flashes of violet light from one plug wire to another. An extremely bad set of wires with moisture on them will give quite the light show. A more sophisticated way, though, is to measure the wire resistance using an ohmeter. A general rule of thumb is 8,000 to 12,000 ohms per foot. This will also test the wire for continuity ensuring that there are no breaks in the copper wire core. An alternative way to check the wires in lieu of an ohmeter is to simply put a test light in series with a voltmeter (or a small 12 volt test light) and the tractor battery. The glowing light or a voltmeter reading the same as the battery indicates continuity of the copper core. A follow up test is to remove the plug wire, start the engine and run a grounded test probe along the plug wire. A wire with a crack or puncture in the insulation will arc a spark to the grounded probe. Replace that wire, it’s trouble!
The other end of the secondary ignition wire is installed in the distributor cap. This cap, made of hard plastic or bakelite, distributes the appropriate pulse of voltage to the correct spark plug as determined by the rotor inside. Proper inspection of the cap will reveal hints about the intergrity of the ignition system. Make sure when removing the plug wires from the cap that you grasp and gently twist the boot as you pull. It might be a good idea to label the wires with tape and a magic marker so they reinstall without error. Visually inspect the cap towers. They should be clean and shiny. Many times the secondary wire to the coil will have heavy corrosion in the terminal tower. This causes erratic operation or a no run condition. If corrosion is present replace the cap and secondary wire boots as they are probably cracked or loose and let moisture into the tower. Before wiping the cap off check for carbon tracking along the tower tops to the cap base, usually where the retaining clips are located. Bad plugs or secondaries will cause an occasional or regular spark to occur from any one of the towers to the nearest ground….the mounting clip. The evidence of this happening is a small powderlike carbon trail that resembles a hairlike tree root running from top to bottom of the cap. Replace the cap if this is present. Now turn the cap over and inspect the inside. Check the center carbon terminal and the individual terminal posts for cracking or burning. Carbon tracking on the inside will look similar to outside except that the hairline fracture lines will run from terminal post to post; evidence of cross-firing, backfiring, or missing. If the posts are burned or grooved the cap should be replaced. Mild scaling, caused from the spark leaping from the rotor to the post, can be scratched off with a sharp object. Try not to groove the terminals as they are probably made from aluminum and are quite soft. When replacing the cap it is best to replace the rotor also. If reusing, gently scratch the rotor prong so as to remove any oxidation or scaling.
The ignition points act as a switch to open and close the primary ignition circuit. When the points are closed, current flows through the ignition coil and builds up a magnetic field. When the points are opened, current through the coil is stopped and the magnetic field collapses, creating a high voltage current. Sound simple? It is if you understand how electricity works. But stay focused in getting the points set correctly and don’t worry about how it works…just as long as it works.
It’s easier to clean and inspect the distributor if it is removed and put in a vise on the workbench. It is understandable, though, if one doesn’t feel comfortable pulling this rotating heart from engine. Getting it reinstalled and timed correctly can be a hair pulling experience if you’ve not done it a few times. In order to save reinstallation time set the distributor cap back on the unit and take note where number one plug wire tower is. Mark the distributor housing side with a chalk mark that corresponds to this tower. Pull the cap back off and note where the rotor firing prong is. Rotate the motor by hand (it’s easy without the plugs in) so the rotor prong lines up with the chalk mark (also note the rotor rotation direction). You might note that when they align, the timing marks on the crankshaft pully will line up with the timing cover arrow. This means that number one piston is at top dead center on the compression stroke and the distributor is firing on that spark plug terminal. Also note that the points are just opening….you can tell because a high spot on the rotating cam on the d-shaft has pushed against the rubbing block at the point base forcing the points to separate creating a gap. When the spark jumps the gap it causes the coil to fire the high voltage spark to the cap rotor and from there to the correct cylinder. With the motor timed to number one you can now remove the mounting bolt and fork that retains the distributor in the motor housing. Observe how the rotor shaft will rotate a little as you remove the distributor. This is because the drive gears are usually tapered and as you pull up on the distributor the unmeshing of the gears causes the shaft to turn. Remember when you reinstall the shaft you will need to be off center to the mark you made by a little to compensate for this gear taper. Make sure and disconnect the battery ground cable so the kid doesn’t roll the engine over while you’re working at the bench. If you forget, and he does, don’t dispair. You can retime the engine by placing your finger over number one spark plug hole, then roll the motor over so you feel compression trying to blow your finger off. Jockey the crank to align the timing marks back up and this will be top dead center (TDC) for number one piston. When putting the distributor back into the bore align the rotor tang with the mark on the distributor casing that you made….make sure to offset the rotor a little to compensate for the gear mesh…..and slide it in. Some engines have a tang at the bottom of the d-shaft which runs the oil pump. Make sure this tang engages the pump shaft. If it doesn’t then put a little downward pressure on the shaft and then rotate the engine a quarter or half turn. The shaft will then drop into postion.
Now that the distributor is in the vise observe the inside condition. Is it clean or is it full of oil, dirt, or carbon debris? If there is oil in it then the bushing or seal at the bottom of the distributor is bad and needs to be replaced. Wobbly motion of the shaft in the housing bore will confirm this. Excess carbon indicates the points have been arcing too much or there has been a lot of cross firing under the cap. Dirt and debris can be caused from an opening in the side of the cap and/or a dry rubbing block at the base of the points. A little bit of high temperature grease on the block facing the cam rotation will reduce that problem. Remove the points and condenser and set aside. Underneath the mounting plate there may be a spark advance system of springs and counterweights. Make sure they freely rotate and that the springs aren’t broken. Moisture buildup tends to rust these gizmos up a lot so you may have to break the distributor down a little farther to get to them. Take note of the proper order as you disassemble. The configuration may look a little different on the bench as it looked in the unit. Wire brush and clean the pieces in solvent before reassembling. Some newer tractors may have a vacuum advance diaphram. Make sure the rotating linkages are free to move and check the diaphram by sucking on the vacuum port. The vacuum plate will rotate as pressure decreases. Hold the pressure on with your tongue for a few seconds to see if the diaphram has any small cracks in it. Now lets check the points. If the points are a dull slate color and if wear is not excessive they can be cleaned with a point file and reinstalled. Don’t use sandpaper or emery cloth as this will scratch the point surface and leave small grit particles imbedded in the points. Use a clean file as this will remove a small layer of metal creating a good flat surface to fire against. If the points are burned or pitted they need to be replaced. If one side of the points is severely pitted and the other side has metal buildup on it then you need to check and make sure the correct capacitor (or condensor) is installed. Also check the coil polarity. Most engines have a wire running from the negative terminal of the coil to the distributor. It is common practice to replace the condensor when the points are replaced. A leaky one can cause excessive point arcing and/or metal transfer on the point surfaces. Reinstall the refurbished or new point set in the distributor. Rotate the shaft so the cam lobe separates the points, creating a gap, and measure the gap opening with a wire type feeler gauge. Set to specs and tighten the mounting screws. Make sure and keep the rubbing block lubricated with a small dab of high temperature grease. Install a new rotor, line up the casing marks and reinstall the distributor in the housing….don’t forget to compensate for the tapered gears. The mark on the distributor casing should now line up with the rotor tang. If it doesn’t then you are probably off one tooth. Pull it out and reinstall it again. The plug firing order is usually stamped somewhere on the engine block. Locate it there or in the manual and reinstall the plug wires accordingly. You know where number one plug is on the cap and you wrote down the d-shaft rotation, so install the wires in their respective order and attach them to the corresponding plug. With the current distributor alignment the tractor should start and run. Additional timing should be checked and adjusted with a timing light. Attach the light pickup to number one plug wire, start the engine, and aim the light on the crankshaft pully timing marks. Adjust the distributor according to specs. If no light is available then you can static time the engine. It won’t be as close a timing light but it will be next best. Look in the book to determine the exact firing timing for that motor. Most of them are 4 or 6 degrees before TDC. Rotate the timing mark on the crank pully to line up with the housing cover arrow according to the specs. Hook up a spark plug to the number one secondary wire and hold it against a ground. With the ignition on, rotate the distributor in the direction of normal distibutor rotation and watch for the exact moment that a spark occurs at the plug. If it caught you by surprise back the distributor up and do it again. The engine is now timed properly. Tighten the distributor mounting bolt and secure all the necessary wires.
One last thing to check is the integrity of the primary wires running to the coil and distributor. Check to see that they are securely fastened and have no exposed strands from broken or crimped insulation. The coil should be clean and dry; check the termial tower for oil or corrosion. If oil is present it indicates internal leakage and the coil should be replaced.