KEEPING YOUR TWO-STROKE ENGINE FLYING
By John Davis and Ralph Shultz
If you really would rather be flying than looking for a place to set her down, it's very important that you know and understand a little about your engine and what it takes to keep it ticking. This article will introduce you to some of the critical parts and components of your basic power package, and provide some insight as to the how, what, and why's of their maintenance.
CHECKING THE POWER PROVIDER'S INTERNAL HEALTH
Two-stroke engines must be well sealed to outside air and internal cross breathing. To ensure this all gaskets and seals must be in good condition. Any air leakage leads to a lean air/fuel mixture and may cause excessive heat in the combustion chamber. This can result in the piston expanding too much within the cylinder wall, causing the piston to rub and/or to seize up the engine. If either of these conditions has happened, the internal cylinder walls, pistons and rings should be evaluated before returning the engine to service.
The intake manifold flange/cylinder joint(s) must be tight, so occasionally check the condition of the screws and their torque. Torque should be within 10% of 90 in/lbs. (6 mm bolts) on a Provision 4 engine and 195 in/lbs (8 mm bolts) on a Provision 8 engine. These numbers, Provision 4 or 8, refer to the number of bolts used to secure the gearbox or the prop redrive tower unit to the case.
The piston carbon should be checked at between 50 to 150 hours of operation. This can be done by removing the intake and exhaust manifolds so the pistons can be seen through the ports. Verify the rings are free by moving the pistons up and down (turning the crank) or pressing them with a probe. Do not use anything sharp that could scratch the rings.
Lower the piston and estimate the thickness of the carbon layer on top of the piston. If more than .020" consider having a de-carbon job done. When the manifolds are reinstalled be sure to thoroughly clean the mating surfaces, use new gaskets and torque the screws evenly while working up to the required values in stages.
The carburetor intake air filter must be kept clean and in good condition. Failing to do so will result in a rough running engine producing less power and contributing to excessive fuel use.
Cleaning of the filter can be accomplished by washing it gently in a cleaning solvent, soap and water or even gasoline and then letting it stand to air dry. Follow up by spraying a light coat of air filter oil on the filter mesh. Don't overdo the oil spray. If washing the filter with premix fuel there is enough oil remaining after the gas evaporates that oiling is not considered necessary
CAUTION WHEN DRYING
"Do not use open flame" "Excess heat will shrink the cotton filter media"
"Do not use heat dryers" "Excess heat will shrink the cotton filter media"
It's a real good idea to safety wire the filter to the carburetor(s) before flying. Some believe it's cheap insurance to replace the filter once a year, but cleaning it every 50 hours or so should keep it serviceable for a number of years.
CHECKING THE IGNITION SYSTEM:
Check all wires and connectors making sure that none are frayed or broken. This includes the kill and temperature probe wires.
CHECKING THE COOLING FAN BEARING & BELT TENSION:
The belt that drives the cooling fan on the Rotax 503 & 447 is located on the magneto end of the engine and is beneath, or behind the black plastic grill. It is important that the belt and fan bearings are in good condition. If the cooling fan bearings are noisy or feel rough, they must be replaced. If the belt is loose but does not show signs of heat glazing, cracking and checking then we simply tighten it. This is best done by using a fan wrench to hold the fan while we loosen the nut. Do not try to loosen the nut without a fan wrench as that usually results in a broken fan.
After you remove the fan nut and forward half of the pulley, you will see a few thin spacer washers between the two pulley halves. Remove one or two washers and reinstall the pulley. The washers you removed should be placed under the lock washer. Replace the nut and torque to within 10% of 530 in/lbs (44 ft. lbs.). The belt should show about 1/4-inch deflection when pushed moderately at mid-span with your fingers.
When installing a new belt, remember to go back in and retighten the belt after 25 hours of running. New belts will loosen up some after this adjustment, so check the belt about every 50 hours or so thereafter. Checking it ensures good insurance!
CHECKING THE ENGINE STARTING SYSTEM:
If you have a pull starter, all you have to watch is the condition of the rope. The rope can and does stretch and fray with time. Keep an eye on the fraying. You will know when it's time to replace it. Don't be afraid to do this. Just remove the cover screws. The insides are pretty much self-explanatory, much like a lawn mower starter.
On electric starter-equipped engines regularly check all wiring and connections. No broken wires or cracked insulation should be allowed. All starter mounting bolts must remain tight.
If the starter has been working correctly but has been gradually slowing down, it's probably due to a bad battery connection, a poor ground or possibly trouble with the brushes. If it locks up, it will probably have to be removed in order to inspect and repair it.
CHECKING THE PROP AND DRIVE SYSTEM:
Some engines have a gearbox to drive and reduce the speed of the propeller. Other engines have a belt and pulley system, referred to as “redrives”, to accomplish this. Regardless of the type, they both must be checked and maintained.
The gearbox must have the oil level checked from time to time at about 50 hour intervals. Always inspect for oil leaks, loose bolts, and make sure the vent is open and in place. Safety wire must be installed in the oil inlet and drain plugs.
The belt redrive units work well if properly maintained, but we have to look after certain things, such as the prop shaft bearings, belt tensioning mechanism, nuts and bolts, and of course, the belt and its proper tension.
The belt tensioning mechanism parts usually include a spreader weldment, a bolt through the spreader used to move it up and thereby spread the pulleys and a jam nut to keep the bolt from backing off. Belt tensioning cannot be done while the “large redrive nut” is fully torqued (150 ft/lbs). Anything beyond snugly tight is too much for tensioning.
Most flyers set the belt tension by spreading the pulleys to the point that will just allow the belt to slide a little forward and aft on a cool top and bottom pulley when a cool belt is pushed fairly hard by the fingers. Another method of measuring belt tension is to measure the belt deflection. Discussions of this method always seem to bog down to how much force, where to apply it and just how the deflection should be measured.
The pulleys should last the life of the engine if they are kept clean; that is no dust, dirt or other debris should be allowed to remain in or on their teeth. The belt will also benefit by being kept clean. In addition to being clean, the belt should be checked for any signs of wear and overheating such as discoloration or loose cords along the edges and cracks. With proper care the belt should serve well for anywhere from 50 to 150 hours and possibly beyond. Be especially watchful for any loose or misshaped teeth. Now after saying all this, many who have experienced belt failures say they saw nothing to foretell the failure! Remember, if the engine suddenly goes to full RPM in flight immediately bring power back to idle. For more very interesting information on the use of these cog belts please read the report located at Challenger Synchronous Belt Report
The prop-shaft bearing package is best checked with the cog belt disengaged from the top pulley. Apply pressure, side to side and back and forth, on the propeller while watching and feeling for any movement from the nut end of the prop shaft. Any movement, gritty feeling, rough running or clinking of the bearings while spinning the prop signals it's time to replace the bearings. For what it's worth, recent discussions on redrive bearing(s) failures seem to indicate that these bearings cannot easily be judged as good or bad until the prop is removed and the prop shaft and bearings pressed out of the bearing housing.
Check the propeller for any damage such as nicks, cracks and looseness, occasionally checking the blade tip tracking. The blade tips should track within 1/8-inch of each other.
For more information on prop damage see: Wood Prop Repair & Balancing
Inspect propeller bolts and their safety wires before each flight. These bolts should be torqued to 144 in/lbs every 50 hours or if the prop feels loose or tracking is found to be excessive. The bolts should be replaced if heavy rust is present around the bolt head, head to shank radius and particularly where the thread ends on the shank.
CHECKING THE EXHAUST SYSTEM COMPONENTS:
There are only a couple of things to keep an eye on here. At every preflight inspection check the exhaust manifold piping joint connector springs or brackets for wear and breakage, making sure the safety wiring and the high temp silicone on the springs are still in place. Inspect the articulating exhaust joint for signs of wear-through and that it is still protected with high temp joint grease.
The engine shroud screws must be kept tight. A little blue Loctite will help keep vibration from working these screws loose.
CHECKING THE OIL INJECTION SYSTEM:
As part of your preflight inspection checks, check that the oil reservoir tank has more than an adequate amount of oil for any engine operations planned. Inspect the tank and pump mounting bracket(s) and hardware for cracks, loose nuts and bolts. Check for any sign of leaks.
Another very important item to check is the fuel/oil mixture ratio put out by the pump. This is easily done using a semi-transparent oil reservoir with ounce marks on the side. Fly a typical two-hour flight, check the fuel and oil consumed and do the math to see what the ratio turns out to be remembering that there are 128 ounces to the gallon. If it works out to be 50:1 that's acceptable. If it's higher, say 40:1 then no real harm will be done, just a little more carbon being deposited on the pistons. If it's lower, say 80:1 then maybe excessive wear is taking place in the engine bearings.
CHECKING THE FUEL DELIVERY SYSTEM:
It's a good idea to filter your fuel when filling your tank. A Mr. Filter funnel is said to be the very best for this purpose. Carrying a Chamois skin or cloth on x-country flights may also save you some grief.
Check for “junk” in the bottom of the tank and remove any found by suctioning it out using a spare bulb pump fitted with a 1/4-inch copper tube and a length of fuel line kept on hand for this purpose.
Make it a point to periodically check the fuel tank for cracks or crazing. Pay particular attention to yellowed, discolored areas in the bottom half of the tank. Look for any signs of fuel leakage or seepage.
Urethane or Tygothane type lines should be your first choice for the fuel line material. Vinyl lines should be avoided as they tend to cut and abrade easier and do not stand up to gas as well.
Inspect all fuel line tubing for condition. The lines should be soft (squeezable), clear (not dark or crazed), and, of course, display no signs of leaking.
All fuel line connections, including the primer lines if installed, must be tight and leak free. Clamps should not cut into the tubing. Many types of clamps leak when over tightened. A length of safety wire wrapped twice around the tube, pulled tight but not cutting the tube and then twisting the ends together works well. Indeed, many flyers believe this makes the best clamp because it seems to form a more perfect circle.
Many flyers think that the squeeze bulb pumps are, or can lead to, more trouble than they are worth. More and more pilots are not installing them, choosing instead to use an electric fuel pump to get fuel to the engine driven pulse pump or just pulling the rope starter a few more pulls to accomplish this.
If you are using a squeeze bulb pump check for a hardening of the rubber as noted by a stiff and hard to squeeze bulb. If the pump will not pump fuel past it or if the pumped fuel won't stay in the line above the pump after pumping it to the carb bowls, replace it. Keep in mind that the check valves in each end of this type of pump are notorious for sticking without warning. Once stuck open or closed, there is no reliable way of unsticking them. If they act up, just replace the bulb!
Fuel filters seldom show signs of contamination. Simply put, if you see any stuff in the filter throw it away! Many think the best medicine for its care is to replace it after say... 35 hours of use, or whenever changing spark plugs, but at least at each annual inspection. Again, if unsure, replace it!
Pulse pumps are blessed with long life and rarely fail. When they do, it's usually the internal diaphragm or a leak somewhere in the pulse line or its connections.
The pulse line connecting the pulse pump to engine crankcase must be checked for any signs of collapse when engine is running. It should be as short as possible, be resistant to internal vacuum and display absolutely no cracks!
Engine controls such as throttle lever(s) and cable(s) can and do stick, slip or otherwise fail to operate properly. When inspecting them, check all lock pins and rings, pivot pins, swaged ends and for frayed surfaces. Check for free and smooth operation. Use a suitable lubricant if needed but keep in mind that in cold temperatures some fluid lubricants will freeze and lock things up on you.
Carburetors can be a mystery to many of us. But there really is nothing to be afraid of about them. As carbs go they are quite simple. The Bing 54 has three fuel circuits, each controlling a specific RPM range. Be sure to check out the Engine (Carb) Related Articles (conveniently grouped and indexed) on the 101 site map page located at: Site Map
Carburetor sockets can work themselves loose or crack with age. They then become a source for air leakage. These sockets must not leak! Cracks, even small ones are not acceptable. Socket leaks lead to high internal temperatures to say the very least. If they show any sign of cracking or crazing don't wait to replace them, do it now.
When replacing them do not use any lubricant on the inside of the sockets. They are installed dry and are done so to prevent them or the carburetor(s) from slipping or being thrown off in flight. Again, don't over tighten the clamps. Check these sockets and their connections before each flight. Replace them at the first sign of failure.
Learning about your aircraft's systems and their proper maintenance should keep your magic carpet flying and purring with contentment.
May all your flights have light winds or be conveniently fast from your 6 o'clock!
** Remember to smile, be happy, and enjoy the gift of flight. **