How to Diagnose

June 17, 2015

1  How to Diagnose a Moped     

by Shaun Strahm, Jan 2008         This is a brief two-page universal diagnosis procedure. Details are omitted.


Engine: Compression, solidness, smoothness can be felt by hand. Good = solid & smooth. Bad =loose or rough.

Locate the magneto flywheel or the center part of the clutch wheel. That’s the crankshaft. It should turn freely with your fingers, except every turn it should require almost a whole hand to squeeze the air above the piston. It should spring back in both directions in the region from mid stroke to top. Removal of the spark plug allows you to feel only the magnetism in the flywheel, which can feel like compression. Compare the compression with and without your finger over the spark plug hole. With excellent compression you won’t quite be able to hold it all in and it will hiss and make a red circle on your thumb. That’s what over-100psi compression feels like. Low compression can be caused by several things, in order of severity: loose head nuts, decompression valve too tight or leaking, gummed-up stuck rings, worn or damaged rings, worn or damaged cylinder, loose brass wrist pin bushing, or loose wrist pin hole in piston. The decomp valve can be checked by removing the head, tipping it up the combustion chamber facing down, and filling liquid such as WD40 behind the decomp valve. None should leak past until you push it open with your finger. The piston and rings can be visually inspected by removing only the exhaust and looking up the exhaust port. Vertical lines across the rings and piston means it needs at least new rings and maybe a piston and cylinder. A solid black coating on the aluminum piston means the ring grooves are probably also black-coated, causing rings to stick and have low compression. In many cases the compression will be enough to run, but with some issues, in order of severity: needing a bigger idle hole in the carburetor to idle, idling badly, loss of power, making the spark plug get black and need frequent changing , dying out often at idle, even with over-rich idle mixture and a new clean spark plug.

Besides compression, the crankshaft bearings are checked simultaneously by turning the engine over by hand and feeling how solid and smooth it is. Brand new ball bearings feel completely smooth and silent, like a wet ice cube across a smooth countertop. Bad bearings have cavities and pits where the balls roll at, causing them to feel and sound rough, like there’s sand in them. Besides the two main ball bearings on each side there’s a center needle bearing in the big end of the rod. It’s part of the crankshaft assembly, which is expensive $100+, and requires disassembly of the cylinder and piston to isolate its motion and feel it by hand. An engine will run OK with rough bearings but will sound bad, like a blender or a garbage disposal. But there’s a catch. Along with the damaged bearings are also usually bad seals, the rubber rings alongside each crank bearing that seal in the two-stroke crankcase suction and pumping pressure. Leaking crank seals will cause the carburetor to need bigger jets to compensate for dilution of the gasoline-air with fresh air, and to have less power and die out often when idling. One way to check the seals is for one person to pedal the bike, turning the engine rapidly with the spark plug out and carburetor off, while the other person holds their finger over the intake manifold hole and feels how much suction there is. Good is when it’s hard to pull your finger off and it leaves a purple dome suction mark. Besides the seals, the cylinder base and the crankcase can also have compression leaks.

The next thing to check for is looseness of the piston and/or connecting rod. This can be done by lightly touching the top of the piston with the tip of a pen or pencil, through the spark plug hole. With the piston near top, wiggle the wheel back and forth slowly and feel the piston move along with it. There should be no detectable looseness. If there is, then disassembly is needed to tell if it’s the piston pin hole, rod little-end bushing, or rod big-end needle bearing.

The last thing to talk about is the first thing you notice. The engine is frozen and the crankshaft flywheel does not move at all. This happens often when the moped sits outside for a long time and rusts inside the engine, especially after being run without enough oil in the gas. The first step is to put penetrating oil in the spark plug hole and in the intake, then try to rotate the flywheel. If it’s just frozen because of tar, then it will melt away and be OK if it was already OK before it sat up. Mopeds stored outdoors are usually rusted solid and need crankshafts, pistons, and sometimes flywheels, cylinders, and whole engines. It’s ecommended to make a tool to hold onto the flywheel to get it to rotate first before disassembly. Sometimes it “wakes up” with just that. Other times it’s stuck tighter than 100 lbs hanging on a 3 foot bar, about the breaking point of the woodruff key. That’s when you remove the head and exhaust, raise the cylinder and see if it’s rusty in the crankcase or just the piston & cylinder.

Ignition: Does it have spark? Is it blue? Does it happen at the right time (17-25 degrees before top)?

Check for spark by removing the spark plug, plugging the wire on it, laying it on the metal of the engine or frame, and then rotating the engine (turning it over) while observing the spark plug gap. There should be little blue or lavender sparks making audible snaps, every time the wheel goes around. Having a fouled plug can falsify the test. The white porcelain insulator must be clean, dry and white to dark brown, not wet, black, silvery, or bridged with carbon balls. The corners of the electrodes should be sharp, not rounded, with a 0.020” gap, less than a car. When there’s no spark at all, more often than not, on ‘70’s US model mopeds Peugeot, Piaggio/Vespa, Garelli, Cimatti, Derbi, Tomos, and others, it’s a wiring problem with the brake light. The first thing to do is find out if there is a “secret” magneto wire that needs to be grounded. It’s the blue wire on most, except on Peugeot it’s black and on Puch and some Sachs it’s blue and black. With the external ground bypassed directly to ground the spark is re-checked. One more bypass is to disconnect the kill switch, in case it’s “off”. The kill switch grounds the coil wire to kill the engine. After checking the points-to-coil-only wire and coil-to-ground, what’s left is inside the magneto, usually dirty points and sometimes a shorted wire. You can see the points opening and closing through the windows in the flywheel. First determine which way it rotates by rotating the rear wheel and engaging the clutch. See if the arrow on the flywheel is correct. Then find top by feeling the piston through the spark plug hole. It’s hard to find the exact point. Now the points should be open fully. Go forward all the way around again. As the piston rises the points are closed until 20 degrees before top, or about 7/8 inch along the edge of the wheel, after that they’re open to a gap of .012 to .018 inch for about 90 degrees. The spark happens at the moment of opening. You can tell visually if the timing is close enough to run, if the points open a little before top.

Carburetion: Does it have gas? Is it getting through? Or does it have too much?

Once the engine has compression, both primary and crankcase, and it has spark at the right time, then all it needs is some flammable stuff mixed with fresh air. Moped carburetors are frequently crusty with tar or glaze from dried up gas and oil mix. It clogs the tiny holes the gasoline passes through and makes the float stick. Besides tar there’s usually some rust powder from the gas tank and maybe white zinc oxide from the carb itself rusting. So it automatically needs a
carburetor service or at least an inspection. For diagnostic purposes, a flammable spray such as starting fluid (ether), carb spray, or WD40 can be used to start the engine, with a wide-open-throttle no-choke carburetor or without a carburetor. Hearing it run for a few seconds until the spray is consumed is good way to quickly know if it will “live”.

Very often a moped will become flooded. This is a condition where excess gasoline and oil have built up, and not enough fresh air is getting in. The thing to do is to keep cleaning the spark plug with carb spray. Once it gets wet it becomes ineffective. Pedal starting rapidly with a dry spark plug, wide open throttle, no air filter, no choke. Once it starts popping you should keep the throttle wide open until it cleans itself out, in maybe 15 to 30 seconds. Healthy bikes can become flooded from leaving the choke on or lying over with the gas left on. Not healthy bikes can become flooded from gasoline leaking past the float, or from too big of a jet,  clogged exhaust, faulty ignition, low compression, or a little of everything. Removal of the exhaust really helps a lot. It’s often full of oil or black gasoline anyway. You have to start a flooded bike to know whether it will clear out after it runs awhile or whether it will keep getting flooded.

The test ride: Does the transmission work? Is it making full power? Are the frame, wheels, brakes and lights OK?

Now that the engine runs, a test ride is needed to learn what’s working or not. First the tires are aired. The wheels are checked for looseness. Cables, throttle, and chain are oiled. The forks and handlebars are checked for looseness and adjusted straight. When riding, see if it pulls to one side (bent fork), is wobbly (loose rear wheel, spokes or wheel bearings), oscillates (bent rim or lumpy tire), stops poorly (oil film or rust on brake drums), goes slow (clogged exhaust, timing way off, something rubbing). Ride it long enough, say 10-15 minutes, to let it wake up. Check the head light, speedometer light, tail light, brake light, horn, and speedometer. Finally, see if it goes full speed, usually 25-30mph.