Service Basics

September 5, 2008

CONTENTS:  Welcome   1  Owners Manual   2  Gallery of Manuals   3  Spark Plugs   4  Service Manuals

5  Tire Mounting   6  Threads   7  Seized Engine   8 Oils

Welcome to Myrons Service Department. Mopeds are easier to work on than cars. You don’t have to lay under them and get dirt in your eyes and oil in your hair.

Myrons Mopeds has stopped doing most moped service, except for new Tomos. That has made things difficult for many people. So here and throughout the website, plenty of useful information is provided, or is being planned, to help people repair mopeds and find parts. Things like how to cut a hole and install a universal bulb socket in a burned out metal backed headlight bulb to recycle it. 

1  Moped Owners Manual and Introduction 

by Shaun Strahm, Myrons Mopeds, 1879 W Commonwealth #L Fullerton CA   714-992-5592   Dec 1999


Mopeds have two-stroke engines. Outboard boats, chain saws, weed wackers, and many small motorcycles also have two-stroke engines. It’s called “two-stroke because the spark plug fires every two strokes of the piston (down & up). Four-stroke engines, like cars & lawnmowers have, fire the spark plug every four strokes of the piston (down, up, down, up). This is why two-stroke engines sound more like buzzing insects, while four-strokes sound more like a drum roll.

Two-stroke engines run on gasoline with two-cycle oil mixed in, or have oil injection. A two-stroke engine does not have motor oil in the crankcase, like a four-stroke does. Instead the crankcase contains the gasoline and air mixture, with a little bit of two-stroke oil added in. After the two-stroke oil coats the crank bearings and piston it is burned along with the gasoline. The burning of oil makes a little bit of smoke in the exhaust, which is normally barely noticeable.


The most important thing you can do to keep your moped healthy is to use a good quality two-stroke oil and mix it in the correct amount of 50 to 1 (2.5 ounces per gallon). This will make the engine stay strong and last a long time, and help prevent carbon build-up inside the exhaust. The oil we sell and recommend is called Champion 2-Cycle Oil and comes in small 2.5 oz bottles for $2, and large 12.5 oz bottles for $5. Some of the highest mileage mopeds I have seen over the years have used this oil, which is a petroleum based oil with expensive synthetic additives. Pure synthetic oils are also very good. You buy good quality two-cycle oil at motorcycle and lawnmower shops. Auto parts stores usually only carry ordinary less expensive blends. Grocery and drug stores also carry cheap two-cycle oil.


If your moped has oil injection, then you don’t have to mix oil in the gas. Instead you add two-stroke oil to an oil tank located usually under the seat. A small oil pump squirts about one drop of oil every few seconds into the intake port, where it is ultimately burned up. The oil gets used up as you go, and so the oil tank must be refilled every so often, usually about every 3 gas fill-ups (300 miles). If the oil tank is allowed to run out of oil after a few minutes the engine
will seize up and suffer piston damage.


The best way to mix the oil and gas is with a gas can. If you have a small pill-bottle-size 2.5 ounce bottle of two-stroke oil, you just pour that into an empty one gallon gas can and then take the gas can to a gas station and put one gallon of gas in it. By adding the oil first you have eliminated the need to shake the gas can afterwards, because the force of the gas pump has already stirred up the mixture and dissolved all of the oil. If you add the oil last it will at first settle at the bottom, and the gas container must be sloshed around a few times. Two-stroke oil is usually colored green so that you can see the green tint of pre-mixed gasoline. Without shaking or stirring the gasoline would be dark green at the bottom and clear at the top. Once the oil dissolves the color becomes very light green throughout, and the oil will stay dissolved and never settle out so there’s no need to ever shake it up again.

The other way to mix the oil and gas is directly into the gas tank. Most moped gas tanks hold about one gallon maximum and they usually don’t get below 1/4 gallon before they’re refilled, but the tank should not be filled all the way to the max because of gas leakage out of the gas cap vent hole, so the actual amount of gasoline needed to refill is usually about 1/2 to 3/4 gallon. If you buy 1/2 gallon of gas then you need 1/2 of a bottle of oil. If you add the oil just before adding the gas, it will be mixed automatically by the force of the gas pump and you won’t have to shake the bike up afterward. If you add the oil last then just make sure to shake the entire bike sideways and back and forth to splash around all of the gas in the tank. Always do this with the kickstand up so you don’t bend the center stand.

You should carry with you a small 2.5 ounce bottle of Champion 2-Cycle Oil that’s enough for one whole gallon. Most mopeds have a compartment that holds a small bottle of oil, which you keep refilled from a big bottle you keep at home. That way you always have oil with the bike. You cannot add gasoline and then drive home and add the oil later because after about two minutes of running without oil the engine will seize-up and damage to the piston, rings, and cylinder walls will occur.

A lot of people ask if it’s better to use too much oil than not enough. The answer is yes but too much oil, say 5oz/gal, can foul the spark plug causing the engine to not run or start until a new spark plug is installed. Way too much oil in the gas, say 10oz/gal, will clog the carburetor and the engine won’t run until the carburetor is cleaned out. Too much oil causes excessive exhaust smoke and leads to premature muffler clogging. Not enough oil, say 1.5oz/gal, will not hurt a normal moped that’s driven slowly. For all these reasons its best to use 2.5oz/gal and never less than 2.0 or more than 3.0.


All mopeds have a gas shut off valve at the bottom of the gas tank. The valve knob has three positions, OFF (closed), ON (open), and RES (reserve). You should always keep the gas valve OFF when you’re not using the moped, or else gas can leak out of the carburetor. If the gas is left on, it’s up to the float valve inside the carburetor to stop the gas from leaking. On a new bike or one that has a good working carburetor float the gas valve can be left on for days without any
leaking. Moped carburetors hold about 5 spoonfuls of gasoline, enough to go about a half a block. So if you forget to turn the gas valve on the bike will start and run for about one minute and then stop running. Turning the gas valve OFF and running the engine until it uses up the 5 spoonfuls of gas in the carburetor is what you should always do before transporting or storing the moped.

The RES (reserve) position is for when the tank is very low or getting near empty. The bottom two inches of the gas tank does not come out when the gas valve is ON. So if youre going along and the gas gets too low it will “hit reserve” and run out of gas. Then you can switch the gas valve to RES and start heading towards a gas station. On most mopeds the reserve is enough to go a few miles. You can run on RES all of the time, but its recommended to use ON instead because it does not draw gas from the very bottom of the tank where rust powder and water tend to settle at. Most gas valves are marked or labeled. In case yours is not, OFF is almost always to the right as your facing the valve, ON is always down, and RES is either to the left or up.


Tomos mopeds since 1976 have always had way of starting that’s different from all other mopeds. On a Tomos you kick either pedal backwards to start the engine. On all other mopeds you kick either pedal forward while pulling a lever. Also unlike all other mopeds, a Tomos does not need to be on the center stand when doing a stationary kick start. You should always have your left hand on the rear brake lever when stationary starting off the center stand. On a normal Tomos you don’t need to give it any throttle at all, but if you do give it throttle, when it starts it will immediately lurch forward. Holding the left (rear) brake on prevents the bike from moving forward before you’re ready. 


Put the moped on the center stand so that the rear tire is off the ground. If the bike has a bent or worn out center stand or if there’s too much weight on the back it will not be able to start this way, because the rear tire must kept from touching the ground. Stand on either side of the bike with both hands on the handlebars. Turn the pedals backwards until the one on your side is almost straight up.


Puch, Sachs engine, Minarelli engine, Morini engine, Garelli, Batavus, Trac, Foxi, and others: Pull start lever on the left handlebar and push the pedal down while holding the start lever. When the engine starts let go of the start lever.


Derbi, Vespa, Honda PA50 Hobbit, Kinetic, Trac (Dai Lim engine), Angel, and others: On these mopeds there is also an engage lever either on the engine (Derbi only), or rear wheel, that disengages the rear wheel from the motor, so the bike can be pedaled or pushed without the automatic starting clutch trying to start the engine. Pull the decompression lever and push the pedal down. As your foot is going down, let go of the lever. The engine will start the moment you let go of the decomp lever.


Motobecane & Peugeot only. Same as Type B, except decomp lever is on the right side handlebar. The left lever is a choke on these French made mopeds. These mopeds have an engage switch on the belt pulley that disengages the rear wheel from the motor, so the bike can be pedaled or pushed without the automatic starting clutch trying to start the engine.

In all 3 types A, B, & C, you should always have your left hand ready to squeeze the left (rear) brake lever in case the rear tire touches the ground while its turning fast.


If you are already on the bike and it’s off the center stand you can start it by pedaling up to about 5mph and then pulling the start lever while continuing to pedal. If your moped has a decomp lever then you pull it first, then pedal, then let go of the decomp lever. Pedal starting is the only way to start a bike that has a bent center stand or too much weight on the back. If the pedals are not working the moped can be push started by walking it forward instead of pedaling.

2 Gallery of all Moped Manuals:  Project Moped Manual is where to go to find original manuals. Clicking on that is like entering the library or museum. It’s a big collection! The manuals are listed A to Z, with details. Most are free downloads that take a couple of minutes. Some cost a small fee. So there is no need to list a hundred different manuals here, thanks to Linda at Project Moped Manual. 

                                   3 Spark Plugs

Spark Plugs are important. They’re also cheap and easy to change. The spark plug is the first thing you look at when a motor won’t start. Spark, compression, and fuel are the three main ingredients. Spark plugs come in different sizes and styles. See Parts Department to learn what sizes and styles there are.

Reading spark plugs: The condition of the spark plug tells you about the condition of the engine. A normal spark plug has a light brown coating on the white porcelain insulator, and sharp corners on the center electrode. A high mileage spark plug in a healthy engine looks the same but the electrode corners are rounded. Sparks like to jump from pointy things. A rounded center electrode or way too big a gap might make the engine hard to start. 

Checking for Spark: Remove the spark plug, connect the cap to it, and lay it on the engine so the metal shell touches the engine. Turn the engine over either by pedal starting or by flicking the flywheel with your hand. Little momentary light blue sparks should be jumping the gap, making a “snap” sound each time the piston goes up and down. Modern CDI ignitions are hard to see in bright sunlight, so check for spark in the shade. A bright white, pink, or yellowish spark is bad. That usually indicates a fouled plug. Dim and blue is good, especially if the snap snap snap sound is loud.     

4 Free Service Manuals for Viewing:

Fred’s Guide – How to Fix your Moped:  Read this first to build a foundation of general knowledge. Highly recommended.

Minarelli Service Manual:  Read page 17 and 18, Two Stroke Engine Theory.

Moped Repair Handbook:  These are the first two chapters of an excellent book by Paul Dempsey, Nov 1977. Very highly recommended.

Moped Army – Wiki – Repair Tutorials:  Hundreds of good user supplied articles on many service topics. Pick and choose what you want to read about.

Almost All Kinds – General Moped Repair – Large List of Online Moped Manuals:  Wow! A collection of useful manuals: Avanti, Batavus, Casal, Cimatti, Garelli, General, Honda, Jawa, Kreidler, Minarelli, Morini, Motobecane, Murray, Peugeot, Puch , Rizzato, Sachs, Tomos, Yamaha, Zundapp. Big thank you to MRA!

Batavus Moped – Owner Service/Repair 1976-1978:  Covers Batavus one-speed mopeds with the Laura M48 engine. For home mechanics, by Clymer Publications. All Clymer manuals have a good Chapter 1. General Information, and Chapter 2. Basic Hand Tools, before they get into the main course.

Derbi 49cc Flat Reed “Start” Engine 1986-1989:   What was the king of US model mopeds, the Derbi Variant Sport and it’s cousin the DS50. This was their factory (engine only) service manual. There was nothing else but parts manuals and wiring diagrams for the 1986-1989 Derbi’s.

Garelli Moped – Owner Service/Repair 1976-1978:  Covers one speed and two speed Garelli mopeds from the 1970’s and 1980’s. For home mechanics, by Clymer Publications. Good quality.

American Garelli Service Manual:  Excellent factory service manual for 1970’s and 1980’s (USA models with brake light and horn) Garelli mopeds with the Garelli one speed automatic 49cc horizontal cylinder engine.

Motobecane Repair Operations – Models 40, 50, 7:  This is the real thing, with five pages of special tools at the beginning. Let that be a hint that Motobecane does things their own way. The bike may be hard to work on, but the manual is well written.

Peugeot Moped – Owner Service/Repair 1976-1978:  Covers Peugeot 103,  US models, from 1976 to 1980. For home mechanics, by Clymer Publications. Good quality.

Puch 1980-1981 Service Manual :  The best Puch moped factory manual. Covers 1970’s models as well, one-speed and two-speed.

Tomos A35 Service Manual: This is the Tomos factory manual for the early two-speed A35 engine 1991-1993 that had points ignition and the carburetor in the center. The rest of the info applies to the later A35 engines 1994-2006. The transmission info applies to the A55 engine 2004-later. Excellent technical drawings, specs and procedures.

Tomos Streetmate A52/A55 Information and Tuning Manual:  This is a technical masterpiece, the best and most complete. By R.L. Vuyk. From the Netherlands, where mopeds rule.


5   Tire Mounting

This is not a complete set of instructions. You can read how to change a moped tire in most manuals. This is an additional item, very often overlooked that needs mentioning.

Left,  a tire has just been mounted on the rim. Before air is added, the area near the valve stem is checked. Sure enough, the tube is pinched between the tire and the rim. If ignored, this will cause a bulge in that area.

Right, the same tire after pushing the valve stem up inside the tire. Now the tire will be straight when inflated. Pushing the valve stem inside the tire before the final mounting will also make the mounting easier, since the pinched inner tube is making the tire tighter on the rim on the opposite side. When your tires are mounted correctly, smooth roads will not feel like bumpy ones, and your bike will feel solid, smooth, and more enjoyable.

6  Threads

Here are the most common metric threads, and several views of the Myrons Thread Pin Cushion:

Metric Bolts

Read all about motorcycle thread repair

Read all about bicycle threads here:





7   Seized Engine: How to unfreeze it

A seized engine is when the crankshaft, rod, and piston are frozen solid and cannot move. You try to pedal start it and it acts like a brake when you pull in the start lever (on a manual starting clutch type), or when the automatic starting clutch engages. It never makes that chug chug chug sound of the piston going up and down.

Here are the steps to diagnose and/or recover from a seized engine.

1. Spray WD40 into the spark plug hole. This will get solvent to the piston top and cylinder wall above the rings. If the piston is just stuck with tar from sitting, this will dissolve some of the tar and maybe unfreeze the engine.

2. Spray WD40 into carburetor (after removing the air filter) with throttle held wide open. This will get oil into the crankcase and lower part of the piston and cylinder, below the rings.

3. Locate the flywheel. Some bikes it’s on the left and some it’s on the right. The magneto flywheel on a moped is a 4 or 5 inch wheel that has magnets inside it. Most have wide holes that allow adjusting the points. The flywheel is attached directly to the crankshaft. It is crucial to not confuse the clutch wheel (which turns with the rear tire) with the flywheel (which turns with the engine). Motobecanes and Peugeots have exposed “wheels” on both sides on the engine.

4. Try to rotate the flywheel with both hands. If it moves, try going back and forth, each time a little more, until it goes all the way around. Then spray more WD40 in and go around again many times. When it’s loose enough, switch to pedalling to rotate the engine faster than by hand. When it’s finally free, it will not feel tight anywhere (with the spark plug removed). A normal flywheel that’s “free” should move in a full circle with one strong finger. If it takes a whole hand then it’s “tight”, not “free”. 

5. Determine what is stuck. A small screwdriver in the spark plug hole is used to feel the top of the piston. When just the piston is stuck, the crank will move a tiny bit back and forth, but the top of the piston will not move at all. When something like a loose screw or piece of debris gets stuck in the flywheel, the flywheel might move free within a range. Then the top of the piston will move with the crank. When neither the piston nor flywheel moves at all, then remove the flywheel and try to rotate the crank. If the naked crank is does not move, the engine is stuck internally, and the flywheel will then need to be put back on tight to continue the unfreeze process. If the crank and piston move free, then the flywheel is rubbing or sticking somehow. That can be caused by several different things each with a different remedy.

6. Find or make a tool to hold onto the flywheel, once you’re sure it’s stuck internally. Be careful to not allow the tool to damage the coils or wires behind the flywheel. Apply a rotation torque of, perhaps, 150 ft-lb for an 8mm crank nut, or 200 ft-lb for a 10mm nut crankshaft. These are estimates only. Be advised that too much torque can shear off the woodruff key and/or damage the crankshaft and/or flywheel woodruff key groove, or even break the crankshaft (possibly $500 damage). Applying high torque is risky but beneficial. 

To be continued …

8   All about Oils

Myrons favorite two stroke oil. For premix or injector.

Myrons favorite two stroke oil. For premix or injector.

Two StrokesMost older mopeds and motorized bicycles are two stroke engines that do not have motor oil in the crankcase. Instead the crankcase has a mist of gasoline with 2% or 4% two stroke oil blended in. The crankshaft and piston are lubricated by the gasoline mix. Two stroke oil is designed to be stable when mixed in gasoline, and to burn clean and not make black tar and carbon build up. Regular motor oil will work in a two stroke, but will leave a mess of tar in the engine and or gas tank.

Champion Synthetic Blend 2-Cycle Power Equipment Oil, with fuel stabilizer, is the two stroke oil that Myrons Mopeds recommends. It is a premium quality ashless motor oil for use in 2-cycle air cooled engines (with higher temperature demands). It combines Group II base oils and pure isobutene, rather than commonly used fragmented isobutene which can degrade faster at high temperatures. In addition, it contains several semi-synthetic additives providing excellent detergency and a dispersant to provide protection and performance. Super tough film, anti-foaming, anti-scuffing, fuel stabilizer, all beneficial.  

In a two stroke (2 cycle) engine, separate from the crankcase is the transmission case, which does have a pool of motor oil or automatic transmission fluid in it. The transmission oil or fluid needs changing every 2000 miles. Below is a chart showing what oil goes in what moped engine, and where to add it and check the level.

Four Strokes: Most newer mopeds and scooters are four stroke engines, like cars, with motor oil in the crankcase. The oil for those is the same as for cars, such as 10-40W. Four stroke oil is motor oil. But unlike cars, the transmission shares the engine oil, like most motorcycles. Motorcycle (non catalytic exhaust) motor oils contain ingredients that are beneficial, but harm the catalytic converter. Motorcycles, scooters, 4-stroke mopeds with catalytic converters should use ordinary car motor oil.  


Moped Trans Oil Chart

Moped Trans Oil Chart

Oil Specified in Owners Manual: Often the specified oil is obsolete. In ATF, the old “Type A” (non-Ford) went on to become Dextron (Type B)  in the late 1970’s, and then Dextron II (Type C and Type D), III, IV, V, and now “Dextron VI“, for most GM vehicles and pre-2004 Toyota automatic transmissions. 1970’s Tomos and Sachs mopeds specified ATF Type A. Tomos also specified motor oil 10W30. The old ATF “Type F” (Ford/Mercury) went on to become Mercon in 1987, as did “Type G” (Ford Europe), and then Mercon II,III,IV, and now “Mercon V“, for most Ford, Lincoln, Mercury automatic transmissions. 1970’s Puch and Kreidler mopeds specified ATF type F 

Type A versus Type F: Type A up till the 1970’s contained whale oil as a friction modifier. A modern friction modifier/reducer is glycerol mono-oleate. Type F contains no friction modifiers. Modern vehicles have nine current ATF versions: ATF+4, Mercon V, Mercon LV, Dextron VI, ATF DW-1, SP-III, Matic-S, Toyota ATF-WS, Honda DW (ZF). Any of these will work in a moped transmission. They are only slightly different. ATF Mercon V would be the safest to use, since it does not have friction modifiers. Friction modifiers are better for the gears and bearings, to reduce friction and extend life, because part of the molecule clings to metal and part is a long chain that slides over metal easily. What’s good for the metal parts may or may not be good for the clutch. This is a current area of investigation. 

Detergent versus Non detergent: Motor oils come with a detergent additive that keeps the dirt floating in the oil, so it can be removed by the oil filter. Non detergent motor oils allow the microscopic dirt particles to settle out. They are for engines and machinery that do not use oil filters but instead rely on manually removing the sludge from the bottom of the oil bath, as in machine shop equipment and small gasoline engines. Some rubber parts can get damaged from the detergent in the motor oil, in particular the Garelli 1-speed and 2-speed centrifugal solid rubber clutches. Garelli, Morini MO-1 and MO-2 call for 30W non detergent (ND), while Batavus/Trac, Demm, Derbi, Indian, Minarelli, Morini M-1 call for motor oil 30W, 40W or 10W40. See above chart.