Tomos Moped/Repair

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How do I replace the piston and cylinder?[edit]

Take off side fairings. Remove airbox with one large philips screw and detach from carburetor by unscrewing clamp.

Turn on fuel flow and remove carburetor where it attaches to intake with philips screw. Let carburetor hang to the side. Unbolt oil injection from intake manifold with one small (size?) bolt.

Unbolt exhaust at bottom of engine and consider completely removing exhaust from moped.

On cylinder head, remove spark plug wire and pull spark plug. Unbolt head from cylinder via 11mm bolts and washer. Wiggle cylinder head completely off engine, pulling it past piston and rings. Stuff paper or cloth in remaining hole to keep debris out of engine.

Unbolt intake manifold from cylinder and remove reed valve. Replace intake and reed valve on new cylinder along with appropriate gaskets.

Remove piston by unclipping 2 retainer rings on wrist pin. Slide out wrist pin, tapping if necessary. Remove piston.

Put new rings on new piston by gently working them down over top of piston. Check documentation for ring arrangement. Often, pistons will have a finger in the groove to keep ring alignment at preferred angles.

Slide piston into new cylinder by compressing rings one at a time. The piston is only supposed to go in one way. It should have an arrow on the top which is supposed to point at the exhaust port (away from the carb).


Scrape old gasket from engine where cylinder mounts and install new gasket. Put new cylinder partially on engine while aligning connecting rod with piston. Slide and clip new wrist pin in with needle nose pliers. Slide cylinder completely down onto engine.

Pour a small amount of 2-stroke oil down cylinder to piston. Roll the engine with kickstart or pedals to lube walls of piston and cylinder.

Replace head. Slide washers and nuts on bolts with allen wrench or similar tool. Torque down in X pattern. Be careful not to overtorque. Replace spark plug and wire.

Bolt on exhaust, using appropriate gasket. Replace carburetor and air box. Turn on gas.

Remember to vary speed while break-in and limit full-throttle operation for 300 miles.

Some people recommend mixing a little extra 2-cycle oil into the gas for the brake-in period.

After the break-in period check head bolts and tighten if necessary.

How do I do a plug chop?[edit]

Full warm up the engine by running it for about 10 minutes... Then make a top speed full throttle run for a half mile or more, then kill the motor and stop and pull the spark plug on the spot.

It is important to chop the throttle and turn the engine off with the switch and pull over to stop. You don't want the engine to come to an idle, That will change the plug color.

How do I replace my chain?[edit]

Loosen the tension adjustment tabs at the rear tire by loosening the nut on one side. Tap these until the cams are in the loosest position. Remove the chain guard with two Philips screws.

Find the master link by slowing rotating the rear tire. It will look different than the rest of the links in the chain. Pop it over using a screwdriver and a sharp pop of the wrist.

To avoid needing to pull the left pedal and part of the engine cover, reattach the new chain with the masterlink to the old chain. Keep tension on the chain and slowly rotate the rear tire to pull the new chain forward through the front sprocket.

When the old chain has pulled through, pull the two other parts of the old master link off. Use a new master link to connect the chain.

If you are using different sized sprockets or a universal chain that is longer than stock, you may need to remove extra links. Use a link breaking tool or grind off pins and remove unneeded link sections.

Tighten tension adjustment tabs, making sure to keep them even on each side. Tap with a hammer if necessary. Look for a 10mm deflection in the chain when finished.

Replace chain guard. Use chain lubrication to heavy oil down chain while slowly rotating rear wheel. Considering oiling chain every gas fill-up to lengthen chain life.

How can I tell if my oil pump is working?[edit]

This is an easy one. On the carb manifold there is a hose on a brass fitting. Take the hose off. Then remove the spark plug cap. Crank over a few times. If oil spits out then it works. (look at the line if it is clear it is not working unlless you have just filled the oil tank.)

How do I add an aftermarket exhaust?[edit]

Below the engine are two bolts and washers. Remove these. Another bolts connects the exhaust behind the pedals. Remove this and discard the exhaust. Replace the after-market exhaust, remembering to add an appropriate gasket before tighting bolts and washers. Don't overtorque! *Don't forget to upjet!*

How do I change my rear sprocket? Front sprocket?[edit]

Little advice about this. A (27 Tooth) Count will allow for maximum speed to be reached without going crazy with compression issues, jetting, or even boring your ports larger. This is not necessary. A smaller sprocket will allow for a slower top end. your looking at a very cheap alternative, around maybe 15 to 20 bucks US. However if you're not looking to go fast then i would say use a 25 tooth sprocket for the front. Now if you want to go even faster, get a 20 tooth sprocket for the rear. The reason for this is very simple, in the front we have a larger sprocket to create more pull and a smaller one on the back means it completes it rotation faster therefore occupying less space in the back and more in the front will increase the speed of the chains movement. Absolutely the same thing as a bike sprocket the only change here is we are modifying it to be a bit different either faster or slower with more power. The larger sprocket in the rear will give you a much more powerful pull but less speed. A lot less, not a very good idea for road use here. To be safe use the smaller sprocket in the rear and larger in the front.

How do I plug my oil pump so I can manually add 2-stroke oil?[edit]

Remove tank from under seat after you drain off all oil. Disconnect oil lines at tank and under cover where oil pump is. Remove 2 screws from oil pump and put pump aside. Take off coupling insert and put with pump. Now on the intake manifold there is the banjo bolt. remove it with the line from the pump. Get another 5mm bolt and cut off short so it will not stick into the flow of fuel delivery. Insert screw with a fibre washer and tighten so there is no air leaks. Put the old side cover(round) for the pump back on. Double a piece of fuel line to fill in the holes where the oil lines fed in and out. Now mix fuel at 50:1 ratio (as recommended in all Tomos manuals) and you're finished.

If your moped is stock the oil injection may connect directly to the carburetor. If this is the case, you need to plug the hole the oil hose went into originally. The banjo bolt step in the above instructions could also be ignored.

How do I change my carburetor's jet?[edit]

Turn off fuel flow and remove carburetor where it attaches to intake with philips screw.

Invert carburetor and unscrew float bowl. The jet sits in the middle of the float. Be careful not to tear gasket. Unscrew jet and replace jet.

Reverse steps to reinstall.

How do I change my Oil Injection?[edit]

To remove oil injection you will have to plug up the hole in the intake. Then remove the pump found on the left side of the motor under its little cover and block off the hole with sheet metal bolted on.

How do I fix a flat tire?[edit]

Get a couple of hard plastic tire irons at your local bicycle shop. These avoid damage to the rims (especially the chrome ones). Use ONLY proper (motorcycle) tubes. a good idea is to change the inner tube. Or you can also use fix a flat at a local motorsports store

The spark plug has no spark[edit]

There are many things to check coil, points,wiring.But the major cause of no spark in older tomos mopeds is the tail light bulb.the manufacturer does not want you driving with no taillight. If the filament is burned out or the bulb is missing the primary coil inside the flywheel will not fire the points thus no spark.