Vespa Kits

Vespa Small Frame - Polini 130 Install

The Polini kit, like the Malossi can be bolted right on to the casing, but to get its true potential the crankcase mouth transfers need to be opened up and matched to the barrel. That involves a full engine strip so an alternative is to simply mount the barrel. The directions are the same from here onwards whether you changed the transfers or not. If you are starting from a built up bike, here's a guide to removing the original barrel and piston.

The kit I had came off a running bike so I didn't have to mess around with fitting the rings. If yours is a new kit you'll have to install the rings. A quick guide from a standard small frame piston can be found here and the idea is the same for the Polini piston.

This section starts after the cases have been ported and the original barrel has been discarded. Three things are very important to make this kit run well.

(1) Porting the cases. This really unlocks the power of the kit. If the cases are not ported you'll still get a faster bike but it will the kit won't perform as well as it could.

(2) Changing the stock timing. The Polini kit requires a slightly advanced timing point compared to a stock Primavera. The Polini requires 16 degrees BTDC compared to the stock 24 degrees BTDC.

(3) Changing the main jet in the carburetor. Since the bike can now burn more fuel because of a more efficient cylinder the main jet must be changed to allow that extra fuel to come through the carb.


Use the new Polini base gasket and slip it down over the studs. Make sure it aligns with the transfers. Because my casing transfers were modified, I opened out the transfer areas of the gasket with an exacto knife to match the transfers beneath. A modified Vespa one works just as well as the Polini original. Fit the small end bearing after smearing a little two stroke oil over it. It should slide tightly into the small end of the con rod.


The piston can be installed in two directions and it is imperative that it faces the correct way. Look on the dome of the piston and you should see a small arrow. This must point towards the exhaust pipe which is up and away from the swing arm pivot as shown above.


While supporting the piston, carefully slide the wrist pin into place. If it is especially stubborn you can heat the piston with a hot wet rag which will make the metal expand slightly and allow the wrist pin a little more breathing room. Don't be tempted to hammer it in.


Polini makes wire wrist pin end clips which are a bit of a pain to fit. Make sure they both fit squarely in their grooves. I also rotate them so that the wire end is in line with the piston length (not shown in the picture above).


Now comes the barrel. Carefully look at the bore of the barrel and if you have gasoline and compressed air available give it a good clean and blow out any debris. Rub some two stroke oil around the inside bore, and fit the barrel over the tops of the cylinder studs. The barrel can go on in many different directions as the studs are in a square format. Make sure the exhaust port is facing up and away from the swing arm.


As you slide the barrel down you'll need to find the ring stop pins and locate the ends of the piston rings at each one. Then compress the rings with your fingers as you slide the barrel down over the piston. It should look like above, and the piston should not bind or stick.


Once the barrel is on, turn the flywheel or crank to make sure the piston runs through the bore smoothly. The next step is to set the points gap, and then changing the stock timing to 16 degrees before top dead center (BTDC).




If yours is a points bike then since the flywheel is on the bike you might as well set the points gap before removing the flywheel to play around with rotating the stator to 16 BTDC. (If it is an electronic ignition bike you can skip this step). The gap is the distance between the faces of the points as they open. The correct gap for a small frame is 0.30mm to 0.50mm and can only be measured with the flywheel in place. Pull out the rubber bung in the flywheel so that you can see the points (green arrow) and you'll notice one large screw (red arrow) and also a place where you can use a screwdriver to carefully lever the points apart.



Using a feeler gauge select a 0.35mm thickness and rotate the flywheel. You'll start to see the points open as the window in the flywheel passes over them. At their widest point apart, stick the feeler gauge directly in between them and then loosen the large screw until you feel the points grab the feeler gauge. Then tighten up the screw and remove the gauge. The points should stay where they are, separated by exactly the width of the feeler gauge. If for any reason this doesn't work, you can slide a small screwdriver in between the two notches on the far edge of the points (where they overhang the stator edge) and lever them carefully apart until the feeler gauge is just slightly released. With the gap set you can now move on to the timing.

Changing the timing: With the Primavera it is necessary to determine where 16 degrees BTDC occurs as there are no marks other than the stock 24 degree mark that aligns the stator to the casing. You could use a degree disk after finding TDC as shown on this page, but the more accurate way to do it is to use a dial gauge and convert the degrees into millimeters of piston travel and then mark the flywheel and casing at the correct point. To do this you'll need to fabricate a bracket (we used a block of wood) to hold the dial gauge in line with the piston travel.



Use nuts or washers to space the dial gauge off the barrel so that the piston doesn't hit at TDC, and then tighten the whole thing down so the barrel cannot move on the studs. Slowly turn the flywheel causing the piston to rise up against the gauge. As the piston reaches TDC the dial gauge increase until it slows and then will come to a stop at exactly top dead center before starting to decrease as you go past TDC. The point you are looking for is where the piston is at the highest point of travel where the dial gauge display is not increasing or decreasing. At this point scribe a line on the flywheel and on the casing with a sharp object so that the line aligns with itself. Mark this scribe "TDC" on the casing with a marker or just scribe it in.

Without moving anything set the dial gauge to 0 mm. To convert 16 degrees into millimeters of piston travel there is a complex formula but it works out to be 1.25mm. You can check it here if you want to be sure. Now turn the flywheel anti-clockwise until the dial gauge reads 1.25mm and make another scribe on the casing aligning with the previous scribe on the flywheel a new scribe on the casing. This is the mark where the engine should fire. It can now be tested in one of two ways: statically or dynamically.

Statically: A simple way to check the exact point of where the points open is to set up a little circuit tester like the one pictured above. You'll need:

  • A 6V 25W bulb
  • A battery holder from Radio Shack that can hold 4 AA batteries.
  • Two small alligator clips (insulated are best).
  • A length of insulated wire.

Make a small open circuit following the diagram below (or click it for a PDF). Once you have it made up disconnect the one red wire on the HT coil (located outside the engine by the rear shock) and the other contact to the engine casing. If the HT coil is not connected due to a rebuild you can go directly off the red wire coming from the stator. Be warned that there is the possibility of very minor 6V shocks and sparking from doing this - basically it is less voltage than putting a 9V on your tongue and isn't really anything to worry about.

Turn the flywheel to the TDC line you made and then turn it counter-clockwise until you see the light bulb get brighter (or is it more dim - I can't remember right now). Either way the light bulb will change brightness at the exact moment the points open. If this matches your scribed mark for 16 BTDC then you are done. If it happens before you reach your mark (i.e. closer to TDC) then you will need to remove the flywheel and rotate the stator slightly counter-clockwise. If it happens after your mark then you will need to remove the flywheel and rotate the stator slightly clockwise. Unfortunately to check the timing each time you will need to refit the flywheel until you get it on the mark.

Dynamically: The second way to check the timing is do the same process to find your TDC and 16 BTDC marks and then build the rest of the motor, put it back in the frame, and fire it up with a strobe light. The strobe will flash every time the spark flashes and will show you exactly where the motor is sparking in relation to your marks.


Finally install the cylinder head and torque down the cylinder head nuts a 1/4 to 1/2 turn at a time in the pattern above (I forgot to shoot this image on the Polini engine that's why the cylinder nuts are different).


Changing the carb main jet: The final step on the install is to change the main jet in the carb. Here's a section on how to get your carb out so the main jet can be changed. Once the carb is out of the bike drain any residual gasoline from it, and turn it upside down. There are two screws holding the float chamber bottom on to the main body. Remove these screws and you'll see the main jet right in the middle of the float (arrowed above). Use a flathead screwdriver to remove the standard jet and install a the new jet. The size of the jet depends on what else you might have done to the engine, but as a starting point Polini recommends a 76 main jet if you just bolted the barrel on the existing casings. Reinstall the carb, put the motor back in the frame, fill up the oil if you split the cases, and make all the fuel, electrical, and control cable connections.

Once you are happy it is time to take the bike for a spin. Be sure to mix your oil to gasoline ratio at 2%.

Once you are on the road be very careful with your new kit. I always recommend using a brand new plug so they are easier to read. Check the spark plug color often where the ceramic insulation is visible near the tip of the electrode. This should be a nice, dry, chocolate brown color. Motors must be carefully broken in once they are first put together. Keep the RPMs low at all times, shift gears a lot, and never hang with the throttle in the same position for a long time. Following these rules will really benefit your new cylinder. Once you've covered 500 miles, open her up and enjoy!!