Search the internet and you’ll find no shortage of posts from people saying how easy it is to replace the Zuma’s stock 50cc cylinder. Just buy the kit, bolt it on, and go.

And in fact it isn’t all that complicated – the actual top end replacement involves just four nuts, two circlips, and two bolts holding on the exhaust. But there a few things to know and/or avoid if you haven’t done this before, especially if you don’t have experience working on engines. Furthermore, there are some other minor changes and tweaks you must make at the same time you put on the bigger top end, because the stock bike is set up to work with a less powerful top end that makes its maximum power at different RPMs.

That’s is why it’s a good idea to buy from a vendor who knows the Zuma and can make sure you get the right parts for a reliable upgrade, and also who’ll provide tech support if you have questions or run into problems.

Obviously you also want to refer to the instructions that came with your kit, and to the Zuma service manual for an explanation of what all these part names are and where they go. But I’d suggest reading all the way through this post before you start; these are all things I wish someone had told me the first time.

Tools needed

1. 5mm allen wrench
2. very solid Phillips screw driver
3. two 10mm sockets: a fairly deep one that will reach the nuts over the head pins on the cylinder head, and a small one – preferably with a miniature wrench) – for removing/replacing the exhaust pipe from the cylinder (there’s not a lot of space on one side)
4. torque wrench for tightening the cylinder head nuts to the exact tension (hint: if you’ve never used one before, practice on something like a bolt going into wood so that you feel where it clicks at the correct tension; you don’t want to miss it and break the head pins)
5. needle nose pliers to install the circlips holding the wrist pin on.
6. and possibly a compression driver to loosen three Phillips screws holding the air shroud on the right side the first time.

Miscellaneous

Some lithium soap grease to help seal the cylinder head if it uses an O ring (as the Polini does), some light oil for the wrist pin, and some regular oil for the piston and also the wrist pin bearing.

And there’s no shame in wearing rubber scrubbing gloves – in fact it’s a good idea for protection against the inevitable minor wear and tear on your hands – but also know that dishwashing soap and a nail brush takes grease right off.

Removing the following

• The luggage rack, rear plastic, side plastic, seat bucket, and the battery cover.
• The ignition wire from the spark plug.
• On the right side of the bike, four screws holding the fan cover to the air shroud surrounding the cylinder (you may need a compression driver to get those screws out the first time). Note that this side of the shroud – the fan cover – forms a box with the other side of the shroud around the cylinder, and – this is important! – be sure to note how the oil delivery tube fits thorough a slot. You don’t want to pinch the oil tube when you put the box back together later, or you’ll blow your new top end!
• This shroud covering the cylinder. You’ll find it easier to get it off the cylinder if you loosen the two bolts holding the air box onto the crankcase cover (on the left side of the bike).
• Note: this box is important for cooling the engine, so don’t ride without it
• Two bolts holding the exhaust pipe to the underside of the cylinder. The exhaust gasket may fall off, but it’s a good idea to replace it anyway.
• Then you can get at the cylinder to remove the four head nuts and slide the cylinder head and cylinder off, after which you remove right circlip and wrist pin to remove the old piston.

Putting the new top end on is almost identical in reverse, except that the Polini uses a base gasket and O ring (no head gasket). Your cylinder will have installation and break-in instructions; if not, don’t start until you do, because there are some important points.

Other mods that must be done at the same time

• For safety, replace the reeds with carbon fiber ones (at least one kit, the Athena sport, even comes with them). The 70cc engine is powerful enough to break the stock metal reeds; it’s powerful enough to break carbon ones too, but the metal ones can chip and take out your engine when they break. You may need to give the reed cage a very light tap with a rubber mallet or something to free it, but be very careful not to damage the gasket.
• Carburetor adjustments (if you’re not upgrading to a larger carb). You’ll need a bigger main jet, and some kits advise you to raise the needle a notch or two for a richer fuel/air mixture. There are conflicting opinions about air box modifications; the consensus is not to do anything to it if you’re using the stock carb, and probably not to do anything more severe than removing the snorkel if you’re running a large one. That’s for street bikes; you’re unlikely to be reading this if you’re a racer.
• Most likely clutch springs and variator weights. This is a whole separate subject, but you’ll probably want stiffer clutch springs (to take off at higher RPMs than stock) and heavier rollers. Be prepared to be tweaking these things a lot if you’re modding your scooter.

Important things to double check before you start the engine or go on your first ride with the new top end

• No pinched oil delivery hose (see above)!
• Air leaks will blow your new kit! Places to check for a good seal: 1. both sides of the carburetor (especially the clamp on the output side – it must be tight), 2. the intake manifold (i.e. reed cage when you replace the reeds), 3. especially the exhaust manifold (go back and forth between the two bolts as you tighten them to ensure that they’re even and secure), 4. the cylinder base and (less likely) head.

Good luck!