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racing dio

check this page for scootling's modified dio zx

this Dio ZX AF28 started as a standard 50cc, imported by us from Japan

Tuning up the AF28 Dio

All the parts used are available through Scootling. This is a demo model and is not registered for road use.

11/2003 - current top speed = 55km/h
It arrived in a thundercloud grey colour, with no obvious modifications to it. There were a couple of stickers on the side panels and the body was in decent condition overall. These ZX AF28s have a longer wheelbase than the base model AF27 and AF28 SR Dios, and have the front disc brake. These models also have different mufflers and richer jetting in the carburettors.

Honda Dio ZX AF28

03/2004 - New top speed = 70km/h
The first modifications made were to the driveline. It had seen a lot of use and the belt and weight rollers were well worn. These were replaced, with lighter weight rollers for the variator, a high speed pulley and a new factory OEM belt. This gave the Dio a higher launch RPM and keeps the revs higher during acceleraton. The slgihtly lighter rollers (7.7g) sit inside the variator pulley in the photo.
scooter high speed pulley

This went well for a while, with no changing of other parts required. The top speed increased a lot, thanks to the high speed pulley. This pulley allows the belt to be pulled to the extent of the drive face on the clutch, meaning a higher gearing ratio at top speeds. The paths for the weight rollers have a reworked profile too, giving a better acceleration curve.

It was decided that low down torque needed a boost. The cylinder and head were pulled, and planed a few decimals of a mm on the mating faces. The cylinder base gasket actually needed slight packing as there was just the tiniest amount of piston contact on cylinder head, threatening an early end to the bearings. Raising the barrel can give higher RPM performance, at the cost of torque as the power stroke length is reduced. However the raised compression allowed the torque to increase although the potential RPM ceiling will have diminished a little.

pulley fitted to dio
10/2004 - New top speed = 70km/h
Time for modification to the exhaust. Two strokes can gain a lot of power by replacing the exhaust with a well designed expansion chamber. The replacement chosen was a handmade KN Kikaku chamber. The new exhaust provides a lot more power, and at higher RPM. The factory AF28 ZX CDI is limited so no increase in top speed here.
The clutch arm springs were replaced with stiffer items, raising the engage point higher. This allows the engine RPM to lift higher before the transmission is engaged.
As engine power has been bumped up and we are looking for durability, the crank was replaced with an aftermarket item. This crank is 50grams lighter than standard, and has a longer pulley collar for extended movement, meaning higher possible top speed. Also the oil gallery to the conrod is enlarged.
Dio ZX crank
Now that the engine's bottom end can handle it, its time for more serious modification. Our 75cc big bore kit was chosen. This has a ported piston, larger port cylinder and redesigned alloy head. After the kit was bolted up we retuned the carburettor with bigger jets and adjusted the needle clip. A pod power filter was added to aid breathing. The front wheel is now difficult to keep on the ground when opening the throttle.
big bore up kit scooter
A full racing manifold and racing carburettor is lined up for fitment to the Dio. The manifold incorporates a 30mm internal diameter and double racing reed valves with optional fibreglass or metal blades. The flow area through the petals is 3 times more than standard. The carburetor chosen is one of our special PWK 28mm chrome flat slide carbs. This has a manual choke, bottom plug for quick jet swaps and the oil feed after the slide for constant oil supply even when the throttle is closed. The difference in engine response and high speed performance is incredible. Once the engine is fully run in the throttle cable travel will be extended to allow full bore use.
scooter racing carburettor
09/2005 - New top speed = 90km/h
The engine has been fairly well run in so its time for a racing CDI replacement. The unit chosen is easily fitted and removes the factory RPM limit.
The body is given some attention with a new eye-frying yellow paint job.
sports scooter
In order to improve looks and get that important air drag down, the scooter is lowered 4cm by swapping the front forks and replacing the rear suspension with a spring damper unit.
Components are sourced for a custom wet nitrous kit for the Dio. A stainless steel button activates a relay which powers the solenoid valve. Carb jets are used in the brass hose fittings to regulate the Nitrous Oxide and extra fuel flow. This set up will be finalised and tested once we find the right bottle to use.
After blowing a hole in the top of the piston in a high speed run we decided to lower compression a little. The modified standard head was swapped for one of our alloy high fin heads, which has optimum squish area for wide usable power and torque range plus bowl combuston chamber. The cylinder was rehoned and the main bearings and seals replaced again. New #9 NGK plug fitted. Running in is begun using mineral oil for proper bedding in.
alloy head bowl scooter
While the engine is apart we have decided to replace other parts. We now remove the oil pump and plug the hole with a specially designed sealed alloy plug to maintain the primary (crankcase) pressure.  
The clutch centre spring is replaced with one of our 10% stiffer springs. A new Kevlar oversized belt is fitted and the interior of the crankcase ground slightly to avoid rubbing the belt at idle. The centre spring helps to stop bogging with the increased torque from the engine, and improves acceleration when the throttle is released and reapplied. scooter clutch centre spring
The speedometer is replaced with a Kitaco 120km/h version. Yes we will need it. The handle bar grips are replaced with Italian Progrip items that fit the Honda and most other scooters perfectly and are very comfortable.
high speed scooter
The rear suspension is now replaced with our billet alloy version, adjustable height coilover damper unit. The improvement in the rear end is miraculous for high speed cornering and control. coilover adjustable scooter suspension
The tyres are replaced with Dunlop TT91 GP, semi-slick medium-soft compund. These give much improved grip in cornering and more axial stability. slick tyre racing scooter
The exhaust currently used is our chrome variable chamber for AF18E engines, this is capable of handling the volume of gas produced by this engine and gives very good low/mid range response, without producing too much noise. A mixture of 7g and 8g rollers works best with this. For competitive track use the exhaust will be replaced with a lightweight hydroformed chamber. After 200km running-in we have switched to semi synthetic oil. expansion chamber exhaust scooter
New Ceat Rio tyres (Made by Pirelli) are fitted, 3.00/10 at the front and 90/90-10 at the rear. These are a relatively soft compound with good dry performance and a nice profile for the track. Once warm, they provide impressive levels of grip.
Some attention is now paid to the weight of the scooter. The entire headlight, surround, dash and switch unit is removed, along with the speedometer and cable plus its drive unit from the hub. The rear spoiler and integrated stop lamp are removed. The seat is lightened, and the inner leg fairing removed.
The weight balance has now shifted rearward so the battery holder is removed as well as the unused oil tank, and the battery shifted to the front. A lighter battery, CT4B-BS is used instead.  
Now that running in is complete, the compression is raised. After calculating for a static compression ratio of 11.5:1, the head is milled 0.5mm and refitted.

A cold Iridium spark plug, the Denso IWF27, is fitted along with a resistorless NGK LZFH plug cap. This modification cures an occasional misfire and gives better acceleration to the bike. If run as a 50cc, the plug used is IWF22

iridium spark plug

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