For all bikes using Derbi Euro 3 D50B0 & D50B1 engines:
Carburation and fuel tap
I don't recommend trying to re-jet the stock Dell'Orto PHVA 17.5 carburetor. Dell'Orto's are designed to be the worlds most tunable carburetors with the aim to "extract maximum performance" and offer "enhanced crisp throttle response" rather than provide safe balanced A/F (air/fuel) mixture ratios across all throttle slide positions. They are a pain to setup w/o destroying multiple engines in the process, and very sensitive to altitude air density changes in ever-changing warm / cold climates. Dell'Orto jets, Atomiser emulsion tubes, needle tapers and throttle slides cost a damn fortune.
The Dell'Orto PHVA choke lever rotational stoppers snap easily when lifted. The Choke lever base is mounted using a thin metal fastening plate with 2 screws and rubber seal, rather than a robust Keihin PWK threaded type mechanism. Pull the PHVA's choke lever up and the seal leaks because it isn't clamped down sufficiently. Unscrew the choke mechanism and you'll find more dirt in the carb than what is normally acceptable.
I recommend the Polini CP 17.5 carb, this uses a PWK threaded type choke mechanism. 1 simple throttle slide, 1 simple needle and 1 simple Atomiser which don't need swapping for different sizes in order to jet the carb sufficiently for the utmost safe running.
To mount and jet this carb correctly you will need the following parts:
Polini 34mm Airbox boot adapter, part number: Polini 343.0023
Polini CP 17.5 carburetor, part number: Polini 201.1700
Or if you want to continue using the handlebar cable choke' part number: Polini 201.1702 (comes with 34mm adapter)
5 pack of 42 - 52 pilot jets, part number: Polini 372.0004
10 pack of 80 - 98 main jets, part number: Polini 370.0002
2mm silicone blanking cap for cylinder head carb heater port.
4mm cap for the plastic T-piece near coolant overflow bottle (see MVT ignition photo)
Not 100% necessary' but i recommend fitting the CP "Evolution" float bowl for quick and easy main jet access, part number: (Polini 343.0009) rather a shame they chose not to include this with the standard version tbh.
Cut the 2 pieces with green arrows pointing directly to them from the intake manifold so the Polini carb body sits flush with the manifold properly.
Insert the included blanking caps on the carb "fuel cock depression" and "mixer" vacuum ports, the "mixer" is made redundant when switching to premixed fuel, and the other is made redundant when the auto vacuum fuel tap is disconnected. These auto vacuum operated taps are immensely unreliable, diaphragm broke on 3 of them. Fuel flow rate goes on/off while the engine is running - creating a dangerous lean running condition.
Therefore' an old design rugged manual operation non vacuum tap is needed. Tried 3 styles of 15mm manual taps, the only one that doesn't leak while turned to the "OFF" position is the OEM Derbi Senda 125/Cross City 125 tap, part number: Derbi 00H00404181
When purchasing this tap' make sure it comes with the black dial, some places don't even include it.
Tip: Free Line Kart fuel line is manufactured from Tygon, molds over connections and doesn't require retaining clips. Tygon doesn't go rock hard and split like old tech rubber / silicone does.
Solution, trim the pipe down by 15mm. By doing this you still have a safety net as these Derbi models do not have fuel amount gauges, so you still maintain a reserve tank. But your allowing fuel to get into the green pipe and thus into the carburettor.
This feature is a reliable way of knowing when you need to refill the fuel tank.. unlike the digital instrument's low fuel warning light that often fails to flash due to the inferior quality Spanish / Italian wiring harness which usually shuts down the whole instrument dashboard. Once the engine starts cutting out' pull in the clutch lever - reach down and switch to "RESERVE" while keeping eyes on the road ahead.
If the engine cuts out straight away' pull in clutch lever and coast to a stop. Never change down gears while coasting with the engine switched off, this won't do the transmission any favours. Always turn to "OFF" position while the engine is switch off, unless priming a cold engine before firing it up.