Author Topic: 2006> SENDA 50 / GPR 50 De-restriction, Reliability and Performance Upgrades  (Read 4137 times)


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Part 1 (most important): Carburation and fuel tap

I don't recommend wasting time, effort and money trying to re-jet the stock Dell'Orto PHVA 17.5 carburetor. The likeliness of engine seizure while rolling off full throttle to 1 1/2 or 1/4 throttle is very high no matter how much richer you jet with pilot jet, Atomiser, needle taper, needle clip position, main jet etc..

Dell'Orto's are designed to be the worlds most tunable carburetors, they provide fuel/oil in a more vaporized gas manner rather than a fine spray of liquid with the aim to "extract maximum performance" and offer "enhanced crisp throttle response" rather than provide safe balanced AF (air to fuel) mixture ratios across all throttle slide positions.

Dell'Orto jets, Atomiser emulsion tubes, needle tapers and throttle slides cost a damn fortune, others argue their parts are more readily available - somehow deeming them more suitable carburetors of choice for the typical teenage or middle aged moped wrenching newbie. You'll find that the Dell'Orto brand is constantly recommended across most internet forums as the bees knees for every 2 stroke beginner or novice. There are many website bloggers who just copy and paste other blogger's paragraphs into their own blog posts w/o even using the products to provide any backup evidence to support their "superiority over other brands" manufacturer's buzzword Snake Oil claims.

After experiencing 1x engine seizure with the stock Dell'Orto PHVA 17.5 and 2x seizures with their PHBG 21 i resorted to a very simple carb fueling design from the Polini brand' specifically their CP 17.5 carb, this uses 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.

There are 2 versions of the Polini CP carb ranging from 15mm - 24mm choke sizes, the standard CP for setups not exceeding 12,000 rpm and the CP "Evolution" which comes with different needle taper, Atomiser, throttle slide and larger diameter air intake for larger filters and is suitable for setups exceeding 12,000 rpm. For the basis of the "reliability" part of this thread i highly recommend you avoid installing 12,000+ rpm "racing" exhausts and go for the standard model CP 17.5 for correct jetting.

To mount and jet this carb correctly you will need the following parts:
Polini CP 17.5 carburetor, part number: Polini 201.1700
34mm adapter for the OEM airbox pipe, part number: Polini 343.0023
Or if you want to continue using the handlebar cable choke' part number: Polini 201.1702 (comes with 34mm adapter included)
5 pack of 42 - 52 pilot jets, part number: Polini 372.0004
10 pack of 80 - 98 main jets, part number: Polini 370.0002

Not 100% necessary but i recommend fitting the CP "Evolution" float bowl for quick & easy main jet access, part number: (Polini 343.0009) rather a shame they chose not to include this with the standard version tbh.

"Where do these pipes plug into?"

"FUEL PIPE CONNECTION": isn't it obvious already? lol
"FUEL COCK DEPRESSION": auto vacuum fuel petcock port
"MIXER": 2 cycle oil injection port
"BREATHER": float bowl overflow pipe (included in the box)

Insert both included blanking caps on these ports because you won't be needing to use them, part 3 will explain why you don't need the "MIXER". These auto vacuum operated cocks/taps are immensely unreliable' the diaphragm broke on 3 of these which lets fuel flow with engine switched off leaving a puddle of fuel on the floor, and the 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' i have tried 3 styles of 15mm manual taps and the only one that manages to stop letting fuel drip while turned to the "OFF" position is a Derbi Senda 125/Cross City 125 part number: Derbi 00H00404181
When purchasing this tap' make sure it comes with the black dial, for some odd reason some places don't include it...

I had to buy a used one from ebay just to put that dial on a new one  >:(


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/gearbox any favours. Always turn to "OFF" position while the engine's switch off.

Part 2: Exhaust

Don't waste time and effort grinding out the manifold washer and sawing the resonator pipe off and welding it shut on the OEM exhaust, you will still have to saw the expansion chamber in half to remove the silly catalytic converter which not only restricts power' but also creates huge amounts of unnecessary extra exhaust and engine heat. After i removed the catalytic converter baffle the performance difference was hardly noticeable compared to an aftermarket exhaust IMHO.

For the sport bike frames' use a Giannelli/Arrow exhaust
For motard/enduro frames' use a Yasuni Cross ML exhaust

Giannelli/Arrow come with 1 restrictor washer in the front manifold, Yasuni's come with another in the muffler. Here's how to remove those restrictions
Remove factory lacquer finish from the steel exhaust chamber with methylated spirits and paint with a few coats of any good quality tinned brush-on VHT paint as an effective rust deterrent.

Part 3: Disconnect 2T oil injection

Drain off radiator coolant and transmission oil' remove the clutch cover and remove the 2 plastic pump drive gears from their shafts. Unbolt the oil pump and remove the pump drive shaft. Reinstall the pump or simply use a blanking plate like this...

Then remove the pump cable from throttle cable Y splitter and remove the oil tank from the frame...

Part 4: Stiffer clutch springs

Due to enhanced performance gained with the aftermarket performance exhaust' the clutch friction plates will begin to slip under hard acceleration - especially on cold engine before warmed up' many owners install shim washers to counteract this w/o realizing the total pressure plate distance travel will be less and clutch dragging usually occurs. The stiffest quality springs on the market are Malossi MHR's which are used in GP 80 cup bike engines, part number: Malossi 2912018

Part 5: Gear selection restriction

In some countries' these engines are limited to an even dangerously slower 30.58 kph (19 mph) :eek: this is done by restricting owners shifting higher than 3rd gear by means of a different shift cam plate. The correct cam plate for reaching 4th 5th and 6th gear is part number: (Aprilia 847038) you'll also need to swap the 11T front sprocket for a 14T sprocket, part number: JTF1128.14

Refill the transmission with 650cc mineral-based JASO MA2 API SJ specification 10W/40 4 stroke motorcycle engine oil, fill coolant/antifreeze no higher than the overflow bottle maximum mark. If fitting brand new clutch plates don't forget to pre-soak them in fresh oil before installation. Always use OEM Derbi/Aprilia clutch friction and steel backing plates for the most durable service life.

Part 6: Polini CP jetting and Premixing

Drain off all fuel from the tank.
Buy a useful oil measuring bottle, part number: Polini 121.500
Apply a piece of tape next to the 4 1/2 liter mark on the 4% scale and add this amount of oil to 5 liters of fuel, shake the Jerry can vigorously and then pour the premixed fuel into the tank. When filling up at fuel stations' always pour oil into the tank first and add fuel at the quickest rate the nozzle allows w/o creating a "fountain" effect, squeeze the front brake lever and pump the forks a good few times before re-starting the engine.

Take out the carb throttle slide needle and put the clip on 4th notch down.
Remove the float bowl and change the pilot jet for size (49), if using the oil injector pump on a 50cc kit with OEM exhaust then use size (46)
Change the main jet for size (92)
Turn the air screw clockwise until fully seated, then turn anticlockwise by 1 and a half turns (from 12 o'clock to 6 o'clock = half a turn).
Adjust the throttle stop screw to reach the desired idle rpm speed after starting the engine.

Go for a ride' warm up the engine before completing a few high speed full throttle runs, during this you will notice the engine begin to sputter before reaching the rev limiter. While this happens the engine is effectively firing on every 4th cycle "4 stroking". Reduce the main jet size by 2 numbers smaller each time until this 4 stroking stops.

When 4 stroking stops' roll from 1/2 to full throttle in 6th gear and the rpm's will drop slightly. When rolling back to 1/2 throttle the rpm's will raise, reduce the main jet size by 2 numbers until this full throttle bogging symptom clears. You will notice the "4 stroking" appears @1/4 throttle' if it 4 strokes massively, lift the clip to middle notch.. but no higher than 2nd notch. At this point the carburetor is perfectly jetted for cooling the piston, cylinder, bearings and no further jetting adjustments are needed.

If you live in high altitude areas where air is naturally less dense' or using "premix only" oil which is usually a thicker viscosity than injector / premix stuff, you might find an 80 main jet is too rich. If so' try the 60 - 78 pack: Polini 370.0001

2 strokes were designed to burn dirty, emit blue clouds of smoke and sputter through 1st / 2nd gear after idling for more than 3 seconds - especially while running healthy premix ratios. The main reason mx and road racing involves whacking open the throttle many times on starting grids before the gate drops or amber light turns green.

DO NOT in any case rely on spark plug electrode color or piston wash deposits to give you a correct indication on safe air/fuel ratios, Blue dyed 2 cycle oil paints them a dark grey color and modern synthetics don't leave the same tan coloring as base mineral dino oils, meaning false plug and piston wash readings.

Part 7: Replace the OEM ignition

The Ducati ignition only lasts a little while before the stator winding gives up it's ghost' cheap China and Tiwan stators are even worse. When the stator fails, don't go replacing the CDI module as this never fails. Replace the whole ignition with the french made MVT Millenium EXT 111 kit...

...but don't ground the Blue 45 km/h restriction wire...

Part 8: Larger capacity cylinder/piston kit

I recommend Airsal's Nikasil plated aluminium 78.5cc sport kit with dual ring piston...

My slight issue with Airsal's kits is they don't supply an exhaust manifold header gasket like some other kits do. However' if you google "Aprilia SX 50 exhaust header gasket" you won't have to deal with 2 cycle oil spraying all over pearly white race boots LOL.

When installing a new cylinder kit' insert piston rings with ring letter or number facing upwards, make sure piston crown arrow faces the exhaust port. Insert 1 gudgeon pin retaining clip before sliding the pin through' cover the exposed crank area with a rag to prevent dropped pin clips or debris entering the bottom end.

Give the crank rod small end needle cage bearing, gudgeon pin, piston and cylinder walls a nice coating of 2 stroke mineral oil (avoid slippery cylinder glazing synthetics). And apply a generous amount of copper grease to all paper gaskets' this helps them pre-expand before engine running and also a lot easier to remove w/o picking off with a gasket scraper the next time round. Never use a stanley knife blade or anything metal for scraping off gasket material, only plastic scrapers should be used since plastic is softer than aluminium.

If you want to stay with the OEM 49cc kit' remove the cylinder head thermostat and replace the head gasket and O-ring...

If the Malossi MHR clutch pressure plate springs feel too heavy on longer trips' saw the LH switchgear OEM lever mount down and fit an easy pull clutch perch/lever like the perfect but quite overpriced RSC Stunt clutch or the cheaper Pro Taper Profile perch/lever.

Part 9: Remove the radiator un-cooling fan

The Gilera SMT and RCR don't include the typical Comex radiator cooling fan...

...someone at the Gilera end of the Piaggio factory obviously had their Italian electronics head screwed on properly. This fan adds unnecessary weight for no useful reason, even after engine seizure/boiled over coolant the fan does not activate. Unplug and remove it, this allows head winds to escape the radiator's rear grills more sufficiently allowing for better airflow.

Part 10: Preventing a damaged battery

No battery box cover included with motard/enduro models...

Place a few pieces of high density foam underneath and on top of it.

Part 11: Have copious amounts of 2 stroke fun! ;)

Always pull the clutch lever in before decelerating

There is little to no engine braking on 2 strokes' every time you decelerate or descend down a hill with clutch drive plates engaged the engine is still creating heat while the cooling system's being fed less cooling air from headwinds. When closing the throttle grip' the carb needle taper lowers into the atomiser and reduces the maximum amount of fuel an engine can draw into the cylinder with vacuum pressure. Closing the throttle grip while decelerating with clutch lever out @high rpm is neglectful.

Cold starting procedure:

When a bike hasn't been ridden for a few hrs, or parked overnight etc.. always "prime the engine". Some of the fuel/oil mix protective coating on the piston and cylinder will begin to slide off after a few hrs due to gravity's effect. Prime the engine by slowly pressing the kick start lever to the bottom of it's stroke once or twice in order to allow lubricating fuel/oil into the cylinder before starting it up. On RS/GPR models with starter motors: select 2nd gear while pushing it down the garden path or out of the garage and release the clutch lever to the biting point a few times.

In the workshop manual it only specifies: complete piston replacement every 15,000; 30,000; 45,000; 60,000 KM. Piston rings are sold separate for many 2 cycle engines for a typical reason, you don't need to replace a piston every time the rings wear out. Below is a photo of the Airsal piston from the 78.5cc "sport" kit, This piston was reinstalled with new rings' the photo shows how a piston condition should be with minimal score marks and black spots on underside of the crown...

If jetting is too lean from typical user error, air leaks from gaskets, split intake manifold rubber, water pump, damaged radiator or thermostat failure, the crown underside dark spot will cover much more surface area and may also show a little brown like this...

If using a synthetic oil then expect more ring blow-by and also if exceeding the recommended ring change intervals, telltale signs are brown or dark coloring on piston walls below the rings like this...

Here's pics showing the piston condition of the previously used Airsal Cast iron "sport" 70 kit...

This kit was run on the fully de-restricted OEM exhaust' which is limited to around 9,000 rpm or less. Covered at least 18,000 miles on this kit before switching to the Nikasil plated Auluinium 80 kit. Aftermarket "sport" pipes like the Yasuni Cross ML, Giannelli and Tecnigas Enox rev to around 11,500 rpm, higher rpm's = accelerated wear n tear rates and higher engine temperatures. So a piston will typically show more scoring marks and worn rings after around 15,000 miles rather than 18,000 miles.
« Last Edit: January 20, 2017, 12:24:32 AM by Jerry-Built Hustler »


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The Euro 3 D50B0/D50B1 LH engine casing has a captain obvious design flaw, no gasket used to help seal out water, dust and grime. Therefore' removing the flywheel and scrubbing round with a small brush is necessary general maintenance every 5,000 tarmac road miles, every 12 dirt road rides or every 1-3 mud trench rides.

Extraction tool size for MVT Millenium flywheel: 27 P. 1,25

Extraction tool size for OEM Ducati flywheel: M19 P. 1

Torque wrench users:

Recently stripped some crank nut threads using a Draper 3/8" drive. OEM manual torque spec 35 - 45NM. Began with a safe 25NM and it refused to click, but it was already too late...

Many experience the same issues with clickers being far out of calibration - despite following manufacturer care and storage guidelines. The more you use a "clicker" the more they seem to drift out of accuracy. Crankshafts are manufactured from steel, nuts from soft aluminium to prevent ruined shaft threads.

Only 1/2 drive beams available in the UK. So despite being cheaper than clicker designs, i paid nearly double the price due to overseas shipping from murica. And will have to repeat this when i have enough funds to replace the 1/4" drive clicker, which was responsible for stretching one of the cylinder stud bolts on the previously used Airsal iron 70 kit.

^ Arrived yesterday morning after a 2 week shipping wait, torqued the nut to 45NM faultlessly. Rather than typing an endless TL;DR, i'll post a video i found that puts things into an obvious perspective.

« Last Edit: January 20, 2017, 12:19:29 AM by Jerry-Built Hustler »