Aprilia RS 125 Fuel system , no fuel problems

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Aprilia RS 125 parts for sale on Ebay UK

Aprilia RS125 workshop manual for sale Ebay

 UK 

Just thought id do a post on the fuelling side of RS125 .


It a pretty simple system using the Carburettor , Reeds .


Fuel System consists of


-Fuel Tank
-Fuel Tank Tap - inc built in Fuel filter
-hoses which sometimes have a fuel filter added
-Carburettor which meters the fuel via specifically sized jets
-Inlet manifold
- Reed valves

Buy all the New an USED Aprilia RS 125 parts on Ebay UK here 

Carbs fitted to the Aprilia RS 125 in the Past are the

Dellorto 34mm LD   early model unrestricted models pre emissions regulations

Slide 40
Needle K57
Slide Needle Valve 266DP Atomiser
Main Jet 160
Pilot Jet 40
Choke Jet 60
Float Valve 300
Float 12630.1 - 9 grams

The Dellorto PHBH 28BD  Aprilia RS 125 (1996-2012)

Slide 50
Needle X2
Slide Needle Valve 264BN Atomiser
Main Jet 115
Pilot Jet 65
Choke Jet 70
Float Valve 250
Float 9010.1 - 9.5 grams

Servicing the fuel system - check over .

-Clean out the air filter as well , and reoil prior to fitting  ( not strictly fuel but important )
- Check inside the tank for any debris , water etc
- make sure any breather holes for the tank are clear and able to breath in air ( this could restrict fuel flow )
- Check and clean the internal tank tap filter
- After refitting check for leaks and refit hoses making sure
all hoses are tightly fitted and no leaks and no splits or damage to pipes .
- make sure fuel pipe is of adequate diameter and construction so as not to obstruct fuel flow .
- fit and inline fuel filter if needed .
- There is also a small filter on the carb , behind the inlet connection clean this as well .
- if the bike is running badly or will not start very well , then clean the carb out thoroughly
we do a post  HERE if you want to read that on cleaning the carb and jets  .
- Make sure all jets are correct sizes and set you float height correctly and clean the float
valve and hole out as well while you there .
- Check the inlet manifold for cranks and correct fitment , any leakage here will affect the
running of the bike and possibly cause a non start issue .
- check the reed valves hold the reeds up to a light source and you should not see much if
any light source through them if you can its a sign of wear and will affect the running .
- Check compression you might not be pulling enough air into engine due to worn rings or other problems , not enough air me and not enough fuel .

Here is a photo of the internal fuel filter and it clearly shows the fuel filter on the end of the internal tank fuel pipe , its worth stripping the tank tap down and cleaning out internally if problems persist .

Aprilia RS125 fuel taps




The carb inlet filter is fitted behind the inlet connection pipe pull it out and clean it as this could cause a restriction in the fuel line

Aprilia RS 125 Reed valves 

Obviously check for broken petals , correct fitting , and also check for signs of warping  as below.

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see more Aprilia RS 125 posts here 

Aprilia rs125 jets blocked , carb cleaning , repair and stripping down

• Aprilia RS 125 for sale here on Ebay UK 

  
  

  Aprilia RS 125 parts for sale here on Ebay UK

  
  

  
  

  
  

  

  
  

  
  

  
  

  
  
  
  
  

  
  
  
  
  
  
  
  
  
  
  
  
   Buy an aprilia RS125 workshop manual on Ebay UK Here

                                         

     

You could just dump it all in your ultrasonic cleaner , get one they are excellent 

Buy ultrasonic cleaners on Ebay UK here 

dellorto carb manual

Download the manual, read it. Stripping and cleaning carb is one of the easiest jobs to do on a 2stroke bike.

Meccano is harder to get to grips with than stripping and cleaning a carb. Don't let it scare you - it really is easy to do... otherwise, I'd suggest selling the bike and getting a 4T, as any regular maintenance that is a necessity for a 2 stroke bike is gonna scare you, and you're gonna end up forking out hundreds in labour hours to the local bike shop for tasks that really you should learn to do yourself.

Grey residue is likely result of water mixed with 2T oil saturating / flooding your spark plug as you have water in the float chamber that is submerging the main jet (mainjet is in base of float bowl, and petrol floats on water), so when you open throttle, bike gets a lung full of water rather than fuel.

Flip up tank. Turn fuel off. Look down at carb. Either side of it you'll see 2x jubilee clips clamped over rubbers. Undo those. Grab carb... move it backwards and forwards and work it out from between the rubbers. Keeping it the right way up, undo big brass bolt in bottom. Bottom will then come off. This bottom part is the float bowl. Ideally, open the bolt over a glass bowl so that you catch the spilt fuel and can inspect it for signs of water or contamination.

If no signs of globules of liquid suspended in fuel, inspect the bottom of the carb that you removed for signs of contamination - dust, rust, dirt etc. Rinse it out.

The jets are small brass things with a hole in the middle and a flat-blade screwdriver cut - you'll see em. Take them out one at a time, blow thru them, rinse in boiling water. Put em back in again. Tighten em til they nip gently.

Put float bowl back on. Do up the big bolt to secure it in place. (Note, this big bolt has the main jet fitted in the opposite end of it).

Fit carb back in between the rubbers. Put the jubilee clips back on. Turn on fuel tap.

See if it starts. See if problem still exists.

CARB SETUP

Set it to rev at 2000 with the idle screw, then twiddle the air screw just to the left of the idle screw... turn it all the way in, then back out 1.5 to 2.5 turns, listening to engine. As you turn it, revs should increase slightly. Set it to a point that revs are at highest and smoothest. Now use the idle screw to tune it back down to around 1100rpm - 1500rpm .

in = increase

out = decrease revs

=================================================================

Servicing The Fuel System

Here we’ll look at servicing the fuel system, checking the tank, fuel tap, fuel hoses and fuel filter are all working correctly as well as checking for any signs of leakage, deterioration or damage. We are particularly looking to make sure that there are no leakages coming from the fuel hose or fuel union.

Inspecting The Fuel Hose

Firstly you’ll have to remove the tank, carefully removing the fuel hose that is connected to the fuel tap. Here you will have access to check the fuel hose for any damage or deterioration, if there are any signs then you should replace it immediately.

Inspecting Engine Oil Hoses

You’ll want to check the engine oil hoses from the oil tank to the pump, then from the pump to the carburettor. If there are any leakages present then ensure that all the hose clips are tight and replace any hoses that are cracked or have signs of deterioration.

Inspecting The Fuel Tap

With the tank removed inspect the tap and underside of the tank for any signs of fuel leakage. If the tap body is leaking then a new tap will need to be fitted as no individual components are available.

If the tap to tank joint is leaking then follow the proceeding instructions.

If your Aprilia RS 125 is the model between 1993 to 1998, the tap is retained by a union nut; hold the tank union with a spanner to prevent it twisting, then tighten the union nut carefully. On 1999 to 2005 models, the tap is retained by two bolts; tighten the bolts carefully. If leakage persists then remove the the tap and fit a new joint O-ring.

Fuel Filter

The fuel filter is mounted in the tank and is an integral part of the fuel tap system. Clean the filter at appropriate intervals or if you suspect fuel starvation.

With the tank removed you’ll need to drain the fuel into a suitable container.

On 1993 to 1998 models, hold the tank union with a spanner and loosen the union nut. Unscrew the tap and union nut from the tank and withdraw the the tap and filter assembly. It is recommended to fit a new O-ring upon reassembly.

On 1999 to 2005 models, undo the two bolts securing the tap to the tank and withdraw the tap and filter assembly carefully.

Allow the filter to dry, then clean with a soft brush or low pressure compressed air to remove all impurities in the system. Check the gauze for holes, if any are found then a new tap will need to fitted.

Buy Carb cleaner here on Ebay UK

Buy carb jet cleaners here on Ebay UK 

see more Aprilia RS 125 posts here 

Aprilia RS 125 problems / troubleshooting / fault finding ideas RS125

Aprilia RS125 ultimate workshop manual bundle

Aprilia RS125 owners technical survival guide 

 Buy an aprilia RS125 workshop manual on Ebay UK Here

Aprilia RS125: Troubleshooting & Fault Finding Guide

The Rotax 122 engine is reliable if maintained, but its high-strung nature means small faults can lead to a "non-runner." This guide categorizes the most common failure points:

• The "No-Start" Diagnosis:

  • Spark: Check the NGK BR10EG plug. If it's wet with fuel, the engine is flooded. If there is no spark, inspect the HT lead and the CDI (the "Bridge" mod often fails here).

  • Fuel: Ensure the vacuum tap is pulling fuel. A common fault is a blocked tank breather, creating a vacuum that starves the carb.

• The "Bogging" Issue:

  • Power Valve: If the bike won't rev past 7,000–8,000 RPM, the RAVE valve is likely carbon-locked or the solenoid has failed.

  • Air Leaks: A cracked intake manifold (rubber boot) causes a lean "hanging idle" and erratic revving.

• Electrical Gremlins:

  • The Rectifier: If the battery isn't charging, the voltage regulator/rectifier has likely overheated.

  • Fuses: Check the 15A and 20A fuses located near the battery before stripping the loom.

• The "Splutter": Usually caused by a dirty carburetor pilot jet or an incorrectly seated needle clip.

you might also be interested in this post  - Aprilia RS 125 will not

 start and common Problems RS125

Aprilia rs125 troubleshoot / fault find

just ideas not comprehensive and not necessary
in order of a sensible test procedure ill try to tidy up another time .
will update with new knowledge as i go !!

Engine won’t turn over:
Piston seizure
Crankshaft bearing seizure
Connecting rod small end seizure
Connecting rod big end seizure
Transmission gear or bearing seizure

or starter motor trouble see our post here Aprilia RS 125 motorcycle motorbike starter motor circuit testing ( will not turn over , no crank issue )

No fuel:
No fuel in tank
Fuel tank vent blocked
Fuel tap turned off
Fuel tap clogged
Fuel line clogged
Float valve clogged or stuck closed
fuel filter block
fuel filter at carb blocked

Buy engine rebuild parts here 


Engine flooded:
Fuel level too high
Float valve worn or stuck open
Starting technique incorrect
(when flooded, kick with the throttle wide open to allow
more air to reach the engine.)
allow to stand for while before retrying to start
do not keep starter motor engaged for long give it
a rest between tries .


see also our Aprilia RS 125 Fuel system , no fuel problems

No spark; weak spark:
Spark plug fouled, faulty , or wrong gap
Spark plug cap or high tension wiring faulty
Spark plug cap not in good contact with spark plug
CDI unit faulty
Ignition coil faulty
Magneto damaged
Loose wire connector blocks
Engine cut out switch faulty
earth wire loose/missing/high impedance
see also our Aprilia RS 125 No spark - Not sparking troubleshooting

Air/fuel mixture incorrect:
Idle air screw mis-set
Slow jet or air passage clogged or restricted
Air cleaner clogged, poorly sealed, missing or sports
Starter jet clogged or restricted
Incorrect fuel level
Incorrect jet sizes
sports Airfilter or exhaust fitted to standard carb
set up
air leaks
reed valve damage
engine not revving properly / spluttering

Fuel level incorrect
Main jet clogged, restricted or wrong size
Throttle valve does not fully open
Air filter dirty /blocked
exhaust tract restricted with deposits
Water or foreign matter in fuel
Ignition timing not set properly
Overheating
Transmission oil level too high
Transmission oil
viscosity too high
Crankshaft bearing worn or damaged
air leaks inlet

Buy engine rebuild parts here 



overheating

Brakes dragging
Clutch slipping
Transmission oil level too high
Transmission oil viscosity too high
Drive chain misadjusted
Tyre air pressure too 

low Transmission oil level too low
Transmission oil poor quality or incorrect type
Coolant level too low
Coolant old
Coolant not mixed properly
Radiator clogged
Radiator fins damaged
Radiator cap faulty
Water pump not working
Coolant passageways restricted
thermostat failure

Buy engine rebuild parts here 

clutch problems

Excessive clutch lever free play
Clutch cable misadjusted or not working smoothly
Clutch steel plates warped
clutch cable / adjusters incorrectly set / need replacing
Clutch spring worn / tension uneven
Transmission oil deteriorated
Transmission oil viscosity too high
Transmission oil level too high
Clutch housing bearing seized

Buy engine rebuild parts here 

gearbox common problems

incorrectly fitted
missing spacers in gearbox
worn faulty gear bearings
selector arm spring damaged incorrect fitting
bent selector forks levers etc
worn casing bearings
missing circlips

common noises and causes
Knocking:
CDI unit faulty
Carbon built up in combustion chamber or on piston crown
Poor quality fuel or incorrect pre-mix
Spark plug incorrect
Ignition timing not set properly

Buy engine rebuild parts here 

Piston slap
Piston to cylinder clearance excessive
Cylinder or piston worn
Connecting rod bent
Piston pin or piston pin holes worn

Connecting rod small end clearance excessive
Connecting rod big end clearance excessive
Piston ring worn, broken or stuck
Piston seizure or other damage
Cylinder head gasket leaking
Exhaust pipe leaking at cylinder head connection
Crankshaft runout excessive
Engine mounts loose
Crankshaft bearings worn
Primary gear worn or chipped
                              

see non start issues and common problems

wiring diagrams

see more Aprilia RS 125 posts here 

aprilia RS 125 will not start and common problems RS125

Aprilia RS125 ultimate workshop manual bundle

Aprilia RS125 owners technical survival guide 

Aprilia RS125: Common Problems & Reliability Guide

The RS125 is a high-performance 2-stroke that requires "preventative" rather than "reactive" maintenance. Here are the critical failure points identified:

• The "Nylon" Balance Gears: Most Rotax 122 engines use plastic/nylon gears for the balance shaft. These can strip or melt if the engine overheats, leading to severe vibration and eventually internal engine damage.

• Power Valve (RAVE) Carbon Build-up: If the bike feels "flat" or won't rev past 8,000 RPM, the exhaust blade is likely stuck with carbon. If left uncleaned, the cable can snap or the solenoid can burn out.

• Con-rod/Big End Failure: This is the most common "catastrophic" failure. It is usually caused by high mileage (over 10,000–15,000 miles) without a bottom-end rebuild or by using low-quality 2-stroke oil.

• Electrical Gremlins: The Rectifier/Regulator often fails, leading to either a dead battery or "blown" bulbs across the entire bike due to over-voltage.

• Oil Pump Failure: While rare, a failed oil pump means immediate engine seizure. Many owners switch to "pre-mix" (mixing oil directly in the fuel tank) for peace of mind.

 Click here see Aprilia RS 125 for sale on Ebay UK

Aprilia RS 125 parts for sale on Ebay UK

I have owned many Aprilia RS 125 bought , sold and owned them .

this is not a comprehensive problems list but the most common problems i have come across.



Fails to start after parking up for a few days/weeks
-------------------------------------------------

Problem usually blocked jets , the jets inside the carb are very small and can clog up  after log periods of parking up. this is why Aprilia have fitted a fuel on and off tap , if you park it up for longer than a couple of day id advise to turn fuel off and let bike run out of fuel .
otherwise you may have to take carb off and clean out the carb and its jets out thoroughly , for longer periods or trying
ti get a bike you do not know about use NEW fresh fuel and a New spark plug lost count of how many bikes i worked on till being
stumped put new fuel in a new spark plug cleaned carb and off it went , but being tight and trying to save the few quid for
the fuel and spark plug cost me an hour of frustration .

--------------------------
electrical problems
low battery , faulty battery. these batteries are fairly poor and if left for long periods without riding can go flat and fail to recharge again needing replacement i used to keep old battery but always found when i come to use them they were completely dead beyond reviving do not know why this is must be some scientific reason for it but i have no idea. Try to run the bike regular or remove battery for trickle charging for periods of long parking or replace battery if you just bought bike and struggling with power.

Another problem can be faulty contacts in the wiring loom (block connectors) i always slit these put an electrical contact cleaner in them to ensure good contacts throughout the whole loom.
id do this yearly as rain , moisture can affect these joints causing a white powdery stuff to form again no idea what this stuff is.

if the bike fails the turn over but clicks and you battery is well charged , this is likely to be the relay situated near the radiator this is a device to turn the low current circuit of the starter switch into the higher current circuit of the starter motor but they are shit 
i hate relays tbh they seem to cause more problems than enough especially on modern cars .

basically this about it ive always found if you buy an old rs125 as long as the compression is good (ie piston rings good)
if you clean out the carb and jets ,replace old fuel with new fresh fuel , fully charge the  battery or replace , make sure all electrical connectors are cleaned and fitted nicely add a new spark plug then there should be no problem firing up .

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


Aprilia rs125 not starting ,

RS125 Non-Starter Checklist

-       NO FUEL please check and put a few quid in any way especially rs125s i have ran them out loads of time even though i see
a bit it the tank , pushing bike to garages cured me of this i never use reserve if possible.

• If there is a nick in a tube or you have a leaky carburettor, your fuel could have escaped and evaporated over the course of a few hours (Lift your filler cap and check that there is fuel visible in your tank and above reserve).

• Turn the engine over and make sure that a good volume of air is coming out of the exhaust ie no blockages to air inlet or exhaust system.


• Make sure the choke is not flooding the engine

• Either remove the spark plug or take a good plug that you are certain works (ive known brand new plugs fail causing havoc and hours of messing about before i tried another new one and bike started !!)
, and connect it to the HT cap , Touch the end of the plug to the frame via the insulated cap or using insulated pliers beware of getting a shock .

Hit the starter button and look for a spark.

• Check the original plug in the same way to make sure that it isn’t the source of the problem.

• Make sure that the HT lead is on plug.

• Make sure that everything is plugged into the CDI as it should be.

if you getting this far see below 



SPARK/COMPRESSION/FUEL/AIR/TIMING

You need all of the above to get the engine to run.

Remove the plug, reconnect it to the HT lead, hold it on the cylinder whilst turning the engine over and you should see a spark.

Whilst plug is out do a compression test. use a decent compression gauge to do the test .
good compression is a must <90psi and bike would not start <100psi starting problems risk of siezure 120/130> good ,

Check fuel is fresh and getting to the carb.

Check air filter is clean.

check timing spark problem - duff plug, cap, ht lead, coil - CDI , check timing .

Try the obvious first.



---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

You think "Ive got fuel" Its always worth actually lifting the filler cap to visually check for fuel. Many people have had £5 of fuel leak out through a nick in a tube or due to a poor carb and this will evaporate over the course of a night, its happened to me a lot on rs125s even though you can see a bit of fuel in take enough for a quick spin next thing you know your pushing bike back
if you can see a bit of fuel in bottom of tank you aint got enough in fill at least half way to eliminate it . 

Kill Switch - Check It Now. We've all done it before left it off, or had someone else fiddle with it unbeknown to you when you get in a panic it can easily be missed and its shit if you take your take off 
or even fairing just to find its the kill switch off !!.

Check The Exhaust for blockages. A friend of mine took his scooter back to the dealership. where they claimed to strip the engine down, before noticing a bottle cap on the exhaust.

If your still not firing by now, you are almost certainly going to need to buy one thing at least before you are. However don't forget to give the bike plenty of chance to fire up. Don't just give the button a little second long tap and expect magic all the time. Check the choke position too.

Now I would say you need to start testing for a spark.

Get Yourself to a point where you can remove both the plug top and the plug from the engine. If like me you have numerous plugs laying about (AND YOU KNOW FOR CERTAIN THAT THEY ARE INTACT) it is often quicker to put one of them in, instead of taking yours out. Whip the plug top off and put the replacement plug into it. Now you must Earth the plug using either the threads or the electrode at the very bottom of the threads. You must get the plug somewhere so you can clearly see if it sparks, and yet it has a perfect contact with the Earth of the bike. I have yet to own a bike with all engine paint still intact so I usually just put the plug on the engine. You could use the frame, or better still battery negative - you're guaranteed an Earth there.
Don't forget the kill switch and ignition switch. You will get a jolt off a plug if you hold it so the spark can get you. if your brave Hold the bakalight at the top, or the plug top or use a hammer as i do to hold plug to earth (frame/engine etc).
Is it sparking?

If yes, (and you aren't already using the original plug from the engine) take the original plug out and check that one. If it doesn't spark now, buy a new plug.

If no spark, (and you are using the original plug) buy a new plug.
If no spark, (and you are using a plug you know is good) you have problems.

Check all the wires are plugged into the Cdi unit. Inspect these wires for chaffing they may have been cut right through from chaffing on something else. Check the HT lead (the fat wire the spark plug top is on) is screwed into the coil tightly and that the plug top is screwed into the HT lead tightly.

if still no spark you need to get WSM out and start test alternator and regulator form correct operation (ill do these tests someday as posts )


just one thing to add.

The starter relay is located just behind the radiator. It is positioned so that the all the wires connect upwards into the unit. This means that it is possible for one (or more) of them to fall out. The symptom is that the starter will not turn the bike over at all but you can bump start it.( If you don't know how to bump start a bike I might do a 'how to' on that if there is any interest) If the wires are all as they should be the relay gets to be suspect. Like all other electrical components , they don't last forever.

Worth knowing I reckon, Oh and don't forget the fuses !

i just done a new post of Spark diagnostics here Aprilia RS 125 No spark - Not sparking troubleshooting

aprilia RS 125 problems / troubleshooting / fault finding ideas RS125 

 Aprilia RS 125 motorcycle motorbike starter motor circuit testing ( will not turn over , no crank issue ) 

Aprilia RS 125 Fuel system , no fuel problems .

if you have a problem ask in comments below and this section may help others .

The Digital 2-Stroke: Aprilia DiTech vs. Cagiva ECS

Aprilia RS125 ultimate workshop manual bundle

Aprilia RS125 owners technical survival guide 

As emissions laws tightened in the late 2000s, Italian engineers faced a crisis: how to keep the 2-stroke alive. Aprilia and Cagiva took two completely different technological paths to solve the same problem.

The Aprilia Approach vs. Cagiva ECS

Did Aprilia have any Electronic Fueling?

Yes, but not on their sportbikes. Aprilia developed the DiTech system, which was a Direct Injection 2-stroke system.

• Where was it used? Mostly on the SR 50 scooters.

• How did it work? It was incredibly efficient, using an air compressor to spray fuel directly into the cylinder.

• Why not the RS 125? Reliability and power delivery. At high RPMs (where the RS 125 lives), direct injection struggled to keep up with the fuel demands without seizing or running lean.

The "Modern" RS 125

In 2011, when emissions laws finally killed the 2-stroke, Aprilia moved to the RS4 125, which is a 4-stroke with Full Electronic Fuel Injection (EFI). It lacks the visceral "smoke and powerband" of the Mito or old RS, but it's far more reliable for daily commuting.

1. The Aprilia DiTech (Direct Injection Technology)

Introduced primarily on the SR 50 scooter (and prototyped for the RS series), DiTech was the most "revolutionary" solution.

• The Tech: It used a system developed by Orbital. Unlike a normal engine where fuel enters the crankcase, DiTech injected fuel directly into the combustion chamber after the exhaust port had closed.

• The Compressor: A tiny, crank-driven air compressor atomized the fuel into droplets as small as 8 microns.

• The Result: It met Euro 2 and Euro 3 standards with ease, offered 100+ mpg, and produced almost zero smoke.

• The Catch: It was incredibly complex. If the air compressor failed or moisture got into the fuel rail, the bike wouldn't start. It was "too smart" for its own good in a 125cc sportbike application.

2. The Cagiva ECS (Electronic Carburetion System)

While Aprilia went for "Fuel Injection," Cagiva decided to "evolve the Carburetor" for the Mito SP525.

• The Tech: It used a Dell'Orto PHBH 28 carb, but with a twist. It featured an ECU-controlled solenoid that adjusted the air/fuel mix based on throttle position (TPS) and engine speed.

• The Goal: The ECS was designed specifically to pass Euro 3 tests by leaning out the mixture during specific test cycles, then allowing the bike to run "normally" elsewhere.

• The Benefit: It felt more like a traditional 2-stroke. You still had a carburetor you could clean, even if the electronics made it finicky to re-jet for racing.

3. Aprilia RS 125: The "EFL" Solution

Ironically, the Aprilia RS 125 never got DiTech. Instead, for its final 2-stroke years (2008–2012), it used a system similar to Cagiva’s but slightly simpler:

• It utilized a Dell'Orto VHST 28 carburetor with an electronic vacuum solenoid.

• The ECU would retard the ignition and adjust the solenoid to reduce emissions at idle and low speeds.