Friday, 20 April 2012

aprilia RS 125 will not start and common problems RS125

aprilia rs125 

ive owned many aprilia rs 125s bought , sold and owned them .

this is not a comprehensive problems list but the most common problems ive 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 dont 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 annd 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 batterys 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 
dont 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 causeing a white powderery 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 saprk 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 espeicially rs125s ive ran them out loadsof timeeven 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 


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 actualy 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 atleast 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 guarenteed 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

or try  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 .

                                                               Checking for a spark 

NON spark diagnosis video 

Aprilia Rs125 parts suppliers

see also fault finding / troubleshooting

or find your workshop manual here

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

see more Aprilia RS 125 posts here 

BUY the ultimate Aprilia RS 125 workshop manual bundle HERE

Visit chinas Ebay and buy motorcycle products direct cheap and safely here 


  1. i have an aprilia rs 125 2001 plate and starts and stops , i checkd for a spark and i am only gettin multipul sparks . what would be the problem

    1. hi mate its hard to help when i dont know full story , is it a new bike thats never started properly or is just started happening ?

      if your sure its spark then try a new plug first obvioulsy then test coil , cdi etc .

      test coil on input side see if its getting 12v to it .

      starting then stopping could be fuel , have you cleaned carb out and cleaned jets out


      You need all 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.

      Check fuel is fresh and getting to the carb.

      Check air filter is clean.

      Unlikely to be timing but rs125s have been known to snap there own woodruff keys

      Prob spark problem - duff plug , cap, ht lead, coil - prob not CDI but never know , timing slipped with woodruff key .

      Try the obvious first and reply with findings!

    2. Sounds like your relay get a multi metre on it and check for brakages in the loom

    3. hi i had this get a coil not to dear but fixed it

  2. hi. your blog is fantastic and really helpful. just got a question for you - my rs125 (bumblebee version) is hard to push if the engine is not running and i'm in first gear with the clutch in. and its impossible to change up to second gear with the engine not running (trying to push start). is that normal?

  3. it can be difficult to change gear without engine running , but it should push freely with clutch in

    doe it try to pull away when started ticking over in first ?


  4. hi I have an aprilia rs125 that I brought in boxes and built myself I have a small amount of skills and decided to upgrade the head n barrel to an itikit 140cc 34mm pull power carb full arrow exhaust derestricted cdi but bike wont run I have a brilliant spark with race plug n coil n new starter mota all parts fitted are brand new bike will start on the button from cold with no choke if left for a couple of days but when she fires up it seems to be chucking out loads of unburnt fuel and she smokes a bit and wont rev up past 4k rpm and then will cut out and not start again please help ? and would u have any idea where abouts the air oil mix screw should be I,e, from wound all the way in how many turns out should the screw be ? thanks for taking time to read thanks ricky.

  5. i belive my ignition timing is out by quite a bit and i have tried to remove the alternator but failed manual says special tool is needed do i have to remove the engine from bike to do what u showed in video, and while i have the alternator/stator out is there anything else i should check for damage or wear that could lead to bike not starting
    i have checked obvious things spark plug, Ht leads, Carb, air filter, earths, battery etc

    1. Update on bike situation upon removal of my enigne my piston which is only a 4-5momths old max has pits in it and the compression rings are seized and the combustion chamber has been badly pited and worn the piston has several score marks in it and similar dmg to combustion chamber walls too, although this could be the cause of the issue i am still going to take of altenator and check the woodruff key as well.Does anyone else have advice for what i can also check

  6. is yours a 123 engine then ?

    sounds like maybe previous engine build was still deposits of metal in the bottom end , needs to be flushed .

    its really hard to diagnose from a distance , sorry , all i can say is do a rebuild and pay close attention to details .

    ie cleanliness , piston to barrel clearances , squish , oil pump settings lightly oil all new parts , compression levels , setting up and then testing of carb settings at all throttle postitions use all new good quality new parts and a new spark plug and fresh fuel .

    these are the differences between a good build and a failure .

    1. it is a rotax122 engine which is strange as i thought it was a 123 engine but was wrong.the bottom end is fine tho no dirt or metal got into while i was re building and working on gearbox, this head is maybe as old as the bike its self and i kinda forgot to check it last time i stipped the engine down (whoops). i am looking at having it re-bored and sorted out that way.

  7. Hi I have a rs 125 on 57 reg I have rebuilt this everything is new but will not run it will start up and run if you hold the throttle open soon as you let go it cut out will not tick over on its own there is plenty of fuel and good spark very good compression help please

    1. copied from above post

      cleanliness , piston to barrel clearances , squish , oil pump settings lightly oil all new parts , compression levels , setting up and then testing of carb settings at all throttle postitions use all new good quality new parts and a new spark plug and fresh fuel .

      these are the differences between a good build and a failure

  8. impossible for me to tell you exact problem .

    but if it has spark , fuel and compression it should start .

    go over everything again , fresh fuel a different new spark plug ( had new plug fail in past ) check
    set up for air leaks etc , check to ensure flywheel is fitted correct .

    it must be something fitted wrong on rebuild or it usually is anyway , its the hardest thing to do with a rebuild getting it set up right , it about attention to every detail , making sure everything is done properly from start to finish , every nut bolt seal and gasket fitted and tightend correctly .

  9. I have a aprilia rs 125 it has been sitting down for a year i put a new battery on it and its coming like theres no battery on it at all none of the electrics work i have checked all the fuses and there all fine so im lost to what the problem can be can any one help me please ?

  10. hi i have a 2005 rs 125 and having problems getting a spark i.e turn bike over ht lead sparkes one time and thats it any idears anyone

    1. We have a none spark post specially for this

      Aprilia RS 125 No spark - Not sparking troubleshooting

  11. Hi, I've really enjoyed reading this blog as I'm new to the aprillia rs 125. The story of my bike (and problem) is I bought the bike about 2 weeks ago from a mate where it was stood for 10 years. I've cleaned out the fuel tank of the old gone off fuel and cleaned out the carb. (28mm I believe as its an s-Reg bike) put a new plug in it with fresh fuel and new battery and it started. The bike wasn't running great but was enough to make a 10 mile trip to mot it. The bike wouldn't Rev very well so I cleaned out the power valve and then it went really well. The bike out of the power band was quite poor and so I replaced some damaged reeds hopefully to help rectify this. My problem is once or twice riding the bike it felt like it had run out of fuel where opening the throttle just made the bikes engine die. I've cleaned the carb out a few times but seems spotless. Ive only ridden the bike twice as I can't get it to run properly. I can start the bike easily. First buzz of the start button. The bike will Rev up quite freely when stone cold (not ideal) but as soon as its been running for a minute it will only Rev to about 5k rpm approx. If I crack the throttle open or gradually open it there is no difference and it will not run properly. It sounds like it's hitting a rev limiter or something. I can't ride the bike now as not enough power to pull off. Any ideas guys? I've changed spark plug, run with no air filter, cleaned carb, checked fuel line for blockages etc.
    Thanks for reading Gareth

  12. Hi, thanks for publishing this blog I've found it a very interesting read. I'm hoping someone can help me with a problem I'm having with my newly aquired aprillia rs 125.the bike was parked up in my mates shed for 10 years. I've cleaned out the fuel tank and carb and put a new battery on it. The bike started up which I was happy with. After riding it I noticed it it didn't Rev very well so I cleaned out the power alive which seems to have helped. I replaced the reeds too as they were damaged/worn. The problem with the bike is when I start it from cold it starts quite easily and for the first say 30 seconds the bike will Rev up freely. After about a minute or so of warming the bike up it won't run properly. It will to lover and Rev to say 4K under very light throttle loads no problem. As soon as I try to Rev it up the bike sounds like it's hitting a Rev limiter or something at about 5-6k rpm. Even if I open the throttle fully it won't Rev past this rpm. I can't ride the bike anymore as not enough power to move. I've changed the reeds, plug, cleaned the carb etc since the problem. The bike has an arrow end can, has a power valve and has had the Cdi mod done previously. ( silicon out at the base assuming the resistor has been cut etc)
    I have only ridden the bike twice now (once to mot it and after cleaned the power valve) and can't use it.
    Any ideas guys? Thanks

  13. Hi did you check the compression ? this is important if the piston / barrel are damaged or worn the air drawn into the engine is not enough to draw in the correct amount of fuel too .

    If i get a bike that does not run , first i check spark , is it a good clean spark . i then check compression do i have a least 120 psi if not replace piston or barrel , then is the carb set up correctly .

    It could be your bike has carb set up for different exhaust or airfilter or even big bore kit meaning the set up is completely wrong .

    1. check spark is good
      check compression is good
      check carb is set to standard to begin with

      post on all this are in the blog


  14. Hi thanks for the reply. I have not checked the compression yet as I'm still waiting to get the tool from my dad but I do intend doing it soon. I've read though if the bike has difficulty starting it could be compression. The bike starts on the button no problem it jut won't Rev at all. When I rode it the second ride after cleaning the power valve it seemed to go really well and Rev up no problem. I parked the bike up for 2 days in a dry garage and now it won't Rev. It's very frustrating as you can imagine haha. The Rev counter is a bit hit and miss/iratic so it's hard to guess the rpm. It won't Rev to the power band though. If I Rev it from tick-over it will Rev up slightly more than the 5-6 k rpm it's maxing out and then instantly falls to the same rpm. Thanks
    (I might have posted the last post twice sorry oops)

  15. make sure you have good fresh petrol as well , dirty petrol can block jets .

    It does sound to me to be fuel related ie dirty carb jets ( you would no believe how hard to clean they are when i was new to bikes i struggle to get carbs clean ) or its the wrong carb jet set up .

    but like i say to mess about with your carb while you compression is low is a bad thing ie your fine tuning your carb to a bad engine .

    another thing to look at is the Coil wires are they loose and well lubed ive know this cause your trouble once before but that was after i knew good compression was present .

  16. Thanks for reply, the carb jets are standard I believe 250, 70,62,120, (unsure what order but the numbers match the blog 28mm carb numbers.
    I'll definitely get round to getting the compression tester and I'll go through the wiring.ive cleaned the jets about 3 times and given the carb a spray to see if carb cleaner sprays out various holes when the jets were removed. The jets look spotless and I can see through a lot of them.
    Thanks again

  17. Bit of an on going update. Today I removed the coil and cleaned up the spade terminal
    Contacts, I also cleaned up the bolts where the earth strap goes. I then squeezed the end cap slightly to ensure a better fit on the end of the spark plug. Then refitted it. I also drained the fuel tank and put in some fresh super unleaded. I started the bike and it revved up fine(good) it then warmed up a bit more and went back to the same problem so it seems it wasn't the fuel at least. I didn't test the coil I only inspected it for damage. Could a worn out coil cause the misfire when under load? I might be getting a second hand cdi to try soon too.
    Thanks for reading 👍

  18. I've also ordered a compression tester so will have one as and when I need one!

  19. Update on my bike, as things are turning up from eBay etc I'm trying parts things one stage at a time. Today I fitted a new ignition coil. The bike started as before but after a few seconds of running again it failed to rev up as before so this isn't the issue. Cdi and compression tester still not arrived. Will update as I go along

  20. Another update. After running the bike a little I thought I'd try turning the lights. The bike then revved freely up to the powerband. I took the bike for a quick spin and it ride ok. The powerband seems very aggressive when it kicks I as there is so little bottom end. The problem was obviously something electrical. The bike also cut out when I did a u turn to the right so I'm thinking a chaffed wire somewhere maybe?
    Will do a compression test as soon as my new toy (tester) arrives. I'm expecting to total lack of bottom end to be low compression maybe.
    Thanks for your help and I'll update if any changes. (In case anyone else encounters the same problem)

  21. Compression is about 90 psi. New piston time it is then!

    1. Yeah good idea , might be even barrel as well .

      at least you can set the bike up to a decent compression value.

  22. Well piston is ordered and in the post. Got myself a bore honing/glaze buster to give the bore a quick clean/deglaze. See how that ends up with the compression. All gaskets and little end bearing arrived just waiting on piston. My piston was the 53.99 one so no full top end kits available on eBay when I was looking. Cheers. Will update my results once fitted.

  23. Today I honed the bore and built up the engine. I took it for a ride where it was quite tight and reluctant to Rev into the powerband. After a few easy miles the motor started to free up a little. It will Rev up into the powerband ok. (Not revving it hard though) there is still a slight 'miss' just before the band kicks in. I know the top end is now new so will need some more miles. I did a quick compression test after the ride and it was showing 75 psi. Hopefully this is as the rings havnt bed in yet! I'll do another after the next ride. I
    Probably did about 15-20 miles.
    Thanks for all the help

  24. Need new cylinder but mine new piston was 70psi was new and I bought the 5400 one and compression was 120 should really get cylinder and bore matched by Pjme

  25. Hi when I turn the fuel tap on fuel starts to drip out of the two vents at each side of the carb what do I do

    1. you need to set the flat height inside your carb or the float valve maybe stuck open and not shutting fuel off .