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Thread: RS 125 wont start

  1. #1
    apriliaforum Member
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    RS 125 wont start

    Iíve posted this on the 125cc sports bike site but am repeating it here in hopes of getting some helpful suggestions for me to pursue. So here it is:

    I could use some new thinking on what's keeping my 2009 RS125 (USA version) from starting. I think I've kind of covered the usual suspects. I've seen the spark on the plug that's installed by grounding on the frame. I've witnessed a spurt of fuel coming out of the new vacuum pet c*ck fuel outlet after removing the outlet tube and pressing the start button very briefly catching the flow on a rag. The bike has sat for about a year in my barn so I pumped out the tank and put fresh fuel in. I found a substantial mouse nest in the air filter and cleared it out. My next suspect is old fuel still in the float chamber or a stuck float valve not filling the chamber. If I could easily remove that nut on the Botton of the carb and drain the chamber I would. It looks like I might be able to get to it if I remove the tank and the air filter assembly but I'm not sure. I'm puzzled at the three nipples on the carburetor that are described as fuel bleed-drains but have no pipes attached and it's not clear to me what they're for. It's curious to note that during my struggle to get it to start again it did start at one point and ran for about 15 seconds. I hadn't done anything that might explain that one-time occurrence that I've been unable to repeat or explain .

    I could use some new ideas short of removing the carburetor or at least some urging to do just that. The bike has less than 1000 miles and here in Massachusetts with the nicest riding days still ahead before winter I'd sure like to get it running again.

  2. #2
    apriliaforum Member asechrest's Avatar
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    In my opinion, a carb clean is always in order after a substantial length of time sitting.
    2002 Aprilia RS50 - RMV125 in progress
    2002 Derbi GPR 50 - EBE075 Engine

  3. #3
    apriliaforum Member
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    Thanks, I do need some urging.

  4. #4
    apriliaforum Junkie chadzilla1's Avatar
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    want to sell it? I have a RS50 and Derbi GP1 Im looking to sell and buy a RS125 to take to the track
    1999 RS 50


    Stock engine, stock pipe, giannelli can


    Dellorto PHBG 17mm AS carburetor
    atomizer = 262 AU
    main jet = 86
    idle jet = 52
    starter jet = 60
    float = gr .4
    needle = W7
    slide = 40

  5. #5
    apriliaforum Member
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    Thanks for asking but I’ve always been better at buying than selling.

  6. #6
    apriliaforum Junkie chadzilla1's Avatar
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    If we dont ask we miss opportunities :-)

    Quote Originally Posted by Moran View Post
    Thanks for asking but I’ve always been better at buying than selling.
    1999 RS 50


    Stock engine, stock pipe, giannelli can


    Dellorto PHBG 17mm AS carburetor
    atomizer = 262 AU
    main jet = 86
    idle jet = 52
    starter jet = 60
    float = gr .4
    needle = W7
    slide = 40

  7. #7
    apriliaforum Member
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    It finally started after overhauling the carburetor. I didn't come across anything that would explain the non-starting except perhaps the strain on the vacuum hose to the petcock which could've lost vacuum and prevented fuel flow. Now I have to get the right combination of settings for idle speed and the fuel mixture screw. I happen to have a right angle remote screwdriver that can reach the mixture screw but could use some advice as to a preliminary setting. I've forgotten where it was when I removed it for cleaning.

  8. #8
    apriliaforum Member Kevlar's Avatar
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    OK, don't laugh, I'm trying to help. Does the owner's handbook or EPA tune-up sticker provide any carb setting information, such as "two-and-a-half turns out from stop position?" The mixture screw will be the hardest to guess at. The idle screw--just turn it in until the bike starts. If it's too high, just back it off. Once you get it running, turn the mixture screw in (lean) & out (rich) to get the smoothest idle at the specified RPM. Just guessing, but could be around 1250 RPM.
    The Patriarch of the SuperNaked: '05 TF

    Aprilias formerly owned: '02 ETV 1000 Caponord, '00 650 Pegaso Cube, '04 RST 1000 Futura, '99 RS 50 (70)

    Too many other bikes to list!

  9. #9
    apriliaforum Member
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    I'm not laughing and 1250 rpm is a great guess as it's exactly what the manual calls for. As long as I use the "cold start lever" I have no trouble starting it and with that lever partly off, keeping it running. However, after warming it up and blipping the throttle earnestly, as I inch the cold start lever upward to close it the engine dies. I can keep it running by finding a sweet spot of the cold start lever but I think I may be just compensating for an air leak somewhere. I saw nothing revealing in the carburetor overhaul as I cleaned it, replaced o-rings, reset the float level, etc. As I said above, it wouldn't start at all before my carburetor efforts. I think the problem was the strain in the vacuum tubing between the petcock and the carb (limiting vacuum and fuel) which I intend to secure even more with slightly longer tubing. The only spec. for the carb in the manual, besides the idle speed, is a call for a 2-3 mm "clearance" for the start lever and instructions on how to adjust it. If I understand clearance to mean play my carb's play is a bit greater but that would only effecting starting not running with the start lever off. I currently have the idle speed adjuster about one turn out and the mixture screw about three turns out. Since it dies with the start lever off but runs with it partially on I conclude the run mixture is too lean either from an air leak or something else. I've made no needle adjustments. I'd do anything to avoid pulling that carburetor out again so any advice is very welcome.

  10. #10
    apriliaforum newb
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    Sounds like your pilot jet may be running lean. Pull your pilot jet and inspect it for any blockage and correct hole size. You may see light passing through it, but that doesn't guarantee it's to size. Could be gunked up from sitting. You're best off if you can get some wire or drills to determine the size and confirm it matches that actual size.

  11. #11
    apriliaforum Member
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    You're likely correct. I've been avoiding pulling the carb again. Maybe the second time is easier. I did spray carburetor cleaner through each jet and blew low pressure air through them as part of my overhaul but "gunk" can be very resistant. I tried spraying "starting fluid" at likely air leak places while it was running on partial cold start lever but didn't notice any change in engine speed. I've tried about every combination of idle speed and mixture screw possible. I can almost get it to idle until I close the start lever the last few millimeters but then it dies. Thanks for helping me to face the most likely culprit.

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