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Thread: Just bought 70cc SR 50, tuning questions!!

  1. #1
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    Just bought 70cc SR 50, tuning questions!!

    SO bought the scooter a couple weeks ago, Its a 1999 aprilia sr50, It has malossi 70cc kit on it with 19mm carb 95main jet and stock exhaust. The bike runs like a bat out of hell, pretty impressed for a gas and go. the catch is the bike wouldn't idle very well and when taking off it would bog down to a stall (too much air) unless you rolled the throttle a bit or rode the bike with the choke half way on. Sometime while at red lights or stops i would have to put the choke on in order for it to idle and or take off. once dealing with take off the bike would run amazing and get to 63mph very fast, clean, and efficient, with no mid or high range problems...

    so me and a friend decided to start trying to tune it, we noticed the carb was pretty dirty and cleaned it very well, and raised the needle one click. after assembling the bike and going for a ride it right away runs like shit. It idles great now and takes off perfect with out the help of the choke but 1/4-1/2 throttle it spits and chokes bad (too much fuel) and will not speed up past 30mph even when full throttle and past the sputtering. almost feels and sounds as if it sucked down the air filter, or lost its MAF (if it had one, like the feeling when a cars goes out) So we put the needle back to where it was before and it still had bad sputtering and power loss, like it cant breath or get air. funny as it sounds i wish the carb was dirty again and hitting 65mph.

    What could of possibly happen? does the ecu need to be remapped? did we pinch a vacuum line or fuck up the ignition or electrical system? could it totally be the carb? will an aftermarket exhaust clean it up? maybe a 21mm carb? im so clueless and irritated that overnight this all happen?

    What do you all recommend or suggest before i take it to the shop and drop some cash? I want it to get back to 63-65mph fast clean efficient and be able to idle and take off fine just like other gas and goes!!! HELP! What are some of your setups?

  2. #2
    apriliaforum Member onu123's Avatar
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    Okay... first of alla there is no ECU to de-restrict.
    On a carbed one there is a CDI & a ignition coil.
    A performance exhaust would make it run faster and makes it breathe more freely, stock one takes away a lot of the 70cc's potential.

    Is this Malossi cylinder cast iron or made of aluminum (Cast iron would be black & alu would be light gray when looket at it....)?

    95 jet seems a bit big for a stock exhaust. If no smaller jet is available then drop the needle step-by-step till it's all the way down.

    There are also 2 screws on your carburetor one would be idle screw (turning in = higher engine speed; out = lower engine speed or wouldn't idle at all.)
    And second screw would be to adjust the idle air-fuel mixture.
    Always try to get the engine at good running temp. before tweaking the carb.

    Spark-plug could have fouled from running too strong mixture.

  3. #3
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    Thanks ONU123

    I believe its an aluminum kit (it is light gray)
    thats was i figured, that it had no ecu...
    I will definetly get an exhaust soon, any recommendation for exhaust?

    Yes plug could be fouled, but if it is fouled would the bike even start and run?
    I will get a new one anyway it could prob use it!

    Hopfully I can figure this thing out, im guessing most of it has to do with the carb...need to get an exhaust! and go from there...

    all suggestions and opionons welcome, Im a old school vespa guy trying to learn about this bike...

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    If the new carb is similiar design as the original:

    You might of put the needle/spring/clip back in the wrong order. It'll idle ok but when you open it up, the needle will float up or down uncontrollably. It'll sputter and won't accelerate past 35 mph or so.

    There is no "1/2 choke". The choke jet is either open or closed.

    Stock exhaust is DEFINITELY too restrictive for a 70cc kit.

    95 is too small for 70cc and 19mm I think. My old 50cc w/ Leo pipe ran spectacular with the 100 jet. It did an indicated 55+ mph

    Do it yourself, it's not that tough. I've torn mine apart several times. You can get some carb jets from Scuderia West. They'll need to dig through their little plastic boxes to find you one though if they've got it. Bring the old jet in...... but of course is after you fix the carb issue. The carb is real simple too. I'm in Daly City.

    Oh and you didn't pinch a vacuum line. If you didn't it more than likely would not run at all.

  5. #5
    apriliaforum Member onu123's Avatar
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    Okay. Since the cylinder is made of aluminum I can't be sure what it exactly is (BUT IT IS GOOD STUFF FOR SURE)...
    Chances are: Malossi MHR Replica, MHR Racing, MHR Team and some other high-end stuff =)

    When you get more info about your cylinder then we can talk exhausts =)

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    thanks all!! going to get an exhaust before i even mess with it again, then I will give it another go...hope i can dial this thing in and enjoy it!

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    you answered the question your self, too much fuel, do a plug chop and have a play with the jetting

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    Quote Originally Posted by 70RSMAT View Post
    you answered the question your self, too much fuel, do a plug chop and have a play with the jetting
    just seems weird it happen so drastically from cleaning the carb? Yes going to do a plug chop and get a performance exhaust and go from there...

  9. #9
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    Quote Originally Posted by onu123 View Post
    Okay. Since the cylinder is made of aluminum I can't be sure what it exactly is (BUT IT IS GOOD STUFF FOR SURE)...
    Chances are: Malossi MHR Replica, MHR Racing, MHR Team and some other high-end stuff =)

    When you get more info about your cylinder then we can talk exhausts =)
    MALOSSI MHR REPLICA!!! thats what i have..

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    also looking to take off stock airbox and upgrading as well...

    what do you all recommend? its such a tight fit what the hell can fit there?

  11. #11
    apriliaforum Member onu123's Avatar
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    Yeah the engine you have (Minarelli horizontal) has the carburetor & air-filter in a really annoying place, so on many occasions to fit a new filter, the plastics should be cut.
    I Think Malossi E5 (prefer the closed one) Cone-filter would fit under there.
    I'm not sure how limited is your budget, but if you slap on a Yasuni C16 exhaust it'll do more than 110km/h.
    Carburetor is big enough as it is, just need some fine-tuning.
    With a new exhaust you'll also need your variator to be re-adjusted.

  12. #12
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    Cool, someone in a similar situation as I. 2 weeks ago I bought a seized up 99 sr50 LC. with vm20 carb, 70cc, leo vince rr. Funny thing is it came with an 80 jet. Seems like it would be theoretically impossible for it to run with a little jet like that. Once I replaced the crank the bike wouldn't run with that jet.

    I have an 85 jet now and it runs sporadically. Its got 2 speeds very slow and very fast. Its hard to start and on acceleration It stutters, backfires then the engine catches and vroom I am gone... then i hit 30mph and the bike looses power as if the fuel was cut. The richer I make it the better it seems to run. What I think happened is that after you cleaned the carb you maybe made it too rich since dirt doesn't impede the fuel anymore. I get my jets from niche cycle supply, brand new, cheap fast delivery. Also, upgrade your pilot jet a few sizes. The poor pilot jet often goes ignored when tuning.

    As for your other problem my sr50 did not come with a filter and I had to make one. I made my own 90 degree free flow air filter but you can find them cheap on ebay.

    Since we have very similar scoots I think we may be able to help each other a lot in the future.

  13. #13
    apriliaforum prov-nov sgw's Avatar
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    Hi, I have just installed a Malossi MHR replica kit on my 2010 SR50, I'm still only running the stock carburettor with a Gianelli extra exhaust.
    I've done all the work myself, when tuning the carby, I started with a 90 main jet and worked back from there.
    Final settings are 68 main jet, needle set at 3 notches down, standard idle jet and it runs fine. Bike is still accelerating past the 80kph indicated on speedo. Plug seems still a little rich, but I think this will improve with a better pipe.

    By the way I'm still running the standard air cleaner and filter.

    Quote:KooLaid
    "95 is too small for 70cc and 19mm I think. My old 50cc w/ Leo pipe ran spectacular with the 100 jet. It did an indicated 55+ mph:"

    If he is running the standard exhaust and air cleaner a 95 main jet will be to far to big.

  14. #14
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    hey guys! thanks for all the imput and help.

    Going to tinker with it with one of the local guy here that has one as well...hopefully we can figure it out? I will keep you all posted

    Lots of people telling me to go with leo vince tt (big bore) exhaust
    prob will just stay with stock airbox unless i find somthing?

    not looking to do 100MPH but would love to have it reliable and have full use of the parts I have (70cc kit). and for everything to make sense!

  15. #15
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    okay..

    so had a local come over that has the same bike to help out a bit. We did a lot today...
    Found out its a 21mm carb with a 95 main jet (weird looking), clean choke jet, 60 pilot jet, and everything clean and working order.
    We tried needle all the way up with air box on, air box off, air screw all the way in all the way out.
    tried needle all the way down with air box on, with air box off, air screw all the way in all the way out...ETC ETC
    the idle screw pretty much never moved its at about 3 turns out? and the bike never really changed in performance....

    To some this all up we can not get it to run like it used too (shitty idle, and only taking off with choke on, but doing a strong 58-62mph)
    the best it ran today was with needle all the way down, air box on, and air screw all the way in. it did about 40mph but mid range sucked, boggy, spitty, then close to top range it would open and clear but not surpass 40...

    Plug is dry, with a grayish white tint almost turning carmel.
    The needle is very aggressive almost to a point.

    Whats next? bigger main jet? bigger pilot jet? less aggressive needle? keep in mind with a stock exhaust and a dirty carb it did 62mph clean with a bad idle and take off so why now that we clean the carb is it acting up?
    After one more day of trouble shooting I will have to give in and pay whatever it takes at the shop to have it sorted out, im out of time and ideas, getting very down and aggravated!! Could it be timing/electrical/CDI??
    What are the setups out there with a 70cc kit, stock exhaust, and 21mm carb? and suggestions before i take this thing to the shop or junkyard?

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