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Thread: RS50 carb advice please

  1. #1
    apriliaforum Member
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    RS50 carb advice please

    Cant get RS50 it to run right


    Vid of the bike not quite running right
    https://youtu.be/2WSw38NGW28




    So Befor you tell me its smoking a lot it will do it’s the first time it has been started its stone cold and I just put some oil in a bottle and added fuel so its running very rich on oil, better safe then sorry on its first start up .
    Now that said when its on choke it will rev ok and will rev for a little while better once its wormed up but will die after a min or so when you try to ride it . I got down the road and it just died started it again and got a back up the road revving it a lot and keeping in it 1st and 2nd gear only .
    If you push the choke in wit will not rev at all just few thou and pull back and it bogs down . It will not ride at all with the choke in not down the road not even down the drive just dies as you let the clutch out .


    I did think could have been oiling the plugs so I toped up the fuel header im using to test it that up under the tank and it’s the same . Thought it may be to rich the jet in the carb was a 110.


    I had a look at the carbs we got with the bike and they are all fitted with 70 and the black one had a 75 in so I fitted the 75 jet and its just the same so its not flooding it up .


    The problem with the other carbs is . First I don’t know what they are for what size they are and if they are good enough for the 70cc kit on the bike . I was told some don’t work some do but the one that does needs a cable choke and the other chokes don’t fit it right . And I moded the cable to run the Rd80LC carb


    Its so far finished now he really wants to get out on it Everything on the bike is mint and the motor sounds sweet . if I have to get a carb for it and a new cable I will but don’t know what to get and it should run with this by rights . May be im over looking something hence this post .
    Last edited by vailon; 07-03-2019 at 06:18 AM.

  2. #2
    apriliaforum Member
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    These are the carbs we got with the bike as well as a very small standard one

    DSC02950 by two stroke Parts, on Flickr

    DSC02949 by two stroke Parts, on Flickr

  3. #3
    apriliaforum Member aphasia's Avatar
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    If it runs better with the choke on, this would suggest main jet may be too small, leading to not enough fuel in the mixture and similar symptoms to what you look to be experiencing.

    Main jet size is dependant on a few things though.

    What type of air filter is being used? Stock air box or cone type?

    What size idle jet is fitted in the carb?

    Atomiser, needle type and notch position will have an impact, but noticeably on the transition from idle to main jet.

    A good size carb for a 70cc would be at least a 19mm but probably better suited to something like a 21mm.



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  4. #4
    apriliaforum Member aphasia's Avatar
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    The bike looks really good in the walk round video you posted too.

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  5. #5
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    Thanks glad you like the look of it .
    It did have a 110 jet in and thats big for this bike from what i found in the big bore carbs so i put the smaller 75 in from one of the above carbs in the silver ones only have 70 fitted but it was just the same thought it was over fueling. Dont have any bigger as they are a half size jet unlike the rest of my Mix hex jets. Going to remove the filter today and try it again . The carb fitted has a duel density phome filter fitter thing you can see it on the rebuild thread that i just up dated .

  6. #6
    apriliaforum Member aphasia's Avatar
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    Looked through your project thread before I posted but couldn't see the air filter. Was using my phone though so probably just missed it.

    I ran around a 110 main in my 17.5mm carb with stock air box on a 50cc so it is feasible. That said the idle jet was only a 32 or 34.

    Sure it's the idle and the main that will combine to give your overall fuel ratio. So if the idle jet is big then a smaller main would make sense.

    The 19mm carb I have at the moment had a large idle and smallish main off the shelf but I couldn't get the mixture dialed in on the idle so had to reduce it. As a consequence of reducing the idle I then had to increase the main to get it to rev the full range at a wide open throttle.

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  7. #7
    apriliaforum Member aphasia's Avatar
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    The black one at the top in your picture looks pretty similar to the dellorto 19mm I use at the moment on a stock 50cc cylinder.

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