Issue - fuel or jetting or other

Discussion in 'The Workshop' started by Buzz, Jul 8, 2018.

  1. Buzz

    Buzz Active Member

    Messages:
    446
    gilera dna50 was running okay but generally slow. I have fitted a 70cc kit, lowered the rollers from 5 to 4g which raised the rpm nicely to the 9500rpm range. I upjetted from an already slightly rich 65 to 75 main jet. Stock is 56.
    Bike has stock airbox and standard aftermarket exhaust from when it was a 50cc.
    Bike fired, ran and we went for a nice 20mile round trip. It rode well with no stutters or bogging. At around 10 miles we refuelled . After sitting for 10minutes the bike started and idled but then struggled to reach the rpms and wouldn’t pull away. It slowly crept up and the 10mile ride back had no issues.
    The next day it fired up and idled but idled slightly lumpy. It would not go over 5000rpm and struggled to get away. It again crept up and slowly reached 8000rpm, flew along but every junction struggled until we gave up.
    Now if I start from cold it idled and runs well for about one minute then cuts as though fuel is low, revs drop and it can only be nursed along at 5mph and 5000rpm.
    It doesn’t improve unless I wait for a number of hours then does the same.
    I’ve tried main jets between 65 and 85 with no change.
    Fuel pump working well.
    Changed carb out for a different 17.5mm dellorto, no change.
    Raised needle one slot, no change.
    It has an auto choke which works fine.
    It’s feels as though it’s running out of fuel but the float is not adjustable and pump works, it was great on day 1.
    Any jetting advice or ideas welcome
     
  2. Stevep

    Stevep Well-Known Member

    Messages:
    5,895
    drop the jet sizes one at a time, sounds like its getting too much fuel now, especially if you think it was rich to start with.
    Just keep an eye on the colour of the plug so not to run it lean.
     
  3. Buzz

    Buzz Active Member

    Messages:
    446
    Plugs the colour of grey primer which is one step away from tan I think.
    I'm thinking it may be a fuel pump issue now. The gilera has a vacuum tap which runs to a vacuum pump to feed the carb. I found fuel in the carb vacuum line so removed the fuel line from the vacuum tap and fuel poured out. I'm sure when the fuel line is removed then the tap should be sealed? Until you run the engine? I'm thinking the leaking fuel tap is backwashing into the vacuum line and causing issues.
    Any thoughts?
     
    Last edited: Jul 9, 2018

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