Speedfight carb jetting

Discussion in 'The Workshop' started by Paul Smith, Apr 13, 2021.

  1. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Hi

    I have a 1997 LC speedfight with a malossi/polini 70cc kit with stage 6 exhaust and 17.5mm carb.

    I am having trouble getting the carb jet right, it used to be awesome and pretty much still is but at some point I clearly thought I was a better mechanic than baines racing at Silverstone and changed something on the carb but hey ho.

    It runs a standard air box opened out slightly to fit the carb and always has done so that's ok.

    What's she doing.

    So when warm she runs for a mile ish then you start to lose power until she finally bogged down so I held the manual choke open and she ran fine on half throttle to get me to where I needed to go. Thinking like she was starved of fuel I checked the jet and it had a 75 in there (not sure what was originally there years ago). Read some bits on line and decided a larger jet was needed so up it went to an 88 which warmed and idled fine but a mile down the road at lights she wouldn't rev up and bogged down. I pulled the manual choke which helped me get away from the lights but I let it go and the bike died and wouldn't re start.
    My thinking is flooded? What are your views.
    I'm kind of thinking the standard box isn't letting enough air in at full throttle thus hesitating and dropping power, so thinking I maybe needed to go down in jets?

    Any ideas welcome :)

    Thank you
     
  2. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    Strip and clean the carb.
    Put the 75 back in.
    Clean or replace the air filter and fuel filter
     
  3. Paul Smith

    Paul Smith New Member

    Messages:
    20
    What is the theory behind re trying the 75 jet? Just so I understand. Air and fuel filter change I get :)
     
  4. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    You say in your original post it ran awesome before you started messing with the jets.
    So the theory is you clean the carb thoroughly, go back to the main jet it had when it was awesome, change the easy stuff first and start from there.
    By using the manual choke while it's warm it's makes it rich in fuel/weak in air, which says the ratio of each is out of whack.
     
  5. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Cool the problem is the 75 jet is what was in there this time but I'm not sure if it was the jet that had it running really well. Been a while since I've tinkered with her lol.

    If the choke makes her rich in fuel and restricts the air would it suggest I need to go bigger than the 88 as she would need yet more fuel?
     
  6. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Stripped the carb and fuel wasn't entering to fill the chamber, it seemed blocked so put some pressured wd40 through and then it was unblocked. Result.

    No fuel from the tank so airline up the fuel valve seemed to do the trick, so guessing it's was stuck closed hence the stalling issue. Trying to find a new one to replace it so hopefully I'll find one and I'll have no more worries. Then I can focus on carb settings again :)
     
  7. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    Oh hell no..................
    Airline up the fuel valve?????
    Dude.......it's a vacuum fuel tap, a one way system.
    Suction from the intake manifold opens the diaphram and allows fuel through, no suction (i.e. engine off) closes the diaphram and stops the carb overflowing with fuel. Before you ask about the carb float, don't rely on it stopping the entire fuel flow completely.
    You may have created a free flowing fuel tap with the airline jobbie now.......
    The fuel tap is pretty standard about £6/£8 from various places.....Pedparts/Racing PLanet/Petrol Scooter etc

    The easy check for tap operation is to place the fuel line into a jar and suck on the vac pipe.
    When you suck fuel flows, when you stop, fuel stops, not instantly but the line will empty.
     
  8. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Hi

    Your right, I did try sucking but with no success so figured I'd probably run the risk but it must have been faulty so wasn't worried if it did create a constant flow. To be fair it's lasted 24 years so done well. I've ordered a new fuel tap/valve so will swap it out.

    Might look to swap the carb out but struggling to find a like for like replacement
     
  9. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    The main thing with the carb is getting one with a float bowl that doesn't bounce off the belt cover.
    The Naraku Black Edition 17.5mm is a good replacement, but get the button or lever choke to go with it.
    There's plenty of room to flick it on and off on the speedfight's.
    I've got a Dellorto 17.5 with manual choke on my 100cc speedy, easy access to the carb choke and easy for jetting purposes
     
  10. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Are there any decent 19mm carbs out there with manual chokes that'd be suitable?
     
  11. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    Again, the Naraku Black Edition. They take 5mm threaded main jets, dellorto type.
     
  12. Paul Smith

    Paul Smith New Member

    Messages:
    20
    My carb doesn't have these circled bits yet all the carbs I'm looking at seem to have them. What are they for?
     

    Attached Files:

  13. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Struggling to find a reasonable phbg19 AS to replace mine with so will try stripping it and running it through my ultra sonic cleaner...fingers crossed
     
  14. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    They are just overflow pipes and balancers. As well as letting fuel out the carb if the float sticks they also balance the fuel/air pressure inside the carb........don't block them off.
     
  15. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    https://www.pedparts.co.uk/product/6351/naraku-black-edition-racing-carburettors

    You will also need the manual choke with either the knob or lever. There are two different sized thread types, you need the smaller threaded one if I remember rightly. Racing Planet have them in stock.
    They take 5mm thread Dellorto type main jets.

    I found the 17.5mm carb easy to jet, the 19mm a bit more finnicky, the 21mm an absolute bitch and it kept overflowing.
    I still have the 21mm and the cow still overflows on me........

    Personally, I would run with the 17.5mm carb. properly set up it'll fly
     
  16. Paul Smith

    Paul Smith New Member

    Messages:
    20
    So using this type of carb would be a no no or I can use it but leave those 2 pipe bits open?

    I have a manual choke so that shouldn't be a problem
     
  17. Stevep

    Stevep Well-Known Member

    Messages:
    5,635
    You can use it
     
  18. Paul Smith

    Paul Smith New Member

    Messages:
    20
    As it's the weekend and if I order a new carb it'll take a few days I figured I'd strip mine and run it through the ultra sonic cleaner
     

    Attached Files:

  19. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Cleaned it all up but won't start . Stripped the inlet to the reed and lots of fuel there so guess the petals not working correctly so ordered a new one and go from there.
     
  20. Paul Smith

    Paul Smith New Member

    Messages:
    20
    Turns out I did a piston
     

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