largest carb for a chinese 80cc bike kit

Discussion in 'Performance Mods' started by dondupuis, Mar 20, 2015.

  1. dondupuis

    dondupuis Member

    Hello. all
    I was wondering what's the largest size carb anyone knows of being installed on these bike kits. I just finnished installing a 42mm Keihin carb on mine and seems to run as well as it did with the stock carb. now it's time to try tune it for top speed. I trilled out the main jet in my stock carb to 1.5mm lol guess what pretty much had to hold a truck box till i was up to speed to match the fuel.. but anyway i figured if i want more fuel ill just install a larger carb. anyway point is will this tune to run right or wasting time.?

    Thanks

    Carb is now install with the help some of hoses and clamps
    FHD0121.jpg
     
    Last edited: Mar 23, 2015

  2. dondupuis

    dondupuis Member

    After some more googleing i found this carb is used for 50 - 110cc 2/4 stroke engines. looks like i have some tuning to do.. :)
     
  3. jaguar

    jaguar Well-Known Member

    it's not just the engine size that counts but also the max RPM.
    20mm is the biggest useful size for your size engine maxing out at 7000 rpm.
    anything more than that offers no advantages.
    but the Keihin is a fully adjustable carb, unlike the crummy stock carb.
    have fun!
     
  4. dondupuis

    dondupuis Member

    cool. i had this one already. thought i would give it a go. so much better than the old one. at 1/4 throttle 2700rpm i now cruise at my old top speed @ 5600rpm. i thought i was having jetting issues as it only seemed to be lacking at the top end. i have expansion chamber from a pocket bike modded to fit. but have not done any porting yet. should I? I am currently running rich it seems as its very boggy. what im wondering is while im cruising alone and it's bogging every once in a while it will start to sound like a 2 stroke should and accelerate some but then back to the bogging. it's weird cause to me it acts like it wants more fuel.

    any thought and suggestions would be great..
     
  5. butre

    butre Well-Known Member

    I like using 19mm carbs on mild engines, highly modified gets 26mm. any bigger doesn't do you any good
     
  6. dondupuis

    dondupuis Member

    i only went with the one i have cause there website said 50 - 110cc use so though has to be better than what it came with.

    also looking around online for some info about modding the CDI and coil from the 110 to work.. not so sure how to connect it up..

    my compression is at 120psi should i bump it up?
     
  7. jaguar

    jaguar Well-Known Member

    I don't recommend any more than 135.
    the chrome plating on these cylinders is weak. more compression means more outward force of the rings on the cylinder, leading to flaking off of the chrome.
     
  8. dondupuis

    dondupuis Member

    if i take a mm of the head would that be to much. is there a rough average as to how much compression 1mm would add. ? I seen another post with the fella that has a very large expansion chamber installed. love that.. looking at some of my sleds to see what one is going to donat a pipe.. lol im just working on uploading a video. i will post soon. maby you can recommend some things..
     
  9. dondupuis

    dondupuis Member

    if i take a mm of the head would that be to much. is there a rough average as to how much compression 1mm would add. ? I seen another post with the fella that has a very large expansion chamber installed. love that.. looking at some of my sleds to see what one is going to donat a pipe.. lol im just working on uploading a video. i will post soon. maby you can recommend some things..

    Do you think this CDI and coil will work with these engines?
    FHD0001.jpg
     
  10. dondupuis

    dondupuis Member

    Here i have added photos of spark plug,head and piston. things don't look to bad. this kits is just coming on my second year with it. it's my sole means of transportation.

    FHD0002.jpg
    FHD0003.jpg
    FHD0004.jpg
     
  11. dondupuis

    dondupuis Member

    Well compression is now 136psi. now retuning the carb, had alot more smoke now that i did not before. infact as rich as i ran before i never seen some come from both tips of my exhaust.. i opened up the intake gasket somemore to better match the head it's no longer blocking the intake.. i am still getting alittle air in through intake tube and carb working on a way to seal it up. I think my exspansion chamber is still a restriction as it's from a 49cc pocket bike and is on the small size maby 2" at its largest point. and looks like about 3/4" tubing..
     
  12. dondupuis

    dondupuis Member

    current leak locations.
    between intake tube and carb.
    no exhaust gasket so leaking at head and exhaust..

    i think i'm going to need to make a new intake tube for this carb. i will probably extend it some to clear the down pipe from the seat i don't have room for the air filter to fit otherwise..
     
  13. dondupuis

    dondupuis Member

    Well finally got a video to upload.. can skip to about 3:33 where i get out on the road.
    @ 4:42 i reliezed my fuel was off. and sorry i couldn't cut the video it kinda ends about 12:30

    Would you guys say it sounds to rich or lean? in this video i believe the needle clip is 4th from bottom..
    it's been moved up 2nd from top.



    On the way home i change the needle setting a little bit leaner..
     
    Last edited by a moderator: Dec 15, 2015
  14. jaguar

    jaguar Well-Known Member

    the needle has no effect at wide open throttle settings, only the main jet does.
     
  15. KCvale

    KCvale Motorized Bicycle Vendor

    Your expansion chamber could very well be doing more harm than good.
    They need to be timed so the shock wave pulse hits the clean new fuel mix coming into the exhaust and shove it back in the cylinder just before the port closes.

    [​IMG]
     
  16. dondupuis

    dondupuis Member

    cool. thanks for the info. i was starting to think that after seeing what some others have installed. however it has improved the lower RPM range i just can't seem to get WOT
     
  17. gary55

    gary55 Active Member

    A mm is to much for the slant plug head. In my experimenting I found by adding a second head gasket of .027 compression dropped 30 lbs.. So that's about 1.1 lb. per .001. I remove the material from the cylinder, and with a slant plug head I make the cylinders 2.730 top to bottom.
     
  18. jaguar

    jaguar Well-Known Member

    there's too much variance between different engines so you first need to get a pressure gauge to see what it has now.
    I think slant plug heads already cause enough compression on most engines.
    If your setup has under 130psi then just remove a little metal at a time and keep rechecking the pressure.
     
    gary55 likes this.
  19. gary55

    gary55 Active Member

    I really gota start looking at the dates on these posts or invest in a time machine.
     
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