Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

bigger carbs

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by sunite, Jun 28, 2005.

  1. how and why do bigger carbs help performance???

    me and old stew had a ili arguement, and came up not knowing swappin a carb helps, if it does or doesnt:S



    my carb atm is a kiehlim (cant spell) and said to swap to a bigger carb:S

    wellz just need help why it does help and if so, what should i swap her to

    thanx
     
     Top
  2. only if the size of the carbs are a restriction to the intake, I would think. If you make them too big, you'll lose precise throttle control.
     
     Top
  3. Bigger carbs help performance by flowing more gas/petrol mixture into the combustion chamber. More gas=more go.
    However, like most modifications, it's best not done in isolation. If you change carbs, ensure that your exhaust system is capable of flowing the extra gases OUT of the combustion chamber too.
    Can't advise you on what carbs to use, talk to a wrench who knows.
     
     Top
  4. I was under the impression that you had to select a carby based on an engines volumetric effieciency or something. Think i remember reading that in street machine... :)

    Although i caould be totally confused just the same...
     
     Top
  5. Of course you are right, Mik, but, as most bikes are fitted with carbs that strangle the engine in a bid to meet EPA requirements, they can accommodate larger carbs without difficulty. Fitting larger carbs can usually be a good way of getting better performance. But you're right, there is a limit too. That's why I suggested contacting a good mechanic.
     
     Top
  6. Yeah sweet. Id ask around at some good reputable places maybe that have racing experience or something. Cause i know in the car world that when talking carbys 90% of people seem to just say "throw a 600 holley on it" before you even explain what type of engine you have :roll:
     
     Top
  7. You may get away cheaper with a change of needles and jets. Like rc36 said the standard ones will be set lean to meet EPA requirements, a good dyno op. will be able to free up some power without changing the whole carb.

    Q. How fast you want to go?
    A. How much money you got?
     
     Top
  8. Oh, Mothra that is SOOOOO true.
     
     Top
  9. I've got a CB250, a parallel twin that runs on a single carb. The bike used to be made with twin carbs one for each cylinder but they changed it to one that had a smaller volume/flow rate than the other two combined for the sake of efficiency. Been wondering whether I can find someone to convert it back to two to give it that extra little something...:LOL:
     
     Top
  10. I'd reckon you would be able to.
    Talk to your local wrench.
     
     Top
  11. I really dont think power modding a CB250 is worth the effort, when you could probably sell your bike and buy an RGV250 for a similar price...
     
     Top
  12. or drop it off a cliff and watch the flames, then go home and buy and RGV :LOL:

    jokes jokes :wink: but i do kinda agree with androo, it wouldn't really be worth modding a CB250 for power. i reckon your money would be MUCH better spent on the upgrade you'll be doing soon :D
     
     Top
  13. Nodz, the best way to get max power out of a cb250 is to keep it properly maintained and leave it alone. Honda use the single carb these days because it they reckon it works better. There's not much proffit in making your newer models run worse thatn the older ones. Anymore fuel going into the cylinder has to go somewhere and unfortunately the standard pipes are not up to the job. If you want an R1 go and by one, it's cheaper than making a bike do a job it wasn't designed for.

    If the bike feels sluggish, give a service, sme new plugs and an air filter. You'd be surprised at the difference. :)
     
     Top
  14. or even better, sell your CB250 and buy a fully sik GSXR1100 off Coconuts :D
     
     Top
  15. fooooooly sik :D

    i'll chuck in a free set of pink fluffy dice too :LOL:
     
     Top
  16. Actually the best way for me to get better performance out of the CB250 would be for it to lose 20kg, namely off my arse...:LOL:
     
     Top
  17. meh wellz thanx:) learned something
     
     Top
  18. About carbys or Nodz's arse?? :? :LOL:
     
     Top
  19. It seems we have 2 threads here now.

    to answere Sunite:

    It depends on the bike. Take my bike for example. It has 4/36mm carbs on it. The slightly later 750s had 4/40mm carbs. So my bike probably would benifit but the later 750's would probably loose so much bottom end the minute gain in the top wouldn't make a difference.

    Also consider the type of carbie. for a given size a CV type carbie doesn't flow as well as a flat slide. To to use another example, a xr650l runs a 42.5mm CV. An excepted upgrade would be a 40mm flat slide. It is apparently both more responsive and produces more power.

    To answer Nods:

    Remeber a single cylinder only uses a carbie for about 280 degrees out of 720. So on a 360 degree twin you can easily get away with the same size single carbie as a twin

    It's only when you get overlap, such as a triple, a v-twin or a 180 degree twin that you need either a bigger carbie or a second carbie.

    In fact there is a good augument for doing the single carbie thing with CV carbies rather then one per cylinder, as the carbie sees a more constant vacuum. This something that seems to have been forgotten in the past 20 years.

    A good mod for the CB may be a large single carbie of a trail bike. Making the manifold may however be a bit of hassle.
     
     Top