Thread: New here and first build - CB250n

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  1. #201
    Without getting too technical,
    The air/mixture screw controls the fueling right at the bottom, just where you start to lift of tickover.
    The pilot jet covers up to about 1/4 throttle.
    The needle affects as stated above.
    The main jet from there upwards.
    With each system, if you have to start going to the extremes of adjustment, bring it back a bit and increase the next system up.
    EG, If the airscrew improves things but you are up to 3.5/4 turns, go back a bit and increase the pilot jet size.
    Needle as high as it will go ? Bring it back to the middle and increase the main jet.
    DO NOT raise the needle so high it clears the hole on full throttle. If it does it can cause the carb to jam open on full throttle and you'll be needing clean underwear ( if you're lucky ).
    The reason some bikes need the filters spacing back from the carbs ? There are pressure waves that go backwards and forwards in the inlet tract, if the length from valve to filter end is seriously wrong it screws it all up and can make jetting a nightmare.

  2. #202
    I'm not an expert on carbs but this bike got cv carbs, but I'm pretty sure that the biggest issue is that you probably not get the vacuum right to lift the trottles. I not 100% sure if I'm thinking right but if you shorten the way for the air and also open it up before it pass thru in to the carbs you got less vacuum to lift the trottles.

    If it was my bike I should try to put some spacers between the air filters and the carbs, look at the airbox rubbers in the stock airbox

    The rubbers will probably work as velocity stacks in the stock airbox and I think if you in some way could connect your pods to the stock airbox rubbers or made spacers with the same dimensions as the stock rubbers you got a better chance to get the vacuum right to lift the trottles and jetting the carbs.
    Last edited by busse; 5th-07-2017 at 04:54 AM.

  3. #203
    Thanks gents, I'll do some more investigation and dig out the original airbox spacers and give them a try tonight.

    I had the carbs off last night and gave them a quick once over. Everything is clean and stock apart from the new jets I put in (140's now).

    Well, I say stock. A while a go I bought this kit

    It never worked as I had problems with the spacers and the lack of a seal so ditched it. I forgot to remove the paper gasket so that went last night. It's back to stock now then and I'll take it out tonight. What I will say is that on starting last night it certainly idled better and revved freely...

    So then pod spacers and maybe a needle raise.

    Thanks again chaps.

  4. #204
    Did you try to run the bike with the airbox rubbers between the carbs and the pods? I'm curious about that if it's work well or if you still got problems to get the bike run properly?

  5. #205
    Pods over the end of the stock airbox rubbers can be very good, the Japanese tend to be rather good at knowing the right "tuned length" for their engines.

  6. #206
    I just read another interesting thing about cv carbs on Swedish forum when a guy says that cv carbs will run best with a "common" airbox to all carbs so they get the same vacuum in to all carbs, with separate filters could each carb rise the trottle to a different height because different vacuum and that will be balanced with a airbox for all carbs.

    This give me an idea, what about to take the original airbox rubbers and then make a small airbox between the rubbers and the pods to make a "vacuum balancer"? I think you perhaps must have a baffle in the small airbox to prevent the air to flow thru the pods "straight" in to each carb.

    What do you think of this idea, stupid?
    Last edited by busse; 16th-09-2017 at 06:03 PM.

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