Carb issue Superdream CB250n Cafe Racer
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Carb issue Superdream CB250n Cafe Racer

This is a discussion on Carb issue Superdream CB250n Cafe Racer within the Technical forums, part of the Caferacer.net Forums category; ok it's been a while since I have been on here but I am well and truly stumped and need to tap in to the ...

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Thread: Carb issue Superdream CB250n Cafe Racer

  1. #1
    Junior Member hallsie's Avatar
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    Carb issue Superdream CB250n Cafe Racer

    ok it's been a while since I have been on here but I am well and truly stumped and need to tap in to the wealth of knowledge that frequents the forum !
    Having finally finished my cafe racer build I am having issues with the fuelling on one side of the carbs ...I am getting a backfire through the intake on one side ..the carbs have been balanced , cleaned , rejetted ( for pods ) ..valves have been checked and checked again , timing is spot on , needles both shimmed but one side is persistently running like a pig , the floats are adjusted and all good and there are no air leaks also the coils and CDI have been replaced to no avail , the plugs are also new ...engine has been rebuilt and compression is very good ...once the bike is revved up it's nowhere near as bad but idling and low revs it's a bag of shite ...I'm out of ideas...any suggestions ...and no I'm not scrapping or setting fire to it ( just yet )
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  2. #2
    Senior Member TrialsRider's Avatar
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    ... is there a vacuum port to operate the carburetors on one intake
    or is there a vacuum control to each individual carburetor?

    If you read through almost any thread on this site you will find that the general consensus is, that vacuum controlled carburetors fitted directly to pod filters works bad enough that you should consider that as a non-functional setup. Your choices are to either replace the carburetors completely with carbs that do Not operate on vacuum to open the slides,
    or install fuel injection
    or put the original air system back on
    or devise a plenum or tubes or airbox that provides a reasonable non turbulent air flow leading into the carburetors.

    and here's the simple reason: Your bike was designed to run on as little fuel as possible and those lean operating carburetors did that, thus reducing pollution emissions and noise :| you know air pollution all that stuff that is slowly killing city people. You have aborted the majority but not all of the equipment intended to do that and you are trying to make the other 30% work. You need to go all in and make it a race bike or farm implement or whatever, or preserve more of the original engineering to make the engine work well with those particular carburetors.
    Last edited by TrialsRider; 06-17-2019 at 03:14 PM.
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  3. #3
    Senior Member TrialsRider's Avatar
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    Quote Originally Posted by hallsie View Post
    ...engine has been rebuilt and compression is very good ...
    Compression numbers were same and in spec? or is good compression a relative term.

    Twin cylinder engines with one lazy cylinder seem to run pretty good until you try and get them to idle.

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  5. #4
    Senior Member Teazer's Avatar
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    Take off the pod filters and see how badly they restrict the air ports in the carb mouth. It is highly likely that one restricts airflow more than the other. Another possibility is a partial blockage of one or more air passages in the carb.

    The trick I use is to spray WD40 or carb cleaner through one passage and then repeat on the other carb to compare flows. It's amazing how often one passgeway is partially clogged and that comparison shoudl reveal any huge discrepancies.

  6. #5
    Senior Member TrialsRider's Avatar
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    If a carb problem is in question on a multi carburetor motor is pretty easy to troubleshoot,
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  7. #6
    Junior Member hallsie's Avatar
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    Compression is identical on both cylinders , jets / needles etc have been designed by Sigma racing to run with pods and straight through pipes . there are no restrictions on the intake at the point the pods clamp on ...the issue is one side only and the carbs are not reversible

  8. #7
    Senior Member TrialsRider's Avatar
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    Quote Originally Posted by hallsie View Post
    ... the carbs are not reversible
    2 different model carbs on 1 bike, well that sucks.

    "jets / needles etc have been designed by Sigma racing to run with pods and straight through pipes"
    <- to me that is like saying; these guys track race these CV carbs and this is the best they found for mostly wide open throttle racing. Race guys don't spend a lot of time idling or sitting in traffic :/ that's why they are revving the sh*t out of their bikes on the start line some times, they don't want it to stall.

  9. #8
    Senior Member TrialsRider's Avatar
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    Air intake restriction is not the problem, the problem is you have messed up the vacuum, you are not getting the correct fuel to air mixes when you need them and the fuel is not atomizing effectively into the air flow.
    Put it this way: your current air flow is like sticking your head out the window of an airplane and trying to breath :| she don't work so good that way.

  10. #9
    Senior Member Geeto67's Avatar
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    Backfires through the intake are usually timing. Maybe the static (idle) timing is spot on, but your advance mechanism is sticking and not advancing? could also be that one of your pulse controllers is starting to go bad. After you check the pod filters, that's the next thing I would check.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
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  11. #10
    Senior Member TrialsRider's Avatar
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    What do the spark plugs look like?

    Bike has CDI conversion, yes? what kind?
    Battery conversion too?

    Geeto what powers the spark in these things, battery coil type? or other.
    Last edited by TrialsRider; 06-18-2019 at 01:22 PM.

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