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Discussion FCR28mm carbs on 4cyl 250cc or 400cc dyno

Discussion in 'Tech Tips' started by daoust, May 24, 2017.

  1. daoust

    daoust Active Member

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    So I m searching dyno of bike 4cyl in the 250cc or 400cc with fcr 28mm carburator.

    I have a set of fcr 28mm that was designed for xjr400 that I m trying on a stock gsf400. I want to see the hp that people have achieve with those small carb. It will help to figure out the limit of those 28mm.

    Thank you
     
  2. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    I may be mis-remembering, so keep that in mind as I don't want to send you off on a wild goose chase, on the net there may be a dyno chart for a CBR250rr with FCR carbs

    I've done a quick google search without success, but didn't look at lots of hits

    Otherwise I have seen figures of a 10% increase suggested because better airflow - I don't however consider these figures to be realistic without proof
     
  3. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    I read you shouldn't replace CV carbs with the same size FCR/flatslides e.g. If the standard bike runs 30mm CV's then 28mm FCR's should be equivalent. I too want to go to FCR's at some stage with a pressurised airbox (force fed), but that's a way off. I think @ZXR Kawasaki has run or does run FCR carbs on his ZXR250. He's got lots of Dyno data that I'm sure he'd be happy to share with us about the pros and cons of FCRs.
     
  4. gregt

    gregt Well-Known Member Premium Member

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    28's are not a small carb for a 250 four. I wouldn't waste time putting them on a stock motor, better cams and a good pipe are required to get the best from this modification.
    On the other hand they are a small carb for a 400. A full house 400 will usually use 33mm FCR's.
    With stock cams and standard pipe you may pick up a little in the midrange on a 400. I'd seriously doubt if you'd get any more right up top as the cams and pipe will be the limiting factors.
     
  5. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    Greg, the ZXR250RR came with 32mm CV carbs (as opposed to the 30mm on the ZXR250). It's apparently the pick of the ZXR250 range, so I guess the engine can cope with the extra air these are bringing into the chamber. There is a slight change in gear ratios and a different CDI, but I believe that's it.

    RE your comment about cams and pipe, where exactly would one find said hot cams for a ZXR250? Would a programmable CDI have the same affect without needing to change the cams? Just asking.
     
  6. daoust

    daoust Active Member

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    Me and my girlfriend have 2 gsf400 and the one of my girlfriend have trouble with is carb, so I take my bst33 and put them on her bike by the same time I have 2 set of fcr carbs 1x28mm from a xjr400 and 1x33mm from a kawasaki 400cc. The 28mm was less trouble to fit, that why I try them first. At this time , the bike have zero mod and it is a 53hp from factory spec. After running them , it look like a lean condition because the power drop fast at around 9500rpm. If I can be shure that this carb can feed the same hp than the stock, I will take the time to find the good jets if not I will put back the bst33 and when I get more spare time, I will work on my fcr 33mm. At this time, the 28mm give more throtle responce at lower rpm but they lost all the punch really fast. So this is my interogation , does the 28mm can feed me the stock 53 crank hp? If yes I will work on the jetting , if no I will fix the bst33 of my girlfriend until I can Install the fcr33.
     
  7. gregt

    gregt Well-Known Member Premium Member

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    I'd actually doubt if the 32mm CV's would flow as much air as 28 FCR's - just my opinion, no flowbench figures to back that up.

    For years now, I've been in the habit of getting cams done for my race engines at my local cam grinders - Kelford cams in Chch NZ. They have a huge library of profiles. When I wanted a bigger inlet for my "half a FZR250 bored to 150" i simply picked what I wanted and they did a grind out of the existing lobes. I have no idea if any of the factories offered a racekit for these motors - or if any of the "name" cam grinders do anything. Even in the 400's I've never been very happy with the kit cams and have used more radical ones several times. I have a constitutional dislike of doing it like everyone else....

    A programmable cdi is useful to fill in flat spots in the range - where for example you can't tune out a poor spot caused by exhaust resonances - you can often get it to pull through better with more advance at that point.
    But they can't compensate for a lack of area under the cam lift curve....
     
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  8. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    It's always awesome to have gurus in your midst - it's like having a direct phone number to the divine
     
  9. gregt

    gregt Well-Known Member Premium Member

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    I HOPE you're taking the piss....I don't even like curry so can not be a guru....
    That said, a big box containing a roadrace sidecar Hayabusa engine turned up here a few months back - Addressed to :
    "The Engine God" and my name and address.
    He's paid for that - in more ways than the obvious.

    If you live long enough and mix with the right people you sometimes learn a few useful things. I'm ancient...
     
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  10. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    Is there something in the water over there? You're not the first New Zealander I've met who has an aversion to normality (when it comes to mechanical things). And, that's a good thing :thumb_ups:

    Quick question for you Greg...Would re-phasing (is that the right term?) a ZXR250 engine have any affect on its power output and delivery curve? I've heard a few NZ folk like to do this mod on 4-cyl engines. To be clear, the Kawasaki 250/4 has a firing order of 1-2-4-3. The 2 Coils fire 1-4 & 2-3 cylinders. I understand re-phasing to be turning the 4-cylinder 4-stroke into basically a 2-cylinder 4-stroke e.g. instead of a dead firing of the corresponding cylinder, the Crank and Cams are modified to make cylinders 1 & 4 and 2 & 3 on the same stroke, hence 1&4 are on a power stroke while 2&3 are on their intake stroke. I think it might also be called a "big bang" configuration. What's your thoughts on doing this mod?
     
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  11. gregt

    gregt Well-Known Member Premium Member

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    Waste of time on anything which can't wheelspin off a slow corner....
    There might be some theoretical gains with the yamaha crossplane configuration - but their last "big thing" - 5 valves - didn't work out so well for them...So until I see the R1 winning everything, I'm not bothered.

    I'm not averse to normality - but back in the day when all the F3 bikes here used racekit cams, if you wanted to win, you had to come up with something better.
    Even factory kit bits are dumbed down to suit what the factory sees as it's base customers. Got an 04 CBR600 here now full of the FIM kit bits for Supersport.
    In no way shape or form is it anywhere near as good as it could be. But it's what the customer wants - and it will suit his application.
     
    Last edited: May 25, 2017
  12. daoust

    daoust Active Member

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    So basicly , the 28mm should not really kill the power band of the gsf400(came stock with bst32mm or bst33mm with intake port 30mm) if jet properly.
     
  13. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Not taking the piss, just recognising knowledge and those who possess it and are kind enough to share it...

    How about Sensei? The definition is appropriate.
    Are you OK with eating sushi?
     
  14. gregt

    gregt Well-Known Member Premium Member

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    The 28's might prove to be difficult to set up on the GSF - with the smaller throat size, gas speed will be up. This should give a stronger signal at the mainjet - meaning that it may not be lean as you suspect - but actually going rich at that point....

    Knowall old kahnt will do as a title...
     
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  15. daoust

    daoust Active Member

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    The fcr28 that I use was made to run on a xjr400, But , yes , they can still be to rich also. I m waiting about some jets that I order. I should be able to give another try in 2 weeks because I work nigth shift at this time
     
  16. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Si El Presidente :bowdown::prankster::dance2:
     
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  17. ShaneP

    ShaneP Well-Known Member Premium Member Contributing Member

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    How did you get on? Did you try a spark plug test?
     
  18. daoust

    daoust Active Member

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    I instal a oxygen sensor, the jetting was too high, it was 125 on the main and end up at around 102. Got more power than with the stock 33mm cv carb and I know by improving yhe air filtration, I can get a litle more. I m running 4 pod filter with the outwear for the rain.
     
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