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flat spot on cbr250rr

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by wheeler84, Jul 25, 2007.

  1. I was test riding a cbr250rr today, and I noticed a definite flat spot...

    basically I was picking up speed steadily in 3rd from about 40k, I gave it full throttle from fairly low revs, and at around 6 grand it lost momentum briefly before accelerating properly in the higher revs. I didn't have a very long ride so I couldn't say for sure whether it was occurring in other gears, it was possibly happening in 2nd as well.

    according to the dealer who has just trucked the bike in from another dealership, it may have been sitting for awhile because when doing a cold start prior to the test ride, it seemed to take some time to sound like it was running cleanly.

    I've been trying to read up on flat spots because I don't have much mechanical knowledge especially with bikes... i've picked up a few possibilites:

    1. the injection system is a bit dirty and basically needs to be blasted out through cleaning/fanging

    2. the fuel has been sitting (will try and confirm whether this is the case or not) for awhile and probably should be drained and replaced if thats the case [on a side note, do you generally run bikes on premium fuel?]

    3. tuning may be out

    4. potentially bigger mechanical problem?

    Overall the performance was basically the same as what i've experienced on two other cbr250rr's i've ridden, just this is the first time i've felt one seemingly struggle to accelerate even briefly.

  2. Third at 40, and trying to give such a highly strung engine full throttle will result in some acceleration problems, a bike like that really should be in second for that sort of action. Even my GTR would baulk if given full throttle at 40 in third. I;d say if the bike did everything else it should, there's nothing wrong with it.
    You needn't worry about the injection system, it is a carbed bike.

    Regards, Andrew.
  3. i'm pretty sure it happen in 2nd from about 40 as well... the only reason I picked up on it was because it initially accelerated but then seemed to lose momentum for about 3 seconds.. but it was definitely occurring at around 6,000 revs which I suppose is relatively low for a cbr
  4. lol yeah u know it makes it to about 80ks in first.
    Sounds like dirty/jammed jet or some simmialr fuel starvation. Nothing a carb clean/balance cant fix
  5. perfectly understood, yeh I will freely admit I am a pretty shithouse rider cos i've spent 10 hours total riding bikes in my life, and those 10 hours were all in proper rider training on a totally different, light, low powered bike. I did open it up a bit as well and didn't have any problems in the revs....

    I probably should mention that if this is not a major issue, i'm gunna be buying this bike tomorrow! I told the guys at the shop about it and the mechanics are gunna take it for a spin and do what they have to do to get it tip top for sale.

    Just wanted to make sure people here didn't get concerned by my description, that it could be a bigger problem rather than just a minor tuning issue.... and I didn't want to fork out for a service/tuneup anytime soon
  6. same issue on my cbr nomatter what gear 5-7krpm it bogs down.. doesnt matter if its small ammounts of throttle of full throttle.. it does it.

    my guess is the cam/s for the bike. just the way the power is delivered thats why i get that feeling.. but i still lub it :D
  7. In my old engine (1991) I dont get decent accel until 8k nomatter what gear. In my new engine (1996) I get goo power as low as 4k but the top end I dont get as much power. This is probably due to the different carby in it.
  8. It's not a flat spot, it's a 250cc motor :p
  9. True you cant call it a flat spot....
  10. If you are keen on buying that one I'd raise your issues with the dealer and get them to do a full service including carb clean and sync.

    When I rode one, it all felt wrong below 8000rpm. Full throttle from lower than that WILL give a delay before you get to power.
  11. ended up buying this bike last thursday, being cruising around on it for a week... all is well I think what it really needed was a good run which i've provided (done about 500k in a week) in fact only yesterday it suddenly felt like it had a turd stuck in it when I was warming up, gave it a bit of a rev and started riding around, all of a sudden everything was smooth as!

    The mechanic at the bike shop put some fuel line cleaner through it in the first tank, i'm guessing that has only just fully flushed out the system.

    It had power all week above about 8 grand, felt like turbo charging when it hit that powerband, but now i'm getting much cleaner acceleration in low revs as well.... I understand the comments about it being only a 250, but still plenty of fun for a newbie :)