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VFR 400 start and idle problem

Discussion in 'Technical and Troubleshooting Torque' started by phongus, Feb 26, 2010.

  1. Hey everyone

    I am having some issues with my VFR400 race bike. It has been happening since I bought it but have been lazy to fix it, but with racing coming back next month, I better fix it.

    Firstly I have major issues starting this bike, especially after sitting for a day or so. I have left the bike sit for a week and go to start it. I'd hold the choke, have the fuel line on and start the bike without throttle. It just turns over and won't start at all. Try again but this time blipping the accelerator a bit or just holding it slightly open and you can hear it trying to start but just won't. Do this a couple of times and sometimes it will start, but once started it won't hold revs and just dies. If I get fed up trying to start it normally, I spray some brake cleaner into the intake and it starts much quicker then usual. Once it dies, I try to start it again and it will start like normal, but I would have to hold the accelerator to hold revs and warm up the bike.

    I've checked the hoses but no blockages in the fuel line. Not too sure what to check on the carbies...kind of new to carbies...well motorcycle mechanics in general. Have given the carbies a clean with carby cleaner, but that didn't seem to do much.

    After a race, left sitting for an hour, it won't start first go, it will the second time while blipping the accelerator and holding the starter for a while. Have to hold choke most of the time to get it going.

    Once the engine has warmed up it will hold idle...but the idle changes every so often :-s.

    For example. I'd start the bike up, hold the accelerator to get it to temp, once up to temp I let go of throttle and it drops to about 1200RPM and stays there, all well and good...once it warms up a bit more for a minute or so, the idle suddenly bumps itself up to about 3000 - 4000RPM. I change the idle screw so that it drops to about 2000RPM...it sits there for a few seconds and then suddenly just stalls.

    I start it up again, with the blip and hold of the accelerator and once I let go it will drop to stall again. I bump up the idle screw and start it up just fine, but sits at about 3000RPM again. I increase the revs to about 5000RPM and watch the revs drop and hang at about 4000RPM and drop to 3000RPM...give it a few seconds and it will drop to about 2000RPM and just idle...blip it to 4000RPM and revs drop straight back to 2000RPM and idle. Change revs again to 6000RPM and it hangs at 4000 and slowly creeps back down to 2000RPM.

    Try again the next day and the whole cycle repeats itself.

    While racing, I do get a bit of flat spotting at certain revs...but not all the time.

    Right now I am pointing my fingers at the carbies since I haven't touched them besides cleaning them. I have changed the spark plugs ($25 each :mad:) and last time I checked, spark was good.

    I was thinking of balancing the carbies first and see how that goes, but don't have a carb balancer tool at the moment...kind of poor.

    Sorry about the long post, but I hope the information will give a better understanding of my conundrum.


  2. Firstly give the carbies a good cleanout, pull all the jets out and soak them in carby cleaner or vinegar overnight, spray carby cleaner down all the carb's passageways and blow them out with compressed air, reinstall and check your float heights and needle valves for any signs of wear..

    Also check your plugs to see if the engine is operating normally.

    All else fails, check the compression. go grab yourself a compression gauge from supercheap or autobarn if you don't have one and report back with the results.

    Good luck with it mate
  3. yeah giving the carbies a clean alone is not going to fix it. Set the float heights and do a balance. The more highly strung a motor is the more likely it is going to be effected by these things.