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VFR750 hesitation on part throttle

Discussion in 'Technical and Troubleshooting Torque' started by VFR750 IRYDE, May 29, 2008.

  1. Just picked up my first non learner bike, a 94 VFR750.

    Very happy with it, but discovered that on part throttle (ie, holding speed at 100kph) it hesitates, with a slight 'power on, power off' shudder, the same as what you would expect if it was having fuel scavenging issues.

    The tank is full and my hand is steady...?

    Dirty carbs?

    Your thoughts please!
  2. more than likely, could also need a carb balance.
    could be the filter too, full of crud, restricting the flow.
  3. Ahh, filter is very possible! I bought the bike off a country boy, so dirt and dusty riding is likely.

    *out comes the manual...*

    I'll let you know what I find.
  4. Started getting the same thing on my NX650 as soon as the winter fuel come on the market...

    My bike and few other people's bikes that I know that are carburated are running real lean... we poped one on the dyno (XL600) and managed to get some fuel/air readings... not good... %31 leaner than it was in summer!!

    I think there is some thing wrong with the fuel these days...
  5. I'll keep that in mind... If she's running lean tho, I'd love to see how quick she is on full juice! :eek:)

    Yeah I know, VFR750's aren't that quick, but coming from a VTR250... CRIKEY!
  6. give it a carb service, tune and balance. If it's still doing it after that I suspect it will be worn needles and/or needle jets.
  7. OK, air and fuel filters are both in serviceable nick, and I hit the Carbs with half a can of carb cleaner, both static and while running.

    It ran like a dream for about 45 mins worth of riding that night, but the gremlin came back the very next day, I thought it was a gonner but it wouldn't stay away...

    You get the drift.

    Some more details on the symptoms:

    It feels exactly like I'm running out of petrol, with the motor suddenly running lean, but only for a few seconds before full power returns. Its most obvious when the motor is fully warmed through. Affects all throttle positions but does NOT affect idle (or at least not so I've noticed?) Can be exacerbated by a few minutes of enthusiastic riding.

    There is no 'rhythem' to it, it's just random 'power on, power off' at different times, and the shift from 'on' to 'off' is significant and sudden enough to alter cornering lines and raise my heartrate by a good margin.

    If it's a 'strip down the carbies' type job, then I'm gonna have to hand it over to the pro's, (unless someone within an hour and half riding time from preston, Melb, would be willing to show a young bloke some mechanical ropes..?) But if there is more than I (being capable but desperately under educated in all things mechanical) can do, then I'm willing to give it a shot.

    Your thoughts please? :?:
  8. kinked fuel lines? improper carburettor float heights? blocked tank intake/fuel filter? (in tank fuel filter/strainer?)

    If it's that violent, and it comes good, you'd assume the fuel isn't getting into the float bowl fast enough.
  9. head on over to ozvfr.net the guys there can tell you the maiden name of the woman who made the third screw on the throttle bodies, and what day it was :LOL:.
  10. What the outside?

    Ok this suggests it's probably not the carbies themselves that's the problem.

    Check all the fuel and vacuum hoses are really good fits on there spigots. Maybe even replace them for good measure. Also check your tank vent is clear.
  11. Lol no! Tank and air filter off, spraying into intake bells. (as per instructions on can. When all else fails, read the instructions!)

    Makes a lovely noise without the filter and tank to muffle it! Highly recommended as audio p0rn!

    All the hoses are firmly secured, but I haven't checked any for blockages.
    *Adds that to the list of things to do*

    Thx mate, I'm awaiting membership confirmation over there. Cheers.
  12. Thankyou to everyone who offered their help with this problem.

    I tried a number of the suggestions, and learned heaps about my bike in the process! Unfortunately, none of the things I tried was the cause of the problem.

    At my wits end I booked it in with a mechanic.

    Turns out it was just fouled spark plugs. :( That's it. All I would have had to do was pull out a socket set while my tank was off and I would have found it myself, but I was so set on it being a fuel/carby problem that it didn't even cross my mind.

    New plugs and a bit of tuning of the carbs (so they dont run so rich) has fixed the problem nicely.

    Thanks again for your help! :)
  13. Here's something for future reference....I have fixed a lot of "must be carb/fuel injection" problems with a simple tune up.
    As you have found out, it is easy to be sucked in to believing something is the cause....it still catches me out sometimes!

    Regards, Andrew.