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VFR400: Hesatation, back firing and on off high idle....

Discussion in 'Technical and Troubleshooting Torque' started by ward_4e, Jul 8, 2007.

  1. Ok here are the symptoms people:

    Issue 1: Hesatation: On the freeway / aterial road at 6k-8k+ in 3rd or 4th. Open the throttle to accelerate... Major hesitation as if no fuel / 2% of fuel is combusting. - Occasionaly randomly folowed by a small pop back fire. Back down a gear to lift the rpm range still no effect....

    does not occur at low speeds >70kmh and around town but the rev range is generaly 2k-6k in town.

    Issue 2: Random High idle: Sitting in traffic and or at toll gates, The vfr will idle at 1.2k then 3k then 2k then 1.2k or 4k or what ever it decieds it wants. I have cranked down the idle on the left hand side of the bike to compensate. Generaly to no effect.

    So there are the problems.... the bike is a er:

    "modified" Honda VFR400 1990
    Full Yoshimura system
    Unknown jets and needles ( reputidley the largest avaliable :shock: )
    Full HRC Ignittion - modified redline 16.5k
    Unknown Gearing ratio's but I have been known to be in 1st in the hills of beaudesert at 110 and 15k rpm :roll:

    Its prety obvious its a fueling issue but er how do I fix it / should i just take it to a good mechanic and say here is my wallet dont abuse me...

    Side note: I am putting together a jaycar O2 sensor display / getting a sensor installed. Later next week i will be able to tell you how rich the cow is running.
  2. Sounds like all of your symptoms are carb related.

    NC30's are known for their lean spot at mid revs, which is what you're feeling. You'll need to see what jets and needles it has before you can assume anything. Mid revs fuelling will be down to needles, main jets will only affect performance after 10k rpm. This lean spot can be alleviated by installing 0.5 mm washers under the needles, a common mod on NC30's. I have plenty of washers, let me know if you want some.

    Erratic idling is often a sign that the carbs need balancing, apparently they can go out of balance frequently although mine don't seem to. This is no easy task on a V4 while the carbs are still on the engine (especially first time!).

    It's easier to take the carbs off and adjust the butterflies by eye, for virtually as good results IMO. They'll almost certainly be better than what yours currently sound. While the carbs are off check to see what main jet size it's running.

    One other thing that can affect midrange performance is any induction mods. These bikes are extremely fussy when it comes to induction. Cut air boxes, snorkels (and even race air filters) will be detrimental to performance. Check to see if all the parts are still there:


    Best thing to do is to whip the carbs off and do it properly, all it takes is a bit of time, the bike will run sweetly after.
  3. Another trick for balancing the carbs -I haven't tried this one myself, is to wind the idle screw up enough to get a very small drill bit under the base carb butterfly. Then use the same drill bit as a gauge to set the rest of the carbs. And also a bit of a confession here on my part. I've done 60,000kms on my VFR 400 and as yet have seen no need to balance the carbs :shock: :shock: Still idles fine and the off idle response is fine to me. If it aint broke -don't fix it :grin:
  4. yes you have the issues as mentioned

    but due to your massive idle fluctuations my bet is there is an air leak at the carbs/manifolds , this would also explain your symptoms

    btw doesnt matter what sprockets you use the speedo will always read 110 at 15K as its driven from the sprocket rather than the front wheel
  5. That doesn't make sense..
  6. Yes it does :LOL: How's the project coming along Wardie?
  7. Only one of two things Vacuum leak ( would be my bet ) or shite in carbies
  8. 1. air leak

    2. balance

    3. it's trying to say "getorf me ya fat bastard" :LOL: :LOL:

  9. at 15K rpm in 1st gear the front sprcket is turning at x speed
    with stock gearing the front wheel will be doing 110kph

    ok now lets change the front sprocket size

    at 15K RPM the front sprocket is turning at x speed so reads 110KPH

    the front wheel however is now only doing 95kph

    since the speedo is driven from the sprocket the speedo still reads the same but the bikes actual speed is different
  10. I thought that with the GTR too........huuuge difference!
    Setting the throttle plates with a drill is a good way to do it, but it should be done off the bike, with the carbs assembled in a rack and held vertically. You insert the drill under the throttle blade, and adjust the idle/balance screw till the drill bit just falls out. I synched the carbs on my Z 650 this way, again for a huuuge improvement, but the vacuum gauges are much faster on the bike, especially if you have a demonic Kawasaki airbox.

    Regards, Andrew.
  11. good she know she is a project now... the whore blew an exhaust gasket and now sounds like a v4 kazoo... :roll:
  12. er sprocket ratios will affect the speedo... to combat this i will be installing a hall effect sensor + magnet for the front wheel to combat this... The bikes that read from the front wheel are generally giving true speeds and are independent of sprocket ratio creeps...
  13. har har... well its probably true..... er how do i check for air leaks?
  14. Cammo Roarn cheers for the info i will be definatley ATTEMPTING that kind of bike abuse really soon.. hopefully under the watchful eye of a more experienced bike molester!
  15. Hehe, you could actually play tunes using the throttle then? I had an exhaust gasket leak develop on the Spada, and she started sounding like a tiny Harley :rofl: Wouldn't mind hearing the Viffer with the Yoshi system, I've never seen it around...
  16. all i know is my hearing hurts with earplugs and the woman beside me on the freeway last night wasnt happy... :LOL: :LOL:
  17. take it to a good mechanic.

    Balance is not going to be youtr problem as balance is not prone to change that quick (unless broken linkage(not likely). I would be more inclined to suggest a vac prob of maybe a sticking slide of float needle.

    Either way, If you want it done right, take it to a mechanic.

    ps. on a a twin it is extremly easy to balance simply by ear. Drill bits?? don't waste your time.... ;)

  18. thanks mate.. i will consider fleshing out for a mechanic... btw its a v4 not a twin.
  19. Bring it to my place, we'll work it out.
  20. Hark do i hear the sound of bottles being opened? lo i must ride the kazoo to the shed of miracles for fixes... :wink:

    I really should try and fix that exhaust gasket first.... hitemp gasket goo here we come.