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VT250F sometimes dies at higher RPMs

Discussion in 'Technical and Troubleshooting Torque' started by Captain115, May 12, 2012.

  1. Hey everyone,

    So me and my housemate are doing up this 1988 VT205F, as detailed here: https://netrider.net.au/forums/showthread.php?t=130035#.T65EysUrJBk .

    Anyway so we have given it a pretty full service, as detailed in the above link, and have been starting it up at idle the past few days to fix various problems (coolant, brake fluid, throttle cables etc). But yesterday we finally took it for a ride around our block of land, and it went fine except a little slow in dropping the revs back down and smelt quite rich.

    The problem began when it just randomly died, even though I was giving it a fair twist of throttle. Thought it was just running out of fuel so poured some more in, started her up and rode around again for a couple of minutes and bam, died again. Seemed to occur above about 3500rpm, and couldn't be stopped by open the throttle? Had a look at the spark plugs and they were quite black with whiter tips (especially the rear) so I'm thinking too rich a mixture, which would explain the strong fuel smell in the exhaust. Alternatively, it had been idling for quite a while beforehand so perhaps that was the reasons for the spark plug colour?

    So I've taken the fuel tank, carbs back off and am a little lost in what could be the problem. I went ahead and adjusted the valve clearances today but surely if there had been out (there were slightly out) that wouldn't cause the bike to die?
    I have also checked the float level which seems ok, and had run a wire through the slow and main jets previously but have just redone that. The place where the slow jet screws in (slow jet holder?) does seem to have like a rusty-brown colour crud in it.

    Last time I took the carbs off I didn't resync them as I burred the screw on the front cylinder where the vacuum gauge tube connects. Anyone have an idea in how to get it out, or if the lack of carb sync could be causing the bike to die?

    Anyway, I have no real idea where to go from here. I guess the motor is ok, and the fuel delivery from the tank is ok (maybe)? So I'm saying perhaps something in the carbs? I just don't understand why it would die at the higher revs when the throttle is open.

    Thanks for the help.

  2. I would be leaning more towards an electrical problem. Especially if it is a sudden die and not a cough cough die.
  3. Cheers, I'll have a look at the ignition leads voltage and resistance.

  4. A few people have experienced unexpected cutout because of intermittent side stand switches. Check there as well.
  5. Maybe the float bowl valve in the carb(s) is sticky so when you open it up the carbs starve of fuel.
    Perhaps there is water in the fuel &/or carbs, so when you open it up it mixes and/or travels into the engine (water can sometimes sit in the bottom of the carb bowls while the engine is running at low revs, but as soon as the revs pick up the flow of fuel mixes with the water etc..).
    Maybe the fuel or carbs are dirty or have contaminates in them. Also the fuel tank has either a filter or even a mesh at the fuel tap which can block up & restrict flow.
  6. Start with the simple...

    *Check for blocked fuel filters ( external and in tank ie. fuel tap )

    *Check fuel tank vent. Vent blocked = starvation after a couple of minutes when vacuum in tank builds up.
  7. Thanks for all the suggestions everyone, I might put a bit more info here as well:

    I've now taken the carb's apart and cleaned all the jets and diaphragms twice, triple checked for worn seals or needles, measured and checked the float levels. So I would like to say it is not the carbs but really, it still could be I guess.

    Marx's, I like your ideas of water in the fuel but I drained the tank and float bowls several times before I reassembled everything so probably not water. Also I cleaned the fuel strainer and the carbs so shouldn't be any gunk in there.

    Mike8863, I've doubled checked all the fuel hoses and vacuum leads so they should be ok.

    I started it up today for quite a while and it seemed fine, didn't happen again. The difference between today and when it was cutting out was the temperature of the bike, as it had been warmed up and ridden around so the temp was about midway/two-thirds of the way up the gauge (about when the fan kicks in). Not sure if this would be a reason for it occur but just a thought.

    I'll let you guys know if it keeps happening again though, thanks so much for all the help!

  8. Sooooo....I was riding around on the bike this morning to see if the problem was still there. And low and behold it did appear again, same as last time: the bike would just drop revs till it died regardless of the throttle.

    I sat there for about 10 seconds trying to diagnose what I had been doing prior to this situation. While I was thinking, I was turning the throttle with the bike off and realised it felt really loose. Looked below and saw the pull throttle cable I had replaced had come off it's holder as I mustn't have screwed the locking nots in hard enough.

    This meant that the cable was pretty slack, so when the throttle was revved it was barely opening the butterfly valves in the carbs so the bike died. I bet it I had pulled the clutch in fast enough it wouldn't have died. So I reattached and relocked the nuts and rode around for a bit more without it happening again.

    Fingers crossed this was the problem, something small and silly as opposed to something within the fuel tank/carbs/pipes.