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Hot weather fuel starvation problem?

Discussion in 'Technical and Troubleshooting Torque' started by jawntybull, Oct 31, 2008.

  1. I've had my GS500F for six months now, and today a new problem surfaced. Its the first hot day since I bought it (36 degree C in Sydney). Started the bike, rode it for 10 mins, then stopped it to do an errand. Came back 5 mins later, started fine, and rode off. Within a minute or so it started to splutter, particularly as I opened the throttle up. Idled ok, but after the first few seconds of acceleration it would feel like fuel starvation. If I kept the thottle open it would choke for a while, then surge, then choke a bit, then surge etc. This happened all the way home, but got progressively worse until it felt permanently starved. The bike never stopped though; it just felt like it was running at half power.

    I just had a major service so new plugs etc in the bike, carbs have been synced - although the air filter is due for replacement in 4000km.

    I have a few thoughts:
    - vapour lock (really hot day, got even hotter sitting still after the first ten mins ride)
    - blocked jets or fuel filter; maybe blocked jets in one carby only, explains the feeling of half power
    - electrical fault, cutting the spark to one plug intermittently.

    So - where should I look first for a solution? I'll let it cool and try it out later this evening, but I want to fix the cause as I don't want this to happen 200km from home!!

    Thanks for any ideas
  2. Just serviced would make me look at fuel and vent lines for any sort of pinching between tank etc when the tank was refitted.
    I once had a Z 650 that had the valve clearances to tight. On a hot day in traffic it'd lose power, idle badly and be really hard to start. So valve clearances could be suspects if they were done too.
    If it hasn't been a problem before logic says look at the last work done on the bike.

    Regards, Andrew.
  3. Problem solved yesterday - so for anyone who has similar symptoms...

    It was not fuel at all, nor was it hot weather. One of the terminals on the primary side of the coil had come loose; this meant one cylinder had no spark. It must have come loose over a period of time, explaining the intermittent surging.

    Diagnostics - it felt that the bike was jumping between half and full power, which made me think of a problem in one cylinder. Easier to check electrics than fuel, so I started there. I took the HT lead off one side, and it ran (but poorly). Took the HT lead off the other side and the bike wouldn't start at all - so it had to be the still-connected cylinder that was the problem. Checked the plug - ok. Traced the circuit (required removing the fuel tank of course) and there it was.