Help!!! Engine misfiring: PROBLEM SOLVED

[dotti]

Took the old GN250 out for an early morning Christmas ride this morning, and all was fine until I hit the open road. At the time I was riding along at 50km/h (as per the speed limit) so I started to accelerate up towards 100Km/h. When I reached about 80km/h my bike started to stutter, shake and misfire. The faster I got the worse things became, so I took it easy, and cruised home at around 70km/h.

The first thing I will check is the sparkplug once it gets light enough (and stops raining). Apart from that can anyone think of any other causes, as to why the bike is misfiring. Also the bike is only missing at higher speeds, and is fine around town, even at higher revs. Can anybody think why this is the case?

Also, for the record, the bike was serviced 2 weeks ago, although the sparkplug wasn't changed.

Thanks in advance

Paul

(Hopefully this won't be an expensive Christmas)
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Dangerous traffic offences spotted on 50-minute trip: Lost count after 100. Number of traffic cops spotted: One, sitting by a speed-trap camera

[mchaggis]

Does it do it at full throttle in lower gears? It's maybe possible that the 3/4-full throttle jet in the carburettor is a little blocked. You need to clean the carburettor out and blow compressed air through the jets to sort that out if it's the case.

The only other thing I can think of right now is the air filter. If it's gunked up, it will be restricting air flow, so might at high rpm be making the fuel mixture too rich and causing misfires like that.
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I must not be a troll...
Mmmm, Guinness
Discovering the delights of Hammerite and a 3/4" brush.

[rizo-sri200]

I'm with mchaggis on that one, possible blocked main jet/needle or a gunked air filter, both items can be cleaned using compress air

HTH
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swift kick to the wing mirror puts em straight..
98' Bandit 600 with Slip on

[dotti]

The bike has a brand new air filter. Also it does not misfire around town, even at very high revs. The problem only occurs at higher speeds and in higher gears. That is what is confusing. It doesn't appear to be an engine speed issue, rather a bike speed issue.

I took the bike out the following day and everything seemed to be okay, so maybe there is some junk in the carb that gets sucked up during sustained periods off acceleration. I will check that out when I get time. In the meantime it is working fine around town, which is where I ride it 95% of the time.
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Dangerous traffic offences spotted on 50-minute trip: Lost count after 100. Number of traffic cops spotted: One, sitting by a speed-trap camera

[dotti]

Thanks for the advice guys. I finally got around to opening up the carburettor, and what a mess there was inside. The inside was pretty corroded and there were plenty of bits & pieces floating around. Might be time to look at the breakers for a new carburettor.

In any case, after a good clean out the problem has gone, the throttle response is a lot better, and the bike accelerates noticeably faster. The best part of all this though, is thanks to the advice I was able to fix the problem myself and save a bit of cash

Cheers for the advice

Paul
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Dangerous traffic offences spotted on 50-minute trip: Lost count after 100. Number of traffic cops spotted: One, sitting by a speed-trap camera