Bike is an '01 MY Duke II. A couple of days ago it started cutting out/missing/stuttering (whatever you want to call it...) when overtaking in 4th at about 5,000 rpm. The bike would hardly accelerate, and problem would only solve itself if you shifted down into third and 6,000+ rpm.
Fuel level and fuel condition are fine. No changes to jetting in the 9,000 miles I've put on it, and the air filter was replaced 1,500 miles ago with no issues since. No fuel coming out of the drain hose or other evidence of leaks in the fuel system. No obviously missing vacuum hoses (though I haven't pulled the tank off yet). Plug and plug cap appear to be properly seated. Plug is factory spec and only about 4k miles old. No difference whether main or reserve selected on the fuel tap (and it's a polycarbonate tank, not steel).
Today it was doing it in all gears with intermittent severity. At its worst, the bike would only accept a slightly cracked throttle and could just about maintain 50mph in top - any more throttle and it would just splutter and decelerate. Would also do it if caned from first through the gears - would do fine through first and second using full throttle and going almost to redline, then when third selected it would start choking and spluttering. When it's really bad, it seems to hunt slightly even on a steady throttle, but behave completely normally on the overrun (normal engine braking, normal sound).
Problem is always present now, but intermittent in severity. Came in really badly today immediately after doing a bit of hooning through twisty bits of the A29 (couldn't get above 55), then by about 10 miles of steady throttle later, it let me creep up to 75 and cruise there (that's quite a bit of load on a Duke, BTW). Immediately after the twisty bits, when it started cutting out, I tried pulling in the clutch and revving it, and it just spluttered and refused to rev. When I got back home about 20 miles later, revving it with clutch in gave perfect throttle response. Full throttle blips from idle, holding it at 5k, 7k, 3k - no problems.
Don't believe it's a jetting or air problem, as (i) I haven't touched anything and (ii) the problem has increased in severity very quickly (over about 75 miles and two days). Plus, if it was jetting or blocked air filter, you would expect it to behave consistently at least on the same day, which it doesn't.
It does sound a bit like the float bowl emptying (the fact that it happens with extended or prolonged use of full throttle, and seems to get better if you run it on a steady throttle for a while), but then I would expect it to cut out completely, not to stutter, and plus I haven't done anything to the carb, so how would the float level magically change?
So that suggests to me the horrible conclusion that it's electrical (just like the TRX - what are the odds
). Where it first appeared and the reaction to full throttle makes me think it's load dependent to at least some degree (full throttle / 5,000 rpm out of 8 in 4th). Can anyone suggest what might be a good place to start looking in terms of finding the fault?
Weird... Turning the tap on, I finally got some petrol through and potentially a bit of dirt, but no water. Strange thing is, having done that, put the little tank back on and drained the same fuel from the old tank to the new one, the problem seems to have resolved. Thank f***. 