I have a 20 year old Kawasaki ZZR600. She was running fine in December. I didn't ride her for a few weeks in January. Left the battery on maintenance trickle charge.
A few weeks or so ago I fired her up again. She started fine, absolutely normal. I had the choke out as normal for the slight cold. With the choke out she idles at 3,000 - 5,000 revs depending upon exactly how far out.
Once she's warm she idles just fine with the choke in, at 500 - 1,000 rpm.
But if I open the throttle to rev the engine, there's a very short burst of revs, for maybe half a second, then the engine cuts out. This happens with both the choke in and out.
Assuming nothing has been done with or to the fuel lines and carbs since she was running fine in December, what possible causes could result in this ?
I write this here as there simply isn't enough room in the comments. I have marked the Seafoam answer as correct, although whether water had anything to do with it I didn't ever find out.
My attempt to apply Seafoam was pretty amateurish, but it still seems to have worked.
The videos I watched varied in their instruction but they all seemed to include getting pure Seafoam directly into the carbs. I struggled to do this. Imagine some comedy moments trying to get a funnel into the fuel pipe, and pouring Seafoam into the funnel while running the engine to suck it through, and then cutting out before any Seafoam has got in there.
In the end, I drained the tank and then filled it with approx a litre of petrol / Seafoam mix - very strongly Seafoam, maybe eighth / quarter of a litre Seafoam. Then I ran the engine for 30 mins, left it a day, ran it 30 mins the next day, waited another day, etc, for 3 days or so.
Whilst running the engine I played with the throttle, moving it micro-amounts looking for the point where it caused the engine to cut out. I experimented with having the choke open to keep the engine running, whilst opening the throttle enough to dip the revs without cutting out.
Eventually, after a couple of days, I found the "bite" on the throttle moving, so that I was able to open the throttle more and more before the engine would cut.
She appears to be running pretty well now. I intend to fill the tank up (she holds 18 litres) and empty the rest of the Seafoam in, and ride her around for a bit, see how it goes.
I probably should still get the carbs professionally cleaned (or get myself properly equipped and on a maintenance course).