Stutttterrriiiinng

[Trev_P]

Not ridden my bike in a while, perhaps its getting back used to it, but under 4000rpm there's a noticeable delay/lag/stuttering even if I give it a decent amount of throttle. Upshot is I can't pull away as quick as I'm used to unless I slip the clutch and rev a bit higher. Over 4000rpm it seems fine. I have had about half a tank of fuel in it, probably some as old as November, the rest around January, so it has been over a month or so. I'm thinking it might be stale fuel doing this. Seemed Ok this morning, only noticed it in the afternoon after getting a minor service (oil). But the tank wasn't off, so unlikely a pinched fuel line.
I only get the stuttering under load, it rev's freely if in neutral. Maybe the cold weather and mix with old fuel?
Anyway I've put some Redex in the tank for the time being and will fill it up to full with newer fuel to see if that helps.

Thinking otherwise could be sparkplugs maybe. Looking online someones suggests IACV hoses, though IDLE seems smooth and fine.

Any thoughts welcome.

Thanks.

more than likely the fuel as already said, with this modern stuff the ethanol evaporates, not like the good ol stuff - 4 star leaded, that used to last years before it went a bit funny if it did you just top it up with number 7 paint thinner
once you fill it up I reckon you will notice it getting better after a few miles

[GezTheHealer]

Cheers all, I'll check the breather, and mix in some newer fuel and try a bit of an italian tune up and see if I have any joy. It would be nice if the redex helped it out. It didn't have an air filter, only an oil filter so I don't think they even had the tank off. Fuel filter sounds like a good place to look. I may just not be used to it and labouring the engine, but I don't remember it riding like this.

Thanks for all the pointers guys, and all the steps WR6133. I'll let you know how I get on.