Engine Keeps cutting out

[Guest]

My cbf250 has been sitting idle for a few months, when I started it again it ran fine but 5 minutes down the road it started lurching and losing power and then cuts out completely and wont start again. I tried bump starting the bike and it turns over for a second or so then cuts out, I tried full throttle, full choke, same results. Once I have let it sit over night it starts again fine but then cuts out like before. It always cut out when using the throttle, never on idle. My battery is a bit knackered but its not flat when the bike cuts out and it will start the bike after its sat overnight. The bikes always had a slightly erratic idle but has always been reliable till now. The air filter and spark plug seem fine (obviously I'm not a mechanic). There is also a knocking noise from around the bottom right crank case. It sounds like a timing chain slapping against the side of the engine but after having a look its not loose and not worn at all. I also replaced the oil pump thinking it might be causing the noise but its still there. The idle and engine noise have been there for a while and I've had no problems with reliability till now so they may well be unraleted.

Any advice would be greatly appreciated.

[Adam]

Sounds like fuel starvation to me, check the petrol cap breather isn't blocked. A simple way to check this is to ride the bike until it cuts out then open the fuel cap and listen for a sharp hissing noise, if it's not that then it's probably the carbs.

[Grumpy Old Git]

Fuel blockage - try disconnecting/undoing the carb base and check for correct (free) operation of the float valve. If nothing else works, then I'd suggest cleaning the carb.

[Guest]

checked the fuel cap breather, it was closed off so hope that was it. I'm also cleaning the carb anyway, might aswell.

thanks for the help folkes, I'll let you know how it pans out.