Posting on behalf of a mate: 

2010 sherco 290. When running it starts fine but when left for a few days it won't start. Plug has a spark but is also wet when removed. New plug fitted, carb cleaned and checked for airbox to carb leaks etc. But nothing seems to solve the issue.

One thought is that it's slowly flooding over a period of time?... There's no fuel tap just a plastic box with what we presume is a pressure feed system to the tear mounted tank. 

Does anyone know what's in this sealed box? .. One way valves or pressure seals?

all pipes to and from this box appear to be 'open' so not sure how it's supposed to work?

 

Disconnect one of the pipes and leave for a week?  Won't solve the issue but may confirm the source of the problem.

Try starting without choke, if it does its flooding.

Is yours a rear tanker?  If so your fuel system is a little bit different to this,  For instance I think your fuel filler cap is elsewhere.

The "box" is a fuel pump.  They don't go wrong.

1.  Disassemble and clean the carb.

2.  I suspect that you have perished "rubber" fuel hose.  It will leak air in/leak fuel out.  Replace the lot.  You will need perhaps nearly a metre.  Take photos first, before disassembly.

3.  You sure that there is no fuel tap where the hose exits the bottom of the fuel tank?  If so, the lever should be forward for on.  They are tiny.  Snapped off?

PS:  When cold, mine is reluctant to start if there is little fuel in the tank.

PPS:  Plastic box? I wonder if you have a secondary (chain saw) fuel filter fitted?  Like this -

Edited January 8, 20188 yr by stpauls

I HAD 2010 ST 125 WHICH WOULD FILL BORE UP IF LEFT 

I PUT AN INLINE FUEL TAP ON IT FROM A LAWNMOWER AS RECOMMENDED BY SPLAT SHOP AND THIS TOTALLY CURED THE FAULT(UNTIL I FORGOT TO TURN TAP OFF)

My Yamaha TY175 needs its tap turned off when parked, or it gradually fills the engine with fuel.  I forgot it one week, and the engine would not turn over as it was hydraulically locked.   After much tipping of the bike, it finally blew half a pint of fuel out of the exhaust, which burned a 4 foot long patch of grass in the lawn.

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