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A quick message in case this happens to you...
I took my old 350 out for a ride, and filled the petrol tank with fuel since it was nearly empty. About a mile (1.6km) later it started running poorly and I noticed petrol being forced out of the right (rear cylinder) carburettor. It was emerging above the height of the float area and covering the base of the carb. I managed to get it back home OK.

On removing the float bowl I was surprised to feel the weight of the float which was much heavier than expected. I then discovered one of the two float chambers was almost full of petrol. By the time I removed the bowl and the float pin some of that fuel seemed to come out (but oddly not the rest). I bought this float no more than 2 years ago from NLM and am disappointed that it's failed so quickly.

Eagle-eyed people will notice the O ring for the choke has some damage too, so I've replaced the float with an older one from a spare carb and also the choke with a better quality of O ring. Here's hoping the bike will behave well.

PS: I lost several litres of fuel in the 5 or so minutes this happened, even though I closed the fuel tap that feeds this carb; (yes I know there's a balance pipe between the carbs for the fuel so it's an imperfect way to deal with the leak and overflow. All in all a bit worrying - thankfully no fires emerged on the ride home.

These new plastic bowls (the older black metalic ones are not more available, as far as I know) often have some ridges (is it the right word?) that do not allow the bowls to move properly. So you have to grind them a bit. But your one is definitely a mess

Julian,

It is the new reserve tank! So much for improving the product over the lifecycle...

Aha! I had a similar problem at Loton Park last month. Got to the assembly area and an eagle eyed marshal spotted fuel was peeing from the left hand PHBH28 on the Forgotten Error. I assumed a stuck float but no amount of tapping would sort it. Went back up to the paddock where dropping the float bowl on that side revealed one of the pair of floats was 3/4s full of petrol.
It didn't want to drain out but I guessed there must be a leak in the 'seam' so I gave it a good squeeze and there was a 'click' followed by swift emptying. Someone had a soldering iron with them that I hooked up to the mains adapter in my camper van so I heat welded the seam well enough to get in three runs that day. I was more than slightly cross however, when the next day, after the bike just spending the night lashed to the trailer, the same thing was happening. The other float of the same pair this time (my bodge had remained intact) and I decided enough was enough.

A new replacement float assembly has been fitted and is behaving itself so far (as is the one on the other side), but it does make you wonder if there was a manufacturing fault on at least a batch of these new style clear floats (mine were supplied with new carbs about two years ago). Naturally I have also acquired another for the paddock spares box, but in light of your experience I might put it in now as a precaution.

It does seem to be a manufacturing fault, I wonder if it's worth asking Eurocarb and/or Dellorto?

I've now found a spare carb to bring with me to the AGM as a precaution (even though I've replaced this float with an older model) as losing fuel at the rate it overflowed isn't a great experience and the bike doesn't even go any faster, so I can't consider it a performance mod

PS: The idea of trying to solder the plastic with the petrol still inside blows my brain - I wonder what'd happen...? Best not explore that idea, eh. Instead I'll have a go at squeezing the petrol out, back into the proper petrol tank.

Could ethanol in petrol be causing this problem?
I have just replaced the fuel sender float on an old VW Beetle because petrol got into it.
Amal carbs are now producing a 'stay-up float' which is supposed to be compatible with ethanol 'contaminated' petrol. See burlen.co.uk then click on 'Amal', then 'technical', then see both 'ethanol fuels' and 'all new stayup floats'.

Richard,
the newer petrol with ethanol *might* be a cause, however that's been for sale for years so if Dellorto sold floats even several years ago that weren't able to cope then they seem to be lagging reality. I'd emailed Eurocarb the UK importer about the problem, I also asked if they'd be covered under warranty; here's their reply:

"Hi Julian
Unfortunately the warranty is only valid for a year so they would not be covered.

It is something that did happen occasionally, though usually it would be when they were first fitted rather than a couple of years later, due to ethanol or additive levels. Dellorto assure us that the current floats are resistant to modern fuels and so it shouldn't be an issue at all any longer."

Paradoxically, it seems the decades-old Dellorto floats cope better and last longer than the new floats...