Hitchcocks Forum

Hi All, I have just completed a road going restoration of a Weslake engined ex-racer, with the 'original' Mk2 Amal concentric carbs refurbished with float kits etc. One carb keeps flooding and I am at a loss to figure why. Owing to the orientation of the manifold (the barrels are vertical) the carbs which should ideally be horizontal slope forward by 12 degrees. Apart from this all seems fine. Any suggestions please? Thanks.

I'm not too familiar with the insides of the Mk2 (although I've used them,) but from the diagram they seem to use the same float needle as the Mk1 but with adjustable tangs on the float pivot to allow you to set float height. Have you tried bending these down to achieve needle seating earlier? With the angle you're using them what works in a horizontal installation might be enough to flood. REgards, Mark

The Concentrics are OK to operate at up to 30 degree off horizontal at the standard float setting. The fact the one works well while the other floods seems to suggest that there is a problem with the setting of the float or its associated parts (or even some other component) in one of the carbs. Are the float needles of the same material? Are the floats themselves of the same material? Alternatively, are the pilot jets in both carbs located in the same position, of the two possible locations? And are there blanking plugs fitted in both the pilot jet locations not being utilized? Are there similar filters in both banjos?

One very likely cause of richness in the MK 2's is a cold staring jet plunger that is not seating correctly, either because of a malfunction of the (very poor) cold start lever or deterioration to the rubber seal at the base of the cold start plungers. When these are not seated correctly there will be a constant rich mixture at all throttle settings, regardless of correctly set float heights.

Probably a bit of debris stopping the petrol from being shut off by the float. (It does not take much.)

I have had the same problem with a MK2. It was the float getting stock on the tube where the jet holder is screwed into. Gentle sand the inside rounding of the float might help you out. It cured my MK2 from flooding.

A flooding carb is (almost) always due to the float valve not sealing.
Either there's some debris fouling the needle (a hair will do it) or the needle/seat is damaged ( a scratch will do it) or the float height is incorrectly set so that the float is not pushing the valve shut at the top of it's movement.

Thank you everyone, some great advice. I put the tank on a rig to act as a remote feed and tested each component. The brand new float bowl I had fitted was the side that was flooding. It appears there is a problem with the machining of the brass valve seat. I have replaced with the original bowl and on the rig all appears OK. Hopefully this will be the case when it is back on the bike.
Moral - something may be new but this does not guarantee it is better than that which preceded it!
Best, Neil

Oh yes, if I had a pound for every new component I've fitted that has caused a problem.......
Never be caught out by the "It can't be that, its brand new" syndrome.