FLOAT LIKE A...
Spike emails to say that, after pinning down tickover trouble on his Triumph Thunderbird to a blocked pilot system in the Amal MKI Concentric carb, he swapped the internals into a different body and everything was fine – except that now it choked up at high speed. “Surely a different – but same size – body shouldn’t need a different main jet?” he asks. The only other clue is that if he runs the engine with the fuel turned off, the engine speed increases.
It’s not that unusual for revs to rise as the fuel level drops, but it could imply that the float height is wrong in the new body. It’s set by how far the valve is pressed into the float bowl – so if Spike replaced the bowl as well as the body, that could be it. With the needle seated in the jet, the float should sit so it is just above the gasket face of the float bowl. The actual level of petrol in the bowl should be 0.210in, give or take 40 thou (ie between 0.170 and 0.250in) below the top of the float bowl with the gasket fitted – which presumably means the gasket is included in the measurement. You can do this by rigging up a standpipe. To do this you need the type of bowl with a drain plug and a spare plug you can drill and tap for a pipe fitting – an old needle jet is ideal. A clear plastic pipe (battery breather is good for this) attached to the fitting and taped up the side of the carb will echo the level inside, so it can then be measured with the carb assembled.
The old plastic floats are not easy to adjust. You can try bending the tabs that hold the needle – they tend to spring back, but it may work if you dip them in boiling water.
Alternatively, a new ethanol-resistant stay-up float from Burlen Fuel systems is a good idea, these have metal tabs that can be bent to suit and cost a pretty reasonable £12.98.