Some while back I retarded the timing on the Square Four and improved starting greatly, but it is still running roughly coming off idle and when coasting. Nothing I do to the carburettor seems to improve this so I have resolved to have another look.
Stripping off the carburettor is quite easy and another inspection revealed nothing new. However looking at the gasket faces (something which I would do every time with an Amal carburettor) revealed a bit of a surprise:
You can see here, by the dark line along the edge of the flange that it is by no means flat, which can lead to a weakening of the mixture. A few minutes on some 200 grit wet and dry paper revealed this:
A few minutes more work improve that even further.
The internal mating face between the throttle body and the float chamber and jet castings was nothing like as bad:
This face did show a little distortion, but it was easily removed. You will note that one of the throttle disc screws is missing - I removed that one but as you can see the other one didn't come out without a fight. The reason I removed it is revealed by this picture:
See the light around the edges? This is ostensibly completely closed! What I needed to do therefore was to remove or loosen the butterfly screws and realign the disc so that it was properly closed. In reality it's not essential that it closes because the throttle stop is there to provide some idle speed and the butterfly will always be partially open. This kind of leakage will prevent you idling as slowly as you might want to so it pays to reduce the gap as much as possible. I replaced the screws and got a marginally better result:
I will work on this some more when the proper screws arrive. These should be raised countersunk M4 screws, around 10 mm long.
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