How to... Check and fix ovalled carbs

A sticking throttle can be casued by a few things, but on Amal carbs distortion is a likely cause

1 First, examine the slide for damage – these black marks were suspicious and had an almost tar-like look – readers have had problems with carbs gumming up due to sealer dissolving in the tank but there’s no sealer in my tank. In any case, cleaning off the stains with wire wool and spirit made no difference.

2 The slide looks fairly new – at least on the sides, the front and back are scored. Is it oval? Slides are delicate and easily distorted with rough handling – although that hasn’t happened here – and measuring revealed it’s not oval, apart from the expected slight wear front and back due to suction from the inlet.

3 First check is to remove the jet block. The slide passes over this centre piece, so if it’s damaged it could chafe here. You can also get this problem when the block’s been removed and not settled perfectly when replaced. But in this case, the slide still stuck with the block removed so it’s definitely the body.

4 Over-tightening the nuts can distort the flange, pulling the chamber out of round in turn. The flange was flat against a straight edge – but it could have been filed back flat. A better test is to measure the choke diameter; when the flange pulls in, it will oval the bore top to bottom. Hmmm, oval it is...

5 To check distortion, I used a telescopic gauge. Locked fore and aft, I turned it 90° and it was a ‘rattling fit’ crosswise. These gauges don’t have dimensional scales so instead I measured the gap with feeler blades. It took a 20 thou blade – half a millimetre, that’s a lot!

6 I needed to reshape the body in the vice – but it’s easy to make a problem like this worse. My vee blocks were too big, but rooting through my odds I found these two ‘collets’. No idea what they’re for, but they were the perfect size to do the job. By gently springing the handle of the vice, I managed to get it back to round.