Rattling fit
Job Hutchings contacted me last year about hot starting problems with his 1958 Indian Westerner (a US version of the 500cc Matchless G80 CS). Job had fitted electronic ignition and a new Amal Monobloc carb – but the problem persisted and I was pleased to hear he’s now cured the trouble by buying a new Amal Monobloc carb... er, wot?
Job says he’d bought the last carb about ten years ago and when dismantling it in search of the starting problem had noticed that after removing the retaining screw, the jet block in the middle of the carb dropped out where usually it’s a snug fit. There is a gasket beneath and as well as the retaining screw, the block is secured by the main jet holder, so it didn’t seem an issue at the time. But the new carb’s block is the usual tight fit and when Job tried the new-new carb’s slide, needle and jets in the old-new carb, the trouble remained.
So presumably the fault lies in the body and he blames the loose jet block.
Air is drawn into a hole at the air filter end of the carb; its quantity is then regulated by the air screw and passed on through a hole that mates up with another in the jet block. Here the regulated air mixes with the fuel to deliver the right balance for tickover into the airstream through a tiny hole on the engine side of the throttle slide, allowing the engine to tick over on a closed throttle. But if the jet block is loose, there’s a good chance extra air could get into the jet block, bypassing the pilot screw to make a lean mixture – and when the engine’s hot, the carb body may expand more than the jet block making the problem worse.
Interesting one this because it’s something that is unlikely to wear but if for any reason there’s a fault in manufacture it could be a real headache; lucky Job found the cure.