| Diagram | Fig. 24 — CARBURETTER · ref 3 |
| Subsystem | fuel |
| Listed in parts books | 1962, 1969–1970 |
| Also fits (as replacement) | 1960–1962, 1964–1965, 1967 · for 4/137 |
Screw plug for top cap
A screw plug that seals the top cap on an Amal carburetter.
The 4/137A is a small screw plug that fits into the top cap of an Amal carburetter. It closes off the top of the carb body where the cap sits, keeping dirt and moisture out of the slide chamber and giving you a place to check or adjust internals without pulling the whole cap assembly.
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This one shows up in the parts books under the carburetter and air cleaner figures, including the Bonneville 120 carburetter listings, so it's tied to Amal instrument work rather than anything Lucas or electrical despite how the catalog numbering might look. Appearances in our records run from the early '60s through 1970, which tells you it was in service across that stretch, but we don't have anything narrowing it to a specific model year.
It's a simple, easy to overlook part, but a stripped or missing top cap screw plug is the kind of small thing that lets grit into the carb or leaves the cap rattling loose. Worth keeping a spare on hand if you're rebuilding a top end on your carb.
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