MGB Carburetter Reassembly Steps 22-34 - Spring-Loaded Needle & Jet Setting
Service procedure page listing carburetter needle, jet and damper refitting steps with a labeled piston/needle figure.
This page (page 425, section 4-9) from an 'Engine Emission' workshop manual (Issue 4, ref. 18959) covers the final reassembly steps for SU-type carburetters. Steps 22–26 detail refitting spring-loaded jet needle assemblies: locating the spring in the needle support groove, a warning never to alter the spring seat position or remove/reposition the centralizing 'pip' on the needle guide, positioning the guide flush with the piston face with its etched locating mark aligned midway between the two piston transfer holes (illustrated, figure DO475), fitting a new (shorter) guide locking screw, checking the piston key, and refitting the suction chamber without winding up the spring. Steps 27–34 apply to all carburetter types: refitting the damper and washer; reassembling the pick-up lever, cam lever, springs, skid washer and pivot bolt tube; hooking the return spring; fitting the brass ferrule and pick-up link to the jet; setting the jet adjusting nut flush with the carburetter bridge, then turning it down to the initial setting from 'TUNING DATA'; refitting the carburetter(s) per the vehicle Workshop Manual; and finally tuning per 'CARBURETTER TUNING—COMPLETE'. A reader learns the correct assembly sequence and critical precautions for spring-loaded needle carburetters.
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Frequently asked questions
- Can I convert a fixed-type carburetter needle to a spring-loaded needle?
- No. The document states spring-loaded needles are supplied complete with shouldered spring seats; no attempt should be made to alter the spring seat position or convert a fixed-type needle to spring-loaded application.
- How should the needle guide be positioned in the piston?
- The lower edge of the guide (76) must be flush with the face of the piston, with the etched locating mark (77) on its lower face in line with the midway point between the two piston transfer holes. Alternative guides with a machined flat must be positioned so the guide locking screw tightens down onto the flat.
- Are guide locking screws for spring-loaded needles the same as needle locking screws for fixed needles?
- No. Guide locking screws for spring-loaded needles are shorter than the needle locking screws used with fixed needles, and a new guide locking screw should be fitted.
- How do I set the jet adjusting nut after reassembly?
- Without removing the suction chamber, screw the jet adjusting nut until the top face of the jet is flush with the bridge of the carburetter, then turn it down to the initial jet setting given in 'TUNING DATA'.
- What happens if the needle guide flat is incorrectly positioned?
- If the guide locking screw has not tightened down on the flat, the head of the screw will protrude from the piston.
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