MGB Tuning Data to 1971 & 1971 - Ignition Timing, Distributor, SU Carb Specs
Factory tuning specs for the MGB 1798cc engine: 8.8:1 compression, idle 900 rpm, 0.015in valve clearance, ignition 20deg BTDC at 1000 rpm, Lucas 25D4 distributor, Champion N-9Y plugs.
This document is page 443 (section 6-9, Engine Emission, Issue 6) of an MGB workshop manual, presenting complete tuning data in two columns: 'To 1971' (engines 18GF, 18GH, or 18GJ) and '1971' (engine 18GK). Both share a 1798 c.c. capacity, 8.8:1 compression ratio, 900 r.p.m. idle, and .015 in. cold valve clearance, but differ in stroboscopic ignition timing (20° BTDC at 1,000 rpm vs 15° BTDC at 1,500 rpm). Distributor data covers the Lucas 25D4 (serial 40897/41155 vs 41339), contact breaker gap .014–.016 in., 57°–63° dwell angle, and detailed centrifugal and vacuum advance curves, which differ between engines. Spark plugs are Champion N-9Y gapped .024–.026 in.; the ignition coil is a Lucas HA12 with 3.1–3.5 ohm primary resistance. Carburetter data covers twin S.U. HS4 units, with specifications AUD 265/AUD 326 (to 1971) and AUD 465 (1971), FX/AAE vs AAL needles, red piston springs, and initial jet adjustments of 14 vs 12 flats from bridge. Exhaust emission limits are 4.5% CO maximum at idle with an air pump test speed of 1,000 engine r.p.m. A footnote notes stroboscopic timing and advance figures are taken with the vacuum pipe disconnected.
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Frequently asked questions
- What is the ignition timing for a 1971 MGB (18GK engine)?
- Stroboscopic ignition timing is 15° B.T.D.C. at 1,500 r.p.m. (vacuum pipe disconnected); static ignition timing is 10° B.T.D.C. Earlier engines (18GF/18GH/18GJ) use 20° B.T.D.C. at 1,000 r.p.m.
- What spark plugs and gap does the MGB use?
- Champion N-9Y plugs, gapped .024 to .026 in. (.625 to .660 mm.) for both the to-1971 and 1971 engines.
- What is the contact breaker (points) gap and dwell angle for the MGB's Lucas 25D4 distributor?
- Contact breaker gap is .014 to .016 in. (.35 to .40 mm.) with a dwell angle of 57° to 63°; rotor rotation is anti-clockwise.
- What carburetters does the MGB use and what is the initial jet adjustment?
- Twin S.U. HS4 carburetters. Initial jet adjustment is 14 flats from bridge (to 1971, needle FX fixed or AAE spring-loaded) and 12 flats from bridge (1971, needle AAL). Piston spring is Red in both cases.
- What are the idle speed and valve clearance for the MGB?
- Idle speed is 900 r.p.m. (fast idle 1,300 to 1,400 r.p.m.) and valve rocker clearance is .015 in. (.38 mm.) set cold for both engine versions.
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