Ownership · Tuning
Tuning Your Stag
Tuning a Triumph Stag means getting two systems working together — carburation (twin Zenith-Stromberg 175 CDSE constant-depression carburettors) and ignition (Lucas distributor, points or electronic). It is a methodical craft, done in a set order, and the Stag has specific quirks — most infamously that ignition timing is set on cylinder No. 2, not No. 1 — that catch out even experienced hands.
Accuracy note
All numeric settings on this page are typical figures for the Stag V8. Emission-spec and export cars differ. Always verify gaps, timing, idle speed and plug specification against the official Triumph workshop manual for your specific engine number and market.
Recommended specialist
AG Classic Car Tuning
“Old school tuning for old school cars.”
Setting up twin Strombergs and a Lucas distributor by ear is a genuine skill. For owners who would rather have it done properly — or who want to learn by watching — AG Classic Car Tuning is a mobile classic-car tuning specialist run by Andrew Grabowski, based in Kirkbymoorside, North Yorkshire. He tunes exactly this carburettor-and-distributor setup, travels to the client, and also overhauls carburettors off the car.
- Website
- classic-car-tuning.com
- Phone
- 07500 103227
- Based
- Kirkbymoorside, North Yorkshire · mobile service
The carburettors — twin Zenith-Stromberg 175 CDSE
How constant-depression carbs work
A constant-depression (CD) carburettor uses a spring-loaded air valve — the piston — that rises with airflow. A tapered needle fixed to the piston meters fuel in the jet below, so the mixture stays broadly correct across the rev range. A dashpot damper in the top of the piston controls how quickly it can rise, which governs transient response.
Damper oil
Top the damper tube with SAE 20 or clean engine oil to about 6 mm below the top. Thicker oil sharpens throttle response and slightly enriches the transient mixture; thinner oil softens it.
Two independent jobs: airflow balance and mixture
On a twin-carb Stag you are always setting two things separately. Airflow balance between the two carbs must be checked with a flow meter — a Unisyn or Gunson — and cannot be substituted by adjusting mixture. Set balance first, mixture second.
Mixture adjustment
Mixture is adjusted with the correct Zenith-Stromberg needle-adjustment tool. Clockwise = richer (it raises the needle) — this is the opposite feel to SU carbs, and catches SU-familiar owners out.
Warning: using anything other than the proper tool risks tearing the diaphragm, which is an expensive mistake.
Lifting-pin test
Lift the piston about 1 mm with the lifting pin and judge the engine response: a brief rise in revs that then settles = about right; revs rise and hold = rich; revs drop immediately = lean.
Jet centring
With the damper removed, the piston should fall freely under its own weight and land on the bridge with a clean “thud” — no binding. If it hangs up, the jet needs recentring.
Cold-start / fast-idle
Set the fast-idle gap to about 1 mm so the choke device works correctly. Note that some units have a summer / winter setting which is easy to overlook.
Colortune check
A Colortune plug is the quickest sanity-check of combustion quality: blue flame = correct, yellow = rich, white = lean.
Stag-specific manifold quirk
The Stag inlet manifold does not split cleanly bank-by-bank. The left carburettor feeds cylinders 2, 3, 5 and 8; the right carburettor feeds 1, 4, 6 and 7. A small balance passage links the two planes, so adjusting one carburettor has a slight effect on the cylinders fed by the other. Set, run, re-check.
Common wear items
- Worn throttle spindles causing air leaks — you cannot tune around this.
- Torn diaphragms (often caused by the wrong adjusting tool).
- Worn needles and jets — check both before chasing a tune.
Points, condenser and spark plugs
Contact-breaker points
Points gap is around 0.015 in (0.38 mm), but dwell angle is the more accurate measure and the one specified by the workshop manual. Set dwell with a dwell meter to the manual figure rather than relying on the feeler gauge alone.
Condenser
Replace the condenser whenever you replace the points. A failing condenser causes misfire and burnt points, and diagnosing it is harder than just fitting a new one alongside the service.
Spark plugs
Standard specification with points ignition is Champion N-series (e.g. N9YC) or the NGK equivalent, gapped at 0.025 in (0.6 mm). With electronic ignition and a strong coil the gap can be opened to around 0.032–0.035 in for a fatter spark.
Reading your plugs
- Light tan / biscuit: healthy mixture and heat range.
- Black, sooty: running rich, or short-run fouling.
- White or blistered: lean, or plug too hot for the engine.
The distributor — Lucas
The Stag uses a Lucas distributor with both centrifugal (mechanical) and vacuum advance. Both mechanisms must be free and working for the engine to tune correctly.
Maintenance
Lubricate sparingly, roughly every 3,000 miles: a couple of drops of oil on the spindle and one drop on each point pivot. Over-oiling fouls the points and costs you a tune.
Wear check
Excessive distributor shaft side-play — more than about 0.002–0.003 in — means worn bushings. The result is erratic dwell and poor running that no amount of tuning will cure. Options are rebush, replace the distributor, or fit electronic ignition into a rebuilt housing.
Advance check
With the cap off, rotate the rotor against the centrifugal springs and confirm it snaps back freely when released. A gummed-up advance mechanism will ruin any tune you put on top of it.
Ignition timing — with the Stag-specific traps
Read this first
Ignition timing on the Stag V8 is set on cylinder No. 2 (left-front) — not No. 1. This is the single most common Stag timing mistake, and it leaves the engine badly out even when the strobe appears correct.
Firing order & cylinder layout
Firing order: 1 – 2 – 7 – 8 – 4 – 5 – 6 – 3. The Stag V8 is not numbered by the convention most V8s use, so before touching leads, plugs or the distributor cap, confirm the cylinder layout for your engine.
Static & dynamic timing
Static timing is typically in the region of 10–12° BTDC, but this varies by year and market — verify the exact figure against the workshop manual for your engine number.
Best practice is to set timing dynamically with a strobe at the specified idle rpm (typically ~800–900 rpm) with the vacuum pipe disconnected and plugged. Once set, reconnect vacuum and confirm that both the centrifugal and vacuum advance mechanisms are increasing timing with revs and load.
Typical settings at a glance
Typical figures only — always cross-check with the official workshop manual for your engine number and market.
| Setting | Typical value |
|---|---|
| Carburettors | Twin Zenith-Stromberg 175 CDSE (constant depression) |
| Damper oil | SAE 20 or clean engine oil, ~6 mm below the top of the tube |
| Fast-idle gap | ~1 mm (some units have summer/winter settings) |
| Points gap | ≈ 0.015 in (0.38 mm) — set dwell angle with a meter for accuracy |
| Spark plugs (points) | Champion N-series (e.g. N9YC) or NGK equivalent, gap 0.025 in (0.6 mm) |
| Spark plugs (electronic) | Gap ~0.032–0.035 in with a strong coil |
| Firing order | 1 – 2 – 7 – 8 – 4 – 5 – 6 – 3 |
| Time on cylinder | No. 2 (left-front) — NOT No. 1 |
| Static timing | Typically ~10–12° BTDC (verify against workshop manual for your engine number & market) |
| Idle speed | Typically ~800–900 rpm |
| Distributor shaft side-play limit | ≈ 0.002–0.003 in |
| Distributor lubrication interval | ~ every 3,000 miles (sparingly) |
Electronic ignition upgrades
The appeal is straightforward: no points to wear or burn, stable dwell and timing, easier starting, and near-zero maintenance. The trade-offs are cost, loss of originality, and the need to match the correct coil (and, on some kits, the correct ballast arrangement) to whatever module you fit.
Options commonly fitted to Stags
- Lumenition — optical trigger, long-established classic-car brand, well-regarded.
- Pertronix Ignitor — the original Ignitor I is a simple points-replacement with no dwell control, and can overheat if the ignition is left on with the engine stopped. The Ignitor II adds current limiting / dwell control and removes that trap.
- Accuspark / Powerspark — budget Hall-effect kits; popular for cost, mixed reports on longevity.
- 123ignition — a programmable distributor with selectable advance curves; some models are Bluetooth-adjustable. 123 conversions can fit their internals into the original Lucas distributor body, keeping the correct look under the bonnet and preserving the mechanical tachometer drive.
DIY or specialist?
Mixture, balance and ignition timing all interact. Chase one out of order and the others move. Working through them methodically, with the right tools, is entirely doable at home — but if the car will not settle, or you simply want it set up perfectly, this is exactly the job a specialist such as AG Classic Car Tuning does — mobile, on your driveway. Contact Andrew on 07500 103227 or andrew@classic-car-tuning.com.
Frequently asked
What carburettors does a Triumph Stag use?
Twin Zenith-Stromberg 175 CDSE constant-depression carburettors, one feeding each bank of the 3.0-litre V8. They meter fuel by a spring-loaded air valve lifting a tapered needle in a jet, with a dashpot damper controlling how quickly the piston rises.
What is the firing order of a Triumph Stag?
1 – 2 – 7 – 8 – 4 – 5 – 6 – 3. The Stag V8 is not numbered by the usual convention, so confirm the cylinder layout before rewiring leads or setting timing.
Which cylinder do you time a Stag on?
Cylinder No. 2 (left-front), not No. 1. This is the single most common Stag timing mistake and will leave the ignition badly out even when the strobe reads 'correctly'.
What spark plug gap for a Triumph Stag?
With standard points ignition, 0.025 in (0.6 mm) using Champion N-series (e.g. N9YC) or NGK equivalents. With electronic ignition and a strong coil the gap can be opened to around 0.032–0.035 in.
Should I fit electronic ignition to my Stag?
It eliminates points wear, gives stable dwell and timing, and makes for easier starting with almost no maintenance. The trade-offs are cost, loss of originality, and matching the correct coil and ballast. Common Stag-friendly options are Lumenition, Pertronix Ignitor I/II, Accuspark/Powerspark and 123ignition — the last of which can be built into the original Lucas distributor housing to preserve the look and mechanical tacho drive.