A misfire on the XPAG or XPEG is diagnosed by its pattern, not by guesswork. An idle-only misfire, a misfire under load, a high-rev misfire and a permanently dead cylinder each point at a different part of the engine — and once you know which pattern you have, the list of possible causes is short.
A rough-running T-Type invites tinkering, and tinkering usually makes it worse. Almost every misfire has a signature: something in the way it appears and disappears that narrows the field to two or three components. Establish the pattern first, then test.
Every figure quoted here is a guide. Confirm the specification for the engine actually in front of you against the workshop manual before you commit a spanner.
Establish the pattern
Drive the car and pay attention to when the misfire is present and when it is not.
| Pattern | Most likely area |
|---|---|
| Lumpy at idle, clears with revs | Carburettor balance, air leak, tappet clearance |
| Stutters under load or uphill | Ignition: plugs, HT, timing, distributor wear — or fuel supply |
| Only above a certain rpm | Coil, plug gap, points bounce, valve springs |
| Constant, feels down on one pot | One dead cylinder: plug, lead, compression |
| Cuts out and recovers at random | Electrical connection, condenser, ignition switch |
Interactive flowchart
Misfire and rough running — narrow it down
Step 1
When does the misfire show itself? Pick the pattern that fits best.
A misfire that only appears under load is a different animal from one that only appears at idle. The pattern is the diagnosis.
The two-minute cylinder test
Before anything else, find out whether one cylinder is dead. With the engine warm and idling, pull each plug lead off in turn using insulated pliers. Every healthy cylinder will make the idle worse when disconnected. The one that makes no difference is already not firing.
If you find a dead cylinder, swap its plug and lead with a neighbour's. If the misfire moves, you have found the fault for nothing. If it stays put, the problem is inside that cylinder — compression, a tappet clearance, or a valve.
Misfire at idle only
At idle the engine is at its most sensitive to mixture and to leaks, because airflow is tiny.
- Carburettor balance. Two SUs pulling different quantities of air give a lumpy, hunting idle that cleans up with revs. Synchronise the throttles with a flow meter before setting mixture, never the other way round. See SU carburettor tuning.
- Unmetered air. A leaking manifold gasket, a tired carburettor flange gasket or a worn throttle spindle weakens the mixture at idle and becomes irrelevant at higher airflow. If the idle changes when you disturb a joint, you have found it.
- Tappet clearances. A tight valve is held slightly open as the engine warms — a misfire at idle now and a burnt exhaust valve later. Set the clearances hot, with a go/no-go feeler pair: the site guide figures are 0.019 in for TB, TC and TD Mk II up to engine 24115, and 0.012 in for TD and TD Mk II from engine 24116, TF and TF 1500, on the original camshaft.
- Compression. If ignition, air and clearances are all right, measure compression. One low cylinder explains an idle misfire that no tuning will cure.
Misfire under load
Under load, cylinder pressure rises and the spark has to work much harder to jump the plug gap. Marginal ignition components fail here first and behave perfectly at idle.
- Plugs. Clean, file the centre electrode flat, and gap to 0.025 in on all T-Series. Check the reach is right for your head.
- HT side. In the dark, with the engine running, look for blue flashes around the leads, cap and coil. Visible arcing is your fault, found.
- Points and condenser. Fresh points set to the guide gap of 0.010–0.012 in, and a new condenser with them.
- Timing. If it pinks or knocks pulling from low revs in top on a warm day, the timing is too far advanced — back it off until the pinking stops. T-Series cars are timed statically, not with a strobe, because the centrifugal advance is already moving by idle speed.
- Distributor wear. Slack in the shaft bushes scatters the spark timing exactly when load is highest. This is where cheap reproduction parts show up badly — see why your distributor matters.
Misfire only at high revs
A misfire that arrives at roughly the same point in the rev range every time, and feels like a soft rev limiter, has a short list of causes:
- Coil. At high rpm the coil has the least time to saturate, so a tired one runs out of energy at the top end first. Substituting a known-good coil is a quick, conclusive test.
- Plug gap. Too wide a gap needs more voltage than the system can deliver when the sparks come thick and fast. Back to 0.025 in.
- Points bounce. Wrong gap or weak spring tension for the distributor cam fitted, and the points start to float. Set by dwell angle rather than feeler gauge alone — Lucas fitted different cams to T-Series distributors and each needs its own figure. The dwell table is in the complete tune-up guide.
- Valve springs. Weak or broken springs allow valve float at the top of the rev range. Check free length against the manual and look for any spring visibly shorter than its neighbours.
Reading the plugs
The four plugs are the cheapest diagnostic instrument you own. Remove them in cylinder order and lay them out.
| Appearance | What it means |
|---|---|
| Light tan or biscuit | Correct mixture and heat range |
| Black and dry, sooty | Running rich, or a partly blocked air path |
| Wet with petrol | Flooding, or that cylinder not firing at all |
| Wet and oily | Oil past rings or valve guides — check compression |
| White, glazed, blistered | Running weak, or too hot a plug grade |
One plug different from the other three localises the fault to that cylinder. All four the same points at something common: mixture, timing, or fuel supply.
When compression is the answer
If ignition is beyond reproach and the misfire persists on one cylinder, take compression readings — engine hot, all four plugs out, throttle propped wide open, ignition disabled, cranking through several strokes per cylinder.
Guide figures: roughly 100–140 psi for TB, TC and TD, and roughly 110–150 psi for TD Mk II, TF and TF 1500, with no more than about 20 psi spread between cylinders. Then repeat the low cylinder wet, with a teaspoon of clean oil down the plug hole: a jump in reading confirms worn rings, no change points to a valve or the head gasket. Two adjacent cylinders low together suggests the gasket between them.
Fix compression before you touch the carburettors. The full sequence — compression, valve clearances, ignition, then carburettors — is set out in the complete tune-up guide.