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
13 steps
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.
With the engine warm and idling, pull each plug lead off in turn with insulated pliers. Does one lead make no difference at all to the idle?
A dead cylinder is already dead, removing its lead changes nothing.
Are the throttles synchronised and the mixture set, or has one carburettor been adjusted on its own?
Not balanced recently, or fiddled with
Next step
Balance the carburettors before anything else
Two SUs pulling different amounts of air give a lumpy, hunting idle that clears with revs. Synchronise the throttles with a flow meter first, then set the mixture, in that order, never the reverse.
SU carburettor tuning →Spray a little carburettor cleaner or run a rag around the inlet manifold joints and carburettor flanges with the engine idling. Does the idle change?
The idle picks up or stumbles
Do this
Air leak on the inlet side
An unmetered air leak at a manifold gasket, a carburettor flange, or a worn throttle spindle weakens the mixture at idle only, because at higher airflow it becomes insignificant. Renew the gaskets and check the spindles for play.
Have the tappet clearances been checked hot, and is there any sign of one being tight?
Site guide figures: 0.019 in for TB, TC and TD Mk II up to engine 24115; 0.012 in for TD and TD Mk II from 24116, TF and TF 1500, all measured hot, on the original camshaft.
Not checked, or one is tight
Do this
Set the clearances hot and re-check
A tight valve holds itself off its seat as the engine warms, which shows up as a misfire at idle and, eventually, a burnt valve. Set every clearance hot with a go/no-go feeler pair and confirm the correct figure for your engine.
Valve clearances and the tune-up sequence →All correct
Result
Go to compression
Ignition is even, air is metered, clearances are right, so measure compression. A single low cylinder explains an idle misfire that no amount of tuning will cure.
How to run a compression check →Under load the ignition is doing its hardest work. When did the plugs, points, condenser, cap and rotor last get looked at?
Not for a while, or parts of unknown age
Next step
Service the ignition first
High cylinder pressure needs a strong spark. Gap the plugs, 0.020–0.022 in factory or 0.025 in modern practice, fit fresh points and a condenser, and inspect the cap and rotor for tracking. Cheap reproduction parts are a common cause of a load misfire that arrives soon after a service.
The hidden cost of cheap ignition parts →Does it also pink or knock under load on a warm day, pulling from low revs in top?
Yes, there is audible pinking
Do this
Back the timing off
Detonation feels like a misfire and destroys pistons. Retard the static timing until the pinking disappears under load, then confirm the setting properly. T-Series cars are timed statically, not with a strobe.
Setting static ignition timing →Does it feel like fuel starvation, a progressive dying away under sustained load, rather than a sharp stutter?
Yes, it dies away and then picks up again
Do this
Fuel delivery, not spark
Starvation under sustained load points at pump output, a partially blocked line or filter, a silted tank, or a restricted tank vent. The pump diagnostics sequence isolates which in a few minutes.
SU electric fuel pump diagnostics →In the dark, with the engine running, can you see any blue flashes or crackling around the leads, cap or coil?
Yes, visible arcing
Do this
HT breakdown under load
Cylinder pressure makes the spark harder to fire, so a marginal lead or a hairline crack in the cap only breaks down when you ask for power. Renew whichever component is arcing, along with any lead with perished insulation.
Nothing visible
Result
Measure compression, then look at the distributor
Take compression readings and note any cylinder more than about 20 psi below its neighbours. If compression is even, suspect a worn distributor: slack in the shaft bushes scatters the spark timing exactly when load is highest.
Distributor wear and its symptoms →A misfire that arrives at a fixed point in the rev range and feels like a soft limiter is a classic. Are the plugs correctly gapped, and is the coil original or of unknown age?
Plugs wide, or the coil is a mystery
Next step
Gap the plugs and substitute the coil
At high rpm the coil has the least time to saturate, so a tired coil or an over-wide plug gap shows up here first. Gap to 0.020–0.022 in, the factory figure, or 0.025 in if you are following modern practice, and try a known-good coil before condemning anything expensive.
Have the points been set by dwell angle, or just by feeler gauge?
Lucas fitted different distributor cams to T-Series distributors, and each needs its own dwell setting, the same feeler gap can give quite different dwell.
Feeler gauge only
Do this
Set the points by dwell
Points bounce at high rpm when the gap and spring tension are wrong for the cam fitted. Set the dwell angle to the figure for your cam, and renew the points if the spring feels weak or the faces are pitted.
Points gap and dwell figures →Set by dwell, correctly
Result
Look at valve gear and fuel supply at rpm
With the ignition genuinely right, a high-rev misfire is either the valve gear, weak or broken valve springs, or fuel supply that cannot keep up at full throttle. Check spring free length and then pump delivery.
Take out the plug from the suspect cylinder. What does it look like?
Light tan or biscuit is right. Black and sooty is rich, wet and oily suggests oil getting in, and a cracked or bridged plug speaks for itself.
Black and sooty, or wet with fuel
Next step
Cure the fouling, then re-test
A fouled plug will not fire reliably however good the coil is. Fit a fresh plug, run the engine, and see whether it fouls again, if it does, the cause is mixture on that carburettor or oil coming past rings or guides.
SU carburettor tuning →Wet with oil
Do this
Compression and oil control
Oil on the plug means worn rings, bore or valve guides on that cylinder. Take a compression reading dry, then wet with a teaspoon of oil down the plug hole: a jump confirms rings, no change points to a valve or the head gasket.
Compression testing, wet and dry →Swap that cylinder's plug and lead with a neighbour's. Does the misfire move to the other cylinder?
Yes, the misfire follows the parts
Do this
It is the plug or the lead
You have found it without spending anything. Renew the faulty plug or lead, and if a lead has failed, look hard at the others of the same age.
No, the same cylinder still misfires
Result
The fault is in the cylinder
With spark and fuel proven good, that cylinder is not sealing or not breathing. Take a compression reading, check the tappet clearance for that valve pair, and consider a burnt exhaust valve or a head gasket leak to the adjacent cylinder.
Compression and valve clearances →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 them. The factory figure is 0.020–0.022 in for both the TD and the TF; modern practice with a healthy coil is 0.025 in. 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, and a new condenser with them. The manual quotes the gap by distributor type rather than by model: 0.010–0.012 in for the TD's first type, and 0.014–0.016 in for its second type and for the TF.
- 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, or the factory's 0.020–0.022 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.
Frequently asked questions
How do I find which cylinder is misfiring?
With the engine warm and idling, pull each plug lead off in turn using insulated pliers. Every healthy cylinder makes the idle worse when disconnected; the one that makes no difference is already not firing. Then swap that cylinder's plug and lead with a neighbour's — if the misfire moves, the plug or lead is at fault.
Why does my MG T-Type misfire only under load?
Under load, cylinder pressure rises and the spark has to work much harder, so marginal ignition components fail there first while behaving perfectly at idle. Check plug gaps, look for HT arcing in the dark, renew points and condenser, check the timing for pinking, and consider wear in the distributor shaft bushes.
What causes a rough idle that clears with revs?
Usually carburettor balance, an unmetered air leak, or a tight tappet clearance. Synchronise the SU throttles before setting mixture, check manifold and carburettor flange gaskets and throttle spindles for leaks, and set the valve clearances hot.
Is my problem a misfire or fuel starvation?
They feel different. A misfire is a sharp, crisp stutter. Starvation is a progressive dying away under sustained load that picks up again when you back off, and points at pump output, a partly blocked line or filter, a silted tank, or a restricted tank vent.
What compression should an XPAG or XPEG give?
As a rough guide, around 100–140 psi for TB, TC and TD, and around 110–150 psi for TD Mk II, TF and TF 1500, with no more than about 20 psi spread between cylinders. Heads have often been skimmed, so treat these as indicative and confirm against the manual for your engine.