Tuning has a reputation as a dark art. It isn't. An engine tune-up is two jobs, done in the right order — once the engine underneath is mechanically sound.
An engine needs three things
To run well an engine needs air and fuel in the right ratio, a spark at the right moment, and good compression to squeeze the mixture. Tuning is about the first two — the spark and the mixture. The third, mechanical health, is the foundation the other two sit on.
Mechanical health first — you can't tune around a fault
This is the rule every good tuner starts with: a worn-out engine cannot be tuned right. Burnt valves, wrong valve clearances, low or uneven compression, air leaks at the manifold, a worn distributor — sort these before you touch a mixture screw, or you are building on sand. A compression test and a check of the valve clearances come before any tuning.
The two pillars
The spark (ignition)
The right spark, delivered to the right cylinder, at the right instant. This is the job of the coil, the distributor, the plugs and leads, and above all the ignition timing.
The fuel (carburation)
The right amount of petrol mixed with the incoming air, correct across every condition from cold idle to full throttle. On a T-Series that is the job of the twin SU carburettors.
Why the order matters
You set the ignition first, then the fuelling. The reason is simple: how the engine sparks affects how it wants its mixture, but the mixture does not change the spark. Get the timing and ignition right first, and the carburettors have a fixed target to aim at. Do it the other way round and you will chase your tail, re-adjusting the carbs every time you touch the timing.
What ‘in tune’ actually gives you
A well-tuned T-Series starts promptly hot or cold, idles evenly, picks up cleanly without hesitation, pulls smoothly, returns its proper economy, and lasts. A badly-tuned one stalls, hunts, flat-spots and fouls plugs — and is precisely where the myth of the unreliable old car comes from. The car was never the problem; the tune was.
The T-Series is made for this
The good news: an XPAG-engined MG wears its tuning on its sleeve. Twin SU carburettors and a simple points distributor — no electronics, no black boxes — mean the whole thing can be understood and set up at home, with a few tools and this guide. The following pages take each part in turn: start with how a carburettor works. The ignition-side pages are coming next.