Tuning twelve carburettor throats and setting up a freshly rebuilt 1969 Lamborghini Miura S

Tuning twelve carburettor throats and setting up a freshly rebuilt Lamborghini V12 seems like a black art. We get a first-hand look at sorcerer Iain Tyrrell’s approach.

Words NIGEL BOOTHMAN

Photography JONATHAN JACOB

12-Cylinder Sorcery 

What does it take to tune and run-in the Miura? We find out

If Aladdin’s cave was a spacious industrial unit on an airfield outside Chester, and the treasure was dozens of exotic supercars, Iain Tyrrell would be the genie of the lamp. We ring the bell and Iain welcomes us inside with a big smile, leading the way through a distracting obstacle course of Ferrari, Iso, Bentley, Aston Martin and above all, Lamborghini, Iain’s major focus for 40 years. He’s already telling us about his art-and-science approach to tuning the Lamborghini V12 by ear, and his enthusiasm is obvious for a job he still loves.

‘There are some standard starting points,’ he says. ‘We set both distributors so the timing is at 18 degrees before top dead centre, and we have the carburettor idle screws between one and one-and-a-half turns out. We’ll get it running, let it come up to operating temperature, then we can begin.’ Iain turns the key, and the engine bursts into noisy life after only a few seconds of churning. The Miura uses four triplechoke Weber 40 IDL 3C carburettors, which Iain says are essentially the same as those used on early Porsche 911s, but not identical, thanks to their installation at 90 degrees to the Porsche fitment and therefore cornering forces act on them differently. The coffees arrive and Iain sets out our objectives.

‘The idea is to balance the air volume and the mixture at idle for each carburettor,’ he says. ‘We begin by assessing the amount of “pop” in each set of three chokes, balancing each carb against the other three. Then we can balance the individual chokes in one carburettor against each other, using the bypass screws.’ The word ‘pop’ needs some explanation in this context. It’s not one I can understand until Iain passes me the length of rubber heater hose he uses as an ear trumpet, and I place one end against my ear and lower the other into a carburettor choke, as the engine idles. There’s a steady ‘pop-pop-pop’ sound; the firing strokes for the cylinder directly below this choke. Compared with the next choke along, it seems louder and sharper. I say as much to Iain, and he nods agreement.

‘We need to bring the “pop” up to the level of the loudest one, because you can’t make that strongest one quieter,’ he says. Before we get into this fine detail of balancing the chokes against each other, Iain listens to each of the four carburettors as a whole and increases the idle setting on those making less noise than the most assertive one. The throttle linkage has an idle setting for each carburettor and for each pair, front and back, so there’s a decent amount of control. But there’s a bit of a linkage issue. The rod that passes across the top of the engine between the front and rear pairs of carbs has an adjustable threaded section which is set marginally too short, and therefore is not allowing the rear pair to close their throttles fully. Having lengthened the rod, Iain replaces it and re-checks the idle, listening to each ‘pop’ on the rear pair once more, until he’s happy that everything is where it should be.

The V12’s engine speed has risen noticeably by this point, so Iain adjusts it back down with the main idle stops for the front and rear pairs of Webers. Now we can move on to balancing the chokes within each carburettor. This is done using the idle bleed bypass screws. Air is allowed to bleed past the throttle butterflies via a bypass passage, as you unscrew the idle bleed screw and raise it from its wedge-shaped seat.

Iain works around the chokes again. Like a conductor hearing a flat note from the second violin, he frowns, pauses, and picks out what’s wrong. He undoes the 8mm locking nut and tweaks open the screw until the volume and clarity of the ‘pops’ reaches the same as the loudest choke, then tightens the locking nut once more. Finally he’s content with the balance within each carb and brings everything back down to the 900rpm target speed. He asks me for a coin and stands it on its edge on the cam cover, where it balances happily on the smooth-running V12. Everything we’ve done so far has involved setting up the flow through the carburettors at idle, and this is important not only for smooth idling but for efficient and seamless transition off idle as the engine speed increases.

‘Fixed-jet carburettors like these Webers transition through phases from idle to one jet to the next jet,’ says Iain, ‘with the accelerator pump to add extra fuel as well. We know what to expect, based on modern fuels and the specification to which the engine has been built, so it’s very rare to have to change a jet size after a build. Jetting issues are fixed by moving up or down a size rather than adjusting the carburettor.

‘Engines will always tolerate slight richness over leanness,’ he adds, ‘and some actually prefer it – Mercedes-Benz 300SLs, Pagodas and so on. Lean mixture is much more apparent through general unhappiness, no clean pickup, hunting at cruise, poor idle, lacking sparkle in performance.’

‘Sparkle’ is a very Iain Tyrrell word; it’s a grand piece of understatement describing a quad-cam V12 howling enthusiastically to its rev limit. What about the ignition side? What has he learned about a Lamborghini V12’s preferences? ‘I like NGK spark plugs for these engines. They suit in terms of performance, wear resistance and tolerance over time; the electrode insulation lasts well. These original, non-precious metal plugs are hardier than modern iridium alternatives.’

The next step is to check everything works in practice – it’s time for a road test. With the rear clamshell still up, Iain reaches for the main throttle linkage and blips the revs, and the engine declines to respond cleanly. It spits and pops, missing angrily as the revs rise. Iain turns it off saying, ‘It’s running out of petrol’.

Investigations begin. After two fresh cans of Super Unleaded, we try again, but with no improvement. Iain still feels it’s a fuelling fault, and he enlists help from colleague Marcus Germany so one can raise the revs while the other examines each carburettor’s function, and before long, Marcus finds one that seems to be dripping. The tedious job of removing the upper half of all four carbs begins, to compare settings. It’s soon established that the float height on one wasn’t set correctly, and this seems to have combined with a sticky needle valve in the float chamber to allow over-fuelling on the adjacent chokes. Yet Iain is dubious that a comparatively minor fault could be causing our problem.

He turns his attention to the ignition side. Miuras have twin distributors, one for each bank of the V12, and with the caps off and various investigations carried out, Iain discovers that the bob weights controlling ignition advance in the front bank’s distributor are not moving as they should. How they’ve come to be stuck is an investigation for another time, but it’s typical of the kind of glitch sometimes encountered with brand-new or indeed freshly rebuilt items. For now, Iain swaps in a distributor from another Miura – yes, he has four of five others hanging about – to prove the theory. After all, this flaw would explain the problem quite well: perfect idling but rough running as revs rise and the spark timing suddenly differs between banks.

Sure enough, with the tops back on the carbs and the ‘new’ distributor in place, it’s much better. Not quite perfect, perhaps, but good enough for a road test to see if it settles down before re-checking the carburettor settings. For ten or fifteen minutes, all is well, with the Miura singing its song through all twelve cylinders. It’s an amazing engine note, somewhat rawer and more serrated than the cultured howl you get from front-engined Lamborghinis with their longer exhausts. Being alongside Iain, you get a running commentary – ‘Yes, this is sounding nice…could be better though. Doesn’t feel perfect yet,’ and so on. Then the fault – or a set of similar symptoms – returns, and we head back to the workshop.

Nothing now seems wrong with the ignition, so it has to be a fuelling issue. Is the filter blocked? No, it’s definitely allowing fuel past. Is the electric pump working? Well, it’s thumping away when the ignition is turned on. But Iain’s first instinct turns out to be correct. Remember he said ‘it’s running out of petrol’ when the engine first spat and popped, on being revved up? So it was, but not because there was too little in the tank. The fuel pump may be brand new, but it’s also faulty, rattling away like a good’un but failing to draw enough fuel through to keep the 4.0-litre V12 fed at much more than idle. Gentle throttle openings allowed it to reach higher revs, but harder acceleration soon exhausted the float bowls. With a new pump in place, the Miura runs like the fighting bull it’s named after.

Fine-tuning any rebuilt engine is challenging enough without having to uncover and correct three unrelated faults, but it’s trickier still for a twelve-choke, twin-distributor V12. It’s been a fascinating process, watching the wizard at work. It may not seem magic to Iain but a few hours in Aladdin’s cave have been enchanting for us.

‘It’s been a fascinating process, watching the wizard at work’

Fighting bull, fighting fit: let’s cover some miles. Listen and learn: hosepipe is vital tuning tool. Coin test on a Lamborghini? Smooth. Basic settings: linkage must allow throttles to close. Road test miles are a crucial part of Iain’s process.

‘Iain Tyrrell is dubious that a comparatively minor fault could be causing our problem…’ Fighting