Autovac – curing an unwelcome buzz from the T-elbow (restrictor valve)
Some years ago a strange buzzing sound started coming from the direction of my engine compartment. At first I heard it while driving and it sounded as though it came from behind the instrument panel, so I looked for an electrical problem, something sparking, something getting hot etc, but to no avail. After a week or two of this persistent and intrusive buzzing, I happened to open the bonnet with the engine idling. Now it sounded as though the engine itself was buzzing. I walked round the front and noticed that buzzing seemed to emanate from the radiator! I jumped into the driving seat, and it definitely came from the back of the instrument panel! It’s a funny thing – noise transmission in a Rolls-Royce. Luckily I fairly quickly traced it to the ‘T’-elbow (restrictor valve) at the end of the vacuum pipe on top of the Autovac, see Figure. When I unscrewed the end cap (A) of the ‘T’-elbow and touched the end of the plunger (B), the buzzing stopped. The plunger itself was vibrating, like a reed in a clarinet, and transmitting the noise all over the engine compartment.
After contacting the Autovac Company, I learned that this “sometimes happens” and might be cured by fitting a new elbow. My reactions were (a) surprise and (b) why? Being a schoolboy at heart and always wanting to find out “why” I decided to investigate. Dismantling the elbow is easy except that you can lose the tiny internal spring clip which holds the plunger in, but this is unnecessary anyway.
Firstly I tried lubricating the plunger and spring with PTFE grease, which is not easily washed away by petrol. The buzzing quickly returned. Then I noticed that, whereas the outer end of the spring is firmly held within the recess in the plunger, the inner end of the spring merely presses on to a relatively wide platform (C) and is free to move side to side, see Figure. So I cut a washer (D) approximately 12.7 mm o.d. and 9.7 mm i.d. from a convenient red fibre washer. After gluing it on to the platform with petrol-resistant sealant, it should hold the inner end of the spring (E) in place. Result: the spring and plunger stopped vibrating and the buzzing has never returned. Recently I replaced the fibre washer with an o-ring that fitted snugly.
While the elbow is detached, test the one-way valve (diagram) by checking that you can blow through it in one direction, but not the other.
The ‘T’-elbow was introduced in 1927 and restricts the amount of suction applied to the inner chamber of the Autovac, especially when the inlet manifold vacuum is high, e.g. when idling or on the over run. The ‘T’-elbow was designed so that higher vacuum pulls in the plunger against the spring, and thereby progressively chokes the vacuum pipe connecting the Autovac to the inlet manifold. This overcomes problems of uneven idling, which can result when large gulps of petrol-saturated air are sucked at intervals into the rear of the inlet manifold.
The whole system for supplying petrol to the carburettor via the Autovac is normally trouble-free, bearing in mind the mechanical precision of the Autovac itself and the age of all the other components. However things do go wrong occasionally with 90-year-old machinery, and when there is a problem it can be frustratingly difficult to diagnose, according to various reports in RREC publications. Our cars were remarkably well designed and built; members have toured all over hot countries like Malaysia and Jordan with no problems. In the 20hp, the petrol supply mechanism starts with the petrol cap, and ends with the needle valve in the carburettor and anything in that long chain can develop a fault. The symptom may be intermittent loss of power under certain running conditions, but it seems that analysis and cure of the problem can involve a shoal of red herrings.