Any advance?
QI own a 1979 Rolls-royce Silver Shadow II. It is impossible to adjust the timing as per the workshop manual. At 1300rpm the timing has to be set at 25°BTDC. If all's well it should now adjust itself to 11°BTDC at 650rpm (idle speed). However, it's around 18°BTDC. When I set the timing to 11° at 650rpm, it idles very smoothly. I replaced the vacuum advance to no avail. Measuring vacuum at the manifold, I get
17psi. At one carb I get 15psi when revving it. However, at the other carb, the pressure is virtually zero. I sprayed the carb and all hoses with starting fluid to test for leaks, but there is no difference in engine speed. Could this be an internal carb problem?
Benoit Leus, Schilde, Belgium
Ed says
AWe assume you're measuring carburettor vacuum (as you rev) at the air filter side of each carburettor – in which case the carb with no vacuum is not opening. First, check its throttle butterfly opens when operated by the accelerator linkage. If so, next raise the piston with a small screwdriver. Check that it is free to rise and fall. You should feel the damping action of the dashpot oil, but there should be no friction or points where it sticks. It should fall under its own weight and shut with an audible click. If not, remove and dismantle the carburettor and align the needle as described in the manual. Check the piston and dashpot for physical damage that may restrict movement. If movement is free, make sure there are no signs of anyone having dressed the piston and/ or dashpot with wet and dry paper or even metal polish, as this prevents a vacuum seal between the two parts. There should be a hole drilled near the base of the piston skirt on the engine side to admit vacuum to the dashpot and another at the base of the dashpot to admit air from the atmosphere.
Make sure both are unobstructed.
There is a second important point to explore here, and that's the spark advance of the distributor. In almost all cases, the vacuum advance at a slow idle is zero anyway, so we think this is not the problem. If the manual states the spark advance at different engine speeds, disconnect the vacuum line and check the timing at different engine speeds. Use a strobe timing light which measures engine speed and has a variable timing setting. It is possible the advance mechanism is sticking and not returning fully, or a spring has broken and so it over-advances the timing at low revs. You'll have to strip the distributor to inspect and service the mechanism, as the weights and springs live under the baseplate. Note that many manuals give the advance as measured at the distributor, which is geared to half engine speed – so you may have to double the given speed and advance angle – not forgetting to add the initial 'static' advance to this figure, which we think is the 11º quoted at 650rpm.