When early 450 SLs have a rough idle it can be vacuum leaks. The eight doughnuts under the intake manifold ducts are often dry and cracked introducing unwanted air to a factory preset mix of gasoline and air. This imbalance will create a lean miss-ﬁre behavior to occur to an otherwise well tuned 450SL engine. In fact all R107 V-8s from the 350 to the 560 engine can skip at idle because of these eight leaky intake manifold gaskets. A more subtle cause of a not so smooth idle on these early 450s can also be uneven valve adjustment clearance. Sometimes not paying very close attention to the feeler gauge measuring tool will cause even half thousandths of an inch difference in neighboring valves and will cause idle issues.
What you are effecting when you adjust valve clearances is controlling when a valve begins to open inside the cylinder head. A half thousandth’s of an inch difference in clearance messes with valve timing and the harmony of each cylinder’s uniform and balanced valve timing. Intake valves open to initiate engine vacuum. When you are performing this valve adjustment - think “even valve timing’’ being the most important thing. An engine with perfect valve timing is a beautiful thing to hear. Valves do get out of adjustment. When all six or eight cylinders are even slightly out of adjustment the nice smooth engine idle doesn’t exist - no matter how well the rest of the tune-up is performed.
If you are lucky and are able to carry out a valve adjustment on one of these early 450s - there is one other thing to consider. Usually I ﬁnd the engine is more responsive when the valves are set on the tight side. This bias allows the valve to open a little on the early side which compensates for worn valve train components. Even a slightly worn timing chain and sprockets will cause valves to open slightly late. The goal is to match piston position with valve opening - so trying this approach often makes for good timing harmony. Good luck and call with any questions!
Peter Vanderveer is our Technical Editor and the owner/operator of Vanderveer Motors in Norwood, Mass. He can be reached at 781-255-0797