On most older overhead valve engines, valve adjustment is the process of adjusting the gap between the top of the valve stem and the rocker arm that pushes the valve open. It is not difficult to do, especially on a vintage 1973 Moto Guzzi El Dorado. The El Dorado has a beautiful V-twin engine with delightfully accessible cylinder heads. On most other motorcycles, half the time required to adjust the valves involves removing things (fairings, fuel tanks, coils, etc.) for access to the valve adjustment mechanism. Not so on the Guzzi — the maestros of Mandello made it easy.
The reason we adjust an engine’s valves is to make sure the valve seats against the cylinder head and closes off the intake or exhaust port fully when the engine is at its operating temperature. When an engine’s piston is at the top of its compression stroke (top dead center), both the intake and the exhaust valves should be seated against the cylinder head for a couple of reasons. When the engine compresses and ignites the fuel air mixture, the combustion pressure drives the piston down; if either the intake or the exhaust valve is not fully seated, the engine loses power. Another reason the valves should fully close is heat dissipation; the valves’ heat will dissipate into the cylinder head. If the valve does not contact the valve seat, the valve will overheat, burn, and leak, possibly damaging the valve seat as well.
Engine designs incorporate a gap or clearance in the valve train to accommodate thermal expansion. As the engine warms, the valve-to-rocker clearance approaches (but should not reach) zero, and everything works the way it is supposed to. The valves close, engine compression is good, and valve heat is dissipated. Over time, though, as the valves go up and down millions of times, very small amounts of deformation occur in both the valve and the valve seat. This microscopic wear grows over time as the engine runs, and it reduces the valve-to-rocker clearance. Eventually, this wear will result in the valve being held off its seat when combustion occurs, and when that happens, hot combustion gases escape around the valve and the valve cannot dissipate enough heat into the cylinder head. We avoid this by adjusting the valve clearance, which keeps the gap within an acceptable range.
Valves should be adjusted at intervals in accordance with the manufacturer’s recommendations (for our Moto Guzzi El Dorado, it is every 3,000 miles). Indications that valves require adjustment include hard starting or silent valves. As valves move up and down and the gap between the valve stem and the rocker closes and opens, it makes a ticking sound. Hearing the ticking is a good thing; it means that clearance exists. (There is an old saying: “Tappy valves are happy valves.”) It means that it is better to have some gap (with its attendant ticking as the valves open and close) than no gap (which means the valves are not fully seating). As Moe Moore, Cycle Garden’s head honcho explains it, properly adjusted valves should “sing.”
There are different approaches to how engines open and close their valves, and the process for adjusting the valves in different engines is different. This article presents the best approach to valve adjustment on a pushrod engine that uses a threaded adjuster screw and nut system for clearance adjustment. The valve adjustment process is different for overhead cam engines that use shims and buckets, or for desmodromic engines. Some engines (e.g., Harley Big Twins) use hydraulic valve lifters; these engines require no valve adjustment. The purpose, however, is the same for all engine designs: to prevent the valve gap from going to zero.
Required tools for adjusting the valves include appropriate sockets, combination wrenches, Allen wrenches, pliers, feeler gauges, shop rags, and a flashlight.
Lindsay, one of the talented expert technicians at Cycle Garden, walked us through the valve adjustment process. The folks at Cycle Garden are the world’s experts for all things related to pre-1976 Moto Guzzi motorcycles, with a particular emphasis on concours-level restorations, service, and routine maintenance of Ambassadors, El Dorados, V700, and V7Sport models.
1 Remove spark plugs and pulley cover
Make sure the engine is completely cold (it is best to let the motorcycle sit overnight before adjusting the valves). Remove both sparkplug leads and both spark plugs. Remove the pulley cover bolts and remove the pulley cover, taking care not to scratch the front fender while doing so.
Remove the valve cover from the cylinder head, using a cross-torque removal pattern. If the valve cover sticks to the gasket, tap valve cover lightly with a soft mallet. Set the valve cover aside on a shop rag where it will not be damaged.
2 Find top dead center
Use the large nut at the front of the crankcase with a socket wrench to rotate crankshaft clockwise (as viewed from engine’s front) to bring right side piston to top dead center. Lindsay accomplishes this by holding a finger over the spark plug hole and feeling the compression and then viewing the piston position through the spark plug hole.
On many motorcycles, finding top dead center can be accomplished by removing an access plug and viewing a mark on the flywheel, or observing marks on the alternator rotor. The 850cc El Dorado engine has a viewing port on the lower right crankcase, but Lindsay and the Cycle Garden crew advise against this on older motorcycles, particularly the Guzzi. “On these 50-year-old bikes, you don’t know how many times the engine has been apart, and you don’t know if it was reassembled with the flywheel oriented correctly to the crankshaft,” Lindsay advised. Steve, another expert Cycle Garden technician, added that the Guzzi flywheel can be installed in any of six positions with respect to the crankshaft, but only one position is correct with respect to its top dead center markings. The motorcycle will run just fine if the flywheel has been mis-indexed to the crankshaft, Steve added, but the flywheel markings will be offset from where they should be. The Guzzi’s flywheel markings are S and D (sinistra and destra are Italian for left and right cylinders), but again, do not rely on these for finding top dead center.
Confirm the piston is at top dead center by grasping and wiggling the rocker arms up and down. Both the intake rocker (at the rear of the cylinder head) and the exhaust rocker (at the front of the cylinder head) should have a small amount of up-and-down play above the valve stem.
3 Check valve clearance, adjust if necessary
Check the valve clearance by inserting a feeler gauge between the valve stem and the valve rocker. For our Guzzi, the intake valve clearance should be 0.006 inch; the exhaust valve clearance should be 0.008 inch (check the shop manual for your motorcycle’s intake and exhaust valve clearances). If both valve gaps meet their requirements, the valves do not require adjustment. This article continues with the assumption that both valve clearances (intake and exhaust) require adjustment.
Grasp the adjuster screw with an open end or Crescent wrench and hold it so that it cannot rotate. Use a box or open end wrench to loosen the threaded adjuster locknut. Turn the threaded adjuster screw so that it unscrews enough to insert the feeler gauge’s 0.006-inch leaf between the intake valve stem and the intake valve rocker. Screw the threaded adjuster screw in such that a slight drag is felt on the feeler gauge if you attempt to pull it from the gap. The 0.006-inch feeler gauge leaf should not slide easily into the gap, nor should it be locked in place. You should feel just a slight drag on the gauge as you attempt to slide it into or out of the gap. Do not overtighten the adjuster lock nut; if tightened too much, the valve will begin to open. Once the adjuster screw has been turned to provide the appropriate valve clearance, tighten the adjuster screw locknut while holding the adjuster screw in place with the wrench to prevent it from turning. Withdraw the feeler gauge and then insert it again to make sure the gap has not changed. If it has, repeat the process.
Repeat the above process for the exhaust valve, this time using the 0.008-inch feeler gauge (check your shop manual for your motorcycle’s exhaust valve clearance). The exhaust valve clearance will usually be larger than the intake valve clearance because the exhaust valve runs hotter and may expand in length more than the intake valve.
After adjusting the valve clearance for the motorcycle’s right cylinder, repeat the process for the motorcycle’s left cylinder. Remove the left cylinder’s valve cover and rotate the crankshaft clockwise (as viewed from the front of the motorcycle) until the left piston is at top dead center (note that the crankshaft’s top dead center position will be different for the left and right cylinders). Adjust the intake and exhaust valve clearances as described above for the right cylinder.
After completing the above, rotate the crankshaft two full turns, find top dead center for each piston, and verify the valve clearance again for each cylinder. If either side is out of specification, repeat the above process.
After adjusting the valves as described above, assembly is the reverse of disassembly.
Special thanks to Cycle Garden and Lindsay, Steve, and Moe!
Cycle Garden is the world’s largest full-service Moto Guzzi restoration and maintenance operation; they keep an extensive inventory of new and used Moto Guzzi parts in stock. If you need any parts described above, visit CycleGarden.com or call 760-972-4800.
