How to Adjust Valve Lash on an Air-Cooled Standby Engine

Why this matters

Valve lash is the small clearance between a rocker arm and the top of the valve stem when the valve is closed. On an air-cooled standby engine it is the difference between a valve that seats every cycle and one that never quite closes. Too loose and the valvetrain ticks, the valve opens late and lifts less, and power falls off. Too tight, and the worse case: the valve is held slightly open, hot combustion gas leaks past the seat, the exhaust valve burns, compression drops and the engine gets hard to start. Exhaust lash on these engines tends to close up with wear rather than open, so a unit that has never had its lash checked is drifting toward the tight, expensive side. Lash, torque values and the check interval are all per the engine maker's service manual.

Before you touch the engine

Lock it out. Controller off, auto-start disabled, battery handled per the shop's procedure, so the engine cannot crank while your hand is on the flywheel or a rocker. See related: The Generator Auto-Start Lockout Before Service SOP.

Let it cool fully, which here is also the spec. The muffler and cylinder heads stay hot enough to burn for a long time after a run. Separately, lash specs on these engines are cold specs: aluminum heads grow when hot and the clearance changes with them, so a lash set on a warm engine is set wrong. If the unit exercised that morning, come back to it or do the rest of the visit first. Room temperature to the touch on the heads is the start condition.

Pull the plugs and keep the leads off the plugs. Removing both spark plugs lets the engine turn easily by hand and removes the chance of a firing. Clip the plug leads to engine ground or tuck them clear of the plug holes.

What you are measuring

  pushrod end        locknut + adjuster
      |                    |
      v                    v
   ===+===================[O]===================+
      |     rocker arm     pivot                | tip
      |                                    [ feeler ]
      | pushrod                                 | valve
      |                                         | stem
      |                                      spring
   lifter on cam                         valve CLOSED

  piston at TDC on compression: both rockers loose

The gauge goes in the gap between the rocker tip and the valve stem tip. Turning the adjuster raises or lowers the rocker on its pivot and changes that gap; the locknut holds the adjuster once the gap is right. Some engines use a screw adjuster at the pushrod end instead of a pivot nut; the principle is the same and the manual shows which.

Step 1: Get the spec and the gauge

From the engine maker's service manual for this exact engine, write down the intake and exhaust lash (they are sometimes different), whether they are cold specs, the locknut torque and the valve cover bolt torque. Use flat feeler gauge blades in clean condition; a kinked or rusted blade reads tight.

Why first: two engines that look identical can carry different specs, and remembered numbers are where a tight exhaust valve comes from.

Step 2: Remove the covers

Take off the enclosure panels and any shroud that covers the valve covers, then the valve covers. Have new valve cover gaskets on hand; these gaskets take a set and a reused one leaks oil onto a hot cylinder.

If you clean old gasket material off with solvent, do it with the enclosure open and air moving, nitrile gloves on, and keep solvent off the hot side of anything. Do not scrape so hard the aluminum cover face gets gouged.

Step 3: Find top dead center on compression for cylinder 1

Lash is only valid with both valves of that cylinder fully closed, which happens at top dead center (TDC) on the compression stroke. At TDC on the exhaust stroke the valves are overlapping and the reading means nothing.

1. Turn the engine by hand in its normal direction of rotation, using a wrench on the flywheel nut, never by pulling on the fan or a belt.
2. Watch cylinder 1's intake rocker. It goes down (valve opens), then comes back up (valve closes).
3. Keep turning slowly. Insert a plastic straw or wooden dowel into the plug hole, not a screwdriver, and feel the piston rise to the top and just start to come back.
4. At the top, both rockers on cylinder 1 should be loose, with a little play you can feel by wiggling them.

If a rocker is tight at what you think is TDC, you are on the exhaust stroke. Turn one more full revolution.

Step 4: Check intake and exhaust

Slide the blade of the spec thickness into the gap on the intake rocker. Right is a light, even drag: the blade moves with a slight pull but does not need forcing and does not drop through loose. Then the same on the exhaust.

Check one blade under and one over the spec. If the spec blade drags, the under blade slides freely and the over blade will not go in, the lash is right and you leave it alone. Every adjustment risks disturbing a good setting, so only adjust what reads out.

Step 5: Adjust and lock

1. Hold the adjuster and loosen the locknut.
2. With the spec blade in the gap, turn the adjuster until the blade drags lightly.
3. Hold the adjuster still and tighten the locknut to the maker's torque.
4. Check again after tightening. Locking the nut often pulls the adjuster and changes the gap by a thousandth or so. If the gap moved, loosen and repeat until the reading holds with the nut torqued.

Step 6: Cylinder 2

Turn the engine in its normal direction until cylinder 2 reaches TDC on compression, found the same way: watch its intake valve open and close, then feel its piston rise with the straw. On a V-twin the two cylinders do not reach TDC 180 degrees apart, so do not count turns; watch the valves. Then repeat steps 4 and 5 on cylinder 2.

Worked example: one tight, one loose

A two-cylinder air-cooled standby at about 900 hours, never had its lash checked. The service manual (illustrative values for this example) gives intake and exhaust lash of 0.002 to 0.004 inch, cold. The shop sets to the middle, 0.003, and checks with 0.002, 0.003 and 0.004 blades, reaching for other blades in the set when a gap reads outside that range.

Two of four valves were in spec and left alone; two were out and adjusted.

Reading the tight exhaust. Cylinder 1 exhaust at 0.001 is the one that matters. Lash on an exhaust valve closes when the valve face and seat wear and the valve sits deeper in the head, so the stem rises toward the rocker. At 0.001 cold, the clearance can close to nothing when the head and valve grow at operating temperature, and then the valve stops seating fully. So while cylinder 1 is still at TDC on compression, the shop runs a leakdown test through its open plug hole, holding the flywheel wrench because shop air on a piston slightly off TDC will try to turn the engine. Air hissing out of the muffler means the exhaust valve is not sealing. Cylinder 2 gets the same test at its own TDC for a baseline, and a cylinder 1 figure well worse than cylinder 2's is head work, not lash. The gauge reading and where the air escaped go on the record.

Reading the loose intake. Cylinder 2 intake at 0.006 opened, which usually means wear at the rocker tip, pushrod ends or pivot, or an adjuster that backed off. After setting it to 0.003, wiggle the rocker and look at the pushrod ends and rocker tip for a cupped or pitted face; worn parts set to spec will open up again quickly.

What would change the next interval. If cylinder 1 exhaust is tight again at the next check, recession is continuing and the valve and seat need attention. On engines running gaseous fuel, seat wear is a known pattern, which is part of why makers set a lash interval for standby units at all.

Step 7: Close up and restore

1. Fit new valve cover gaskets and torque the cover bolts to the maker's value in the pattern the manual gives. Over-torque distorts the cover and it leaks anyway.
2. Reinstall the spark plugs to the maker's torque and reconnect the leads.
3. Refit shrouds and enclosure panels; a missing air shroud on an air-cooled engine starves the cylinders of cooling air.
4. Restore the battery and controller per the lockout procedure.
5. Start the unit from the controller in manual. Listen at the valve covers with the enclosure open and hands clear of the fan and exhaust: a light, even sound is right; a sharp tick from one side is a loose valve or a blade that was set too loose; a rough idle or a miss is worth a second look at the tight side.
6. Run it long enough to reach operating temperature, then stop and check both valve cover gaskets for oil seep.
7. Return the controller to automatic and confirm ready in auto.

How to verify you got this right

• Both cylinders checked at TDC on compression, confirmed by loose rockers on that cylinder.
• Every out-of-spec valve rechecked after the locknut was torqued, with the final reading on the record.
• New cover gaskets, covers and plugs torqued, shrouds back on.
• Clean, even valvetrain sound at operating temperature, no oil seep, unit in automatic.
• Found and final lash for all four valves written on the job record, so the next tech can see whether the exhaust is still closing.

References

• The engine maker's service manual for lash by valve, cold specification, locknut and cover torque, TDC procedure and interval
• See related: The Generator Auto-Start Lockout Before Service SOP; Generator Air Filter and Spark Plug Service for Air-Cooled Units; Generator Oil and Filter Service