Honda GX390 Valve Lash Specs (Adjustment Guide)
On a cold Honda GX390, set intake valve clearance to 0.15 mm (0.006 in) and exhaust clearance to 0.20 mm (0.008 in). Position the piston at top dead center on the compression stroke, with both valves closed. Measure with a feeler gauge, adjust each rocker, tighten its locknut, and recheck the clearances after rotating the crankshaft.
Correct valve clearance helps a GX390 start and run as intended, and it can prevent an incorrect adjustment from adding to a seasonal engine problem. This guide covers the 389 cm³, air-cooled, four-stroke OHV engine. It is not a fuel-mix procedure: the GX390 uses gasoline, not gasoline premixed with two-cycle oil.
I approach a valve adjustment as both a measurement job and a position-checking job. A feeler gauge can show a gap, but only if the engine is cold and the piston is on the compression stroke. Check the model and type code, then consult the manual for the specific engine or equipment. Confirm the procedure and specifications before starting, since an installed configuration or variant may have its own service information.
Equipment Setup and Cold-Engine Safety
This section covers preparation for a safe, accurate adjustment. The GX390 is a four-stroke overhead-valve engine, so its rocker arms and valve stems sit beneath the valve cover. Cool the engine fully, prevent accidental starting, and clear dirt from the cover before opening the valve area.
Tools and preparation
Valve clearance, also called lash, is the small gap between a rocker arm and a valve stem when the valve is closed. A feeler gauge measures this gap. You will need a suitable metric or inch feeler gauge, tools that fit the valve-cover fasteners and adjuster locknuts, and the engine’s manual.
- Stop the engine and let it cool fully. Do not adjust lash on a warm engine.
- Place the equipment on a stable, level surface. Keep the work area clear.
- Disconnect the spark-plug cap so the engine cannot start accidentally.
- Brush or wipe dirt away from the valve cover before removing it.
- Remove the valve cover. Remove the spark plug if needed to make the crankshaft easier to turn.
Do not force a cover or fastener. If the engine is mounted in a machine that blocks access, use the equipment maker’s instructions for safe access and reassembly. Keep loose dirt and debris away from the open valve area.
| Item | What to confirm |
|---|---|
| Engine | GX390, 389 cm³, four-stroke OHV; verify model and type |
| Engine temperature | Fully cold |
| Intake clearance | 0.15 mm (0.006 in) |
| Exhaust clearance | 0.20 mm (0.008 in) |
| Piston position | Compression-stroke top dead center (TDC) |
| Ignition | Spark-plug cap disconnected |
Use the specified cold clearances rather than a guessed value. If the engine’s own manual gives a different instruction for its exact model or type, follow that manual.
Finding Compression TDC and Setting GX390 Valve Lash
This section explains how to locate the correct crank position, measure each valve gap, and make the adjustment. Top dead center means the piston is at the top of its travel. The piston reaches TDC twice in a four-stroke cycle, so identify the compression stroke by confirming that both valves are closed and both rocker arms have clearance.
Find the compression stroke
Turn the crankshaft slowly by hand in its normal direction of rotation. If the spark plug is removed, turning may take less effort. Watch the rocker arms as the crank moves: the intake valve opens and closes during the cycle. After it closes, continue turning toward TDC.
At compression TDC, both valves should be closed, and both rocker arms should have some clearance. A flywheel mark alone is not enough to identify the compression stroke. The same piston position also occurs near the exhaust/overlap part of the cycle, when the valves may not both be closed.
If you are unsure, turn the crankshaft through another cycle and check the rocker-arm movement again. Do not measure or set lash at exhaust/overlap TDC. An adjustment made in the wrong position can leave a valve unable to seat correctly, which may lead to hard starting, lost compression, or valve damage.
Measure and adjust each valve
With the piston at compression TDC, slide the correct feeler gauge between the rocker arm and valve stem. The gauge should move with a slight, even drag. If it will not fit, or moves loosely, the gap is outside the target and needs adjustment.
| Valve | Cold clearance | Inch equivalent |
|---|---|---|
| Intake | 0.15 mm | 0.006 in |
| Exhaust | 0.20 mm | 0.008 in |
Use this sequence for each valve:
- Identify intake and exhaust using the engine’s manual or parts diagram. Do not identify them by guesswork.
- Loosen the locknut for the valve being adjusted.
- Turn the adjuster until the feeler gauge has a slight, even drag at the specified clearance.
- Hold the adjuster in place while tightening the locknut.
- Measure the gap again. Tightening the locknut can change the setting, so correct it if needed.
Do not substitute the intake specification for the exhaust specification. They are different clearances. If the adjuster will not hold its setting, the threads are damaged, or the clearance changes sharply as the crank turns, stop and investigate rather than forcing the adjustment.
Reassembly, Functional Check, and Maintenance
This section covers the checks that follow adjustment. Rechecking the lash after tightening and rotating the engine helps catch a setting that shifted or a crank position that was not correct. Reinstall the cover and plug parts only after the measured clearances remain within specification.
Verify the setting and reassemble
After setting both valves, rotate the crankshaft through at least two full revolutions. Return to compression TDC and confirm that both rocker arms have clearance. Measure intake and exhaust again with the feeler gauge; each should match its cold specification with a slight, even drag.
If either reading has changed, repeat the adjustment and verification steps. A stable reading is more useful than a single measurement taken before the locknut was tightened.
Reinstall the valve cover and spark plug, if removed, using the instructions for the engine or equipment. Avoid overtightening fasteners or damaging the cover gasket. Reconnect the spark-plug cap only after the engine is fully reassembled. Check the engine-oil level using the dipstick or filler instructions for the installed configuration.
The stated oil capacity is approximately 1.1 L (1.16 US qt) when filling a drained engine. That is not a target for topping up an engine that still contains oil. Check the level by the specified dipstick or filler method, and do not overfill.
Start-up check and practical service scenarios
A start-up check can show whether the engine runs steadily after service, but it does not replace a clearance measurement. Start the equipment outdoors, with guards in place and bystanders clear. Listen for stable running and abnormal valvetrain noise, then shut the engine down and look for leaks.
These are practical troubleshooting scenarios, not claims about a particular repair:
- Hard starting after an adjustment: Recheck that the engine was cold and lash was set at compression TDC. Confirm both valves close and remeasure the clearances.
- Clearance changes after tightening: Hold the adjuster steady as you tighten the locknut, then measure again. Do not assume the original reading still applies.
- One valve cannot be set to specification: Check that you are measuring the correct valve and crank position. If the gap still cannot be set, consult a qualified service shop or the model-specific manual.
A noise or starting problem can have causes other than valve lash. If the clearances are correct and the engine still runs poorly, do not keep changing the setting to chase the symptom.
Storage and Seasonal Care
This section keeps attention on preserving the valve adjustment and preparing the engine for later use. Valve lash is only one part of seasonal service. Record the measured clearances and service date, follow the exact engine manual for storage, and recheck the adjustment if symptoms or maintenance instructions call for it.
Use fresh, unleaded gasoline of the grade specified in the applicable Honda manual. Do not premix it with two-cycle oil: the GX390 is a four-stroke engine. For storage, follow the manual’s fuel and engine-preservation instructions rather than relying on a universal procedure.
Keep the valve-cover area clean before any future inspection. If you remove the cover again, prevent debris from entering the engine and use the correct gasket and reassembly instructions. A small note with the intake and exhaust readings can make the next service more useful than relying on memory.
Frequently Asked Questions
These quick answers restate the key adjustment points for the GX390. Use them as a check, not as a substitute for the serial- and type-specific manual. The required condition is a fully cold engine at compression-stroke TDC, with both valves closed.
What is the Honda GX390 intake valve clearance?
Set the cold intake clearance to 0.15 mm (0.006 in). Confirm the exact engine model and type, then use its manual if it specifies different service information.
What is the Honda GX390 exhaust valve clearance?
Set the cold exhaust clearance to 0.20 mm (0.008 in). Do not use the intake value for the exhaust valve.
Should I adjust GX390 valve lash hot or cold?
Adjust it with the engine fully cold. Do not use a warm-engine measurement in place of the cold specifications.
How do I know the piston is at compression TDC?
At compression TDC, both valves are closed and both rocker arms have clearance. Follow the valve movement as you turn the crankshaft; do not rely on a flywheel mark alone.
Can I set the valves at exhaust TDC?
No. TDC occurs on more than one part of the four-stroke cycle. At exhaust/overlap TDC, the valves may not both be closed, which can produce an incorrect adjustment.
What should the feeler gauge feel like?
At the specified gap, it should slide between the rocker arm and valve stem with a slight, even drag. A loose fit or a gauge that will not enter indicates the setting needs attention.
Why recheck clearance after tightening the locknut?
The adjuster can shift as the locknut is tightened. Hold the adjuster steady, tighten the locknut, then measure again to confirm the final gap.
How much oil does a GX390 hold?
The approximate capacity is 1.1 L (1.16 US qt) when filling a drained engine. Check the level using the dipstick or filler instructions for the installed configuration; do not pour in the full capacity as a routine top-up.
Does the GX390 use two-cycle fuel mix?
No. The GX390 is a four-stroke engine. Use fresh, unleaded gasoline of the grade specified in its applicable manual, and do not premix it with two-cycle oil.
What should I do if the engine still starts poorly after adjustment?
First confirm the engine was cold, both valves were closed at compression TDC, and both clearances match specification after tightening. If they do, the starting problem may have another cause; use the engine manual or seek qualified service rather than changing lash without a measurement.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
