Small Engine Valve Adjustment Explained
In This Guide
Valve adjustment is one of the least known small engine services, but incorrect valve lash quietly causes hard starting, weak power, and rough running, so understanding small engine valve adjustment is worth it on today's overhead valve engines. The valves open and close to let fuel in and exhaust out, and a small clearance, the lash, must be right for them to seat and seal. That clearance drifts as parts wear and seat. This guide explains what valve lash is, the symptoms of it being off, and how the adjustment is done.
What Valve Lash Is
Valve lash is the small gap that lets the valves seat fully when closed.
Each valve needs a tiny clearance between its stem and the rocker arm when closed, so it can seat completely and seal, and so heat expansion does not hold it open. Too little lash and the valve cannot fully close, losing compression; too much and it opens late and clatters. The right lash, set to spec, keeps the engine starting and running well.
Symptoms of Wrong Lash
Valve lash out of spec shows in a few ways.
| Symptom | Likely lash |
|---|---|
| Hard starting, low compression | Too tight; the valve cannot fully close and seal |
| Lost power, especially under load | Tight valves leaking compression |
| Ticking or clatter | Too loose; the valve opens late and rattles |
| Rough idle | Lash off from spec upsetting the timing of the valves |
How It Is Adjusted
The adjustment is precise but not complicated with the right tools.
When to Check It
Valve lash is not a frequent job but is worth checking at the right times.
The Takeaway
To recap: valve lash is the small clearance that lets the valves seal, and when it drifts off spec the engine starts hard, loses power, or clatters. Setting it with a feeler gauge at top dead center restores easy starting and full power.
It is an overlooked service, so if you have chased fuel and spark on a hard starting engine without success, the valves are a good next check.
Valve work is precise, so if you would rather have it set correctly, we can adjust the valves at your home. Our compression guide covers checking the seal, and the hard when hot guide a related symptom.
Rather Have a Real Technician Handle It?
A Wildwood technician comes to your home, diagnoses the engine on site, and gets it running, usually in about an hour. More than 10,000 repairs completed since 2019, with flat rates quoted before we arrive so there are no surprises.