ATV Valve Adjustment Symptoms (Top 7 Signs)


When your ATV loses some of the fire in its belly and you can’t quite put your finger on it. It may be time to check the valve lash. Your ATV engine is designed with a ton of power, and it’s still there, waiting to be tapped.

The top seven symptoms of poor ATV valve adjustment, include:

1 ATV won’t start
2 Engine noisy
3 Engine misfires
4 Bike lacks power
5 Engine runs rough
6 Erratic idle
7 Hard on gas

By the end of this post you’ll understand and recognise common valve lash symptoms, know which condition your engine suffers from and how to fix it.

What Is Valve Lash

Valve lash or clearance is a factory specified gap that must be maintained between rockers and valve tips. As an engine wears, the valve clearance usually gets larger and so needs to be adjusted.

Unfortunately on some bikes gaining access to the valves is a bit of a chore and so is regularly ignored, until it can’t be.

All single cylinder engines will have at least two valves, one inlet and one exhaust, however most bikes will have four, two of each.

The valves as you know are activated by the rockers which are driven by the camshaft. The camshaft is driven by the crankshaft and the whole process is sequenced.

The timing of this sequence is mission critical to the optimum performance of your power plant.

ATV 4 Stroke Engine

Only four stroke engines have valves, and valves allow for a more accurate, predictable, smoother and cleaner engine performance.

A four stroke engine is just easier to live with, but because it has more internal moving parts than a two stroke engine, it needs more maintenance.

The 4 strokes are:

1 Intake stroke – Intake valve(s) opens as the piston moves down the cylinder and closes again as the piston bottoms out.

2 Compression stroke – Intake valve(s) now closed as the piston moves up the cylinder, compressing the the air/fuel mix into the combustion chamber at the top of the cylinder.

3 Power stroke – The spark plug fires and ignites the compressed mix causing the piston to move down the cylinder under power.

4 Exhaust stroke – Exhaust valve(s) open as the piston moves up the cylinder pushing the spent gases out. The valve is fully closed as the piston reaches the top of the cylinder, and the cycle starts over.

ATV Won’t Start

An ATV may not start for many reasons, incorrect valve lash will certainty be on the list. Hot start problems are often caused by valve lash issues. As the engine reaches operating temperature, lash tolerances change. If the valves are too tight, they may remain open which will reduce cylinder compression.

An engine without compression won’t start.If the valve lash is excessive your valves won’t open and the engine won’t receive fuel and so won’t start.

Engine Noisy

A noisy engine is a useful clue, the tell tale ticking noise of an idling engine could very well be miss adjusted valve lash.

Loose rockers as you can guess, will make a racket if the gap between the rocker and valve tip is excessive. And you already know that excessive lash will cause poor running or no start.

Excessive lash if neglected can damage the valves and rockers as they hammer together.

Checking the lash is obviously the best test, but I’ll often use a stethoscope to confirm. Especially useful if access to the valve cover is labour intensive.

Engine Misfire

A misfiring engine could be caused by miss adjusted valve lash. Excessive lash will cause the engine to run without enough fuel, and that will cause the plug to misfire.

Running the engine with tight valves can also cause the plug to misfire. If the valves are not fully closing, the engine mix is lean and that will as you know cause a misfire.

Lacks Power

If your engine lacks power, maladjusted valves could be the cause. Incorrect valve lash, causes the valves to open for more or less time than needed.

Excessive valve lash means the valves open late and for less time than needed. This causes a lack of gas which in turn causes a lack of engine performance.

Tight valves can cause the valves to open early and for too long. This causes (depending how tight they are) lean running, hot running, misfiring and general lack of performance.

Runs Rough

A rough running engine is irritating but worse than that, if it’s caused by maladjusted valves, you could cause lasting damage to the motor. If valves are tight it may cause the engine to run lean, a common symptom is popping noise from the engine.

Overheating engine, burnt valves, hammered valve tips, blown head-gaskets are all real risks that will cost a packet to fix.

Erratic Idle

Valve clearance will effect idling big-time. If valves are tight, you might find the engine starts but then dies as it warms up. Or the idling is even rougher when warm.

Whatever the symptoms your engine needs a valve check after xxx hours running. If own a used bike, I’d assume it hasn’t been done.

Sure adjusting the valve lash is a bit of a chore on some bikes, but it’s totally worth it. You will notice the difference right away. The engine will start more easily, quieter and throttle response will be sharp and more power.

I wrote a whole post about adjusting your valve lash, check it out here xxxxxxxxxxxxxxxxxxxxxxxxx

Hard On Gas

When ever performance suffers so to does gas mileage. It simply takes more gas to do the same work. So anytime you notice gas mileage drop, it’s a big clue that your ATV has a problem.

The problem could be something simple like a dirty spark plug or more serious altogether, either way, early diagnoses is always best.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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