Oil changes, brakes, chain and sprocket checks are all part of basic routine motorbike maintenance. Less talked about is the routine exercise of checking the valves on a motorbike. Depending on where and how you ask Google, valve checks can be as regular as every 10,000km, or up to around 40,000km. In terms of Vietnam, valve checks are never done, and most mechanics don’t even know how to do it. The few and far between mechanics that do understand the concept will use locally CNC shims which are then sand papered down to size by the mechanic, which is not exactly a scientific approach to the situation.
Why do a valve check
Valves that are not in spec cause a wealth of motorbike performance issues. Usually when a motorbike is cutting out on the driver, a mechanic will blame the spark plug first, then they will then blame the carburetor second, and if that fails, they will blame the ECU third. By this time the client has lost faith in the mechanic, and moves onto the next mechanic who starts the diagnostic process from scratch beginning with the spark plug again.
The valve diagnosis is left out because mechanics don’t know how they work, this is engine related and out of the comfort zone of most street trained mechanics.
Valves that are too tight or too loose will both cause starting issues and idling issues. When they are out of spec, valves will rapidly start to wear down until failure, at which point the motorbike will lose compression and give up on life. A failure in valves can lead to piston damage, which can lead to cylinder head damage. Depending on your luck at the time of the breakdown, it can be a simple top end rebuild to a very expensive rebuild.
The point of regular valve checks is to have the motorbike running in optimal condition at all times and to avoid a needless catastrophic engine failure.
How much does a valve check cost
Valve checks depend on the motorcycle and how many valves it has. But as a general guideline for most single cylinder motorcycles (which is what we find in Vietnam), it will take two hours of a mechanics time which at Tigit is a total of 700,000VND. If the valves do need adjusting, we use imported HotCam shims and we will get everything in spec for 2,000,000vnd.
Overseas a standard valve check on a motorcycle is in the region of $150 before parts.
How do valves wear out
The above video explains everything in a simple format, but ultimately the valve can wear from the seat when it is cold or from the stem when it is hot. The seat effect causing the valve clearance to get smaller, with the stem effect causing it to get bigger. The take away is that keeping the valve clearance at the right spec helps avoid excessive wear and tear. Through this process a mechanic will be able to detect a worn valve which will save from a catastrophic engine failure. Valves are not cheap, but they are also not hugely expensive. Detecting a failing valve is well worth it, especially when it helps keep the performance of the motorbike in optimum shape.