Eric Gorr

Engine Builder
Jun 29, 1999
384
12
Modern four-stroke dirt bike engines have a strong lineage in F-1 and Indy car racing. Yamaha’s partnership with Toyota and Honda’s collaboration with Mugen has given us some incredible dirt bike engines. The average 250cc four-stroke revs to 13,400 rpm. At that rpm the piston is moving up and down in the cylinder at 223 times per second, that’s 111 valve events per second. Multi cylinder F-1 and Indy car engines are toast after a 2-hour race, so why do dirt riders expect their four-stroke singles to last for years without maintenance? The answer for most riders is the fear of dealing with complicated valve train systems. Modern valve trains wear at about the same rate as the piston and rings. Some models have characteristic problems that require specialized repairs. Here is a guide to how valve trains work, how to know when something is awry, and some choices on how to fix up your four-stroke dirt bike.

WHAT’S IN A VALVE TRAIN?
A valve train consists of all the parts that work together to control the phases of a four-stroke engine, intake, compression, power, and exhaust. The valve train consists of a cam chain, cam and crank sprockets, cam chain tensioner, cam chain guides, camshaft, tappets or rocker arms, valves, valve springs, valve seats, and valve guides.

The crankshaft drives a sprocket with a cam chain that runs on plastic guides and backed by a tensioner mechanism. The cam sprockets spinning at half the speed of the crankshaft are fastened to the cam with bolts or by interference press-fit. The camshaft lobes ride against rocker arms or tappets that look like buckets and contain a shim-pad to adjust the valve lash. At the heart of the cylinder head is the valve assembly that includes a valve sealing against a seat that is cast into the head and machined to a series of precise angles to maximize flow. The valve is loaded by pressure from a spring, and sandwiched by a base and a top keeper, and held together by a set of retainers. The valve slides through a tightly fitted valve guide with an oil seal to reduce the leakage into the ports.

WHAT GOES WRONG
Valve train parts wear out. The cam chain and sprockets wear just like the parts that drive the rear wheel of your bike. The plastic chain guides wear down and the automatic chain tensioners eventually extend so far that they fail. The tappets fit between the cam and valve so if the clearance is too tight or loose they take a beating and can eventually fracture. The valve stems run through the valve guide and both of those parts wear allowing the valve to wallow against the valve seat. Eventually the valve face wears, if the valve is left in too long it damages the valve seat. If the engine still starts and runs it will break the head off the valve. You’re lucky if the engine won’t start because the alternative is massive meltdown!

REASONS FOR BREAKING
Valve train parts break for five main reasons. Metal fatigue, lack of maintenance, dirt contamination, loss of lubrication, and riding errors.

The most common reason for valve train problems is metal fatigue. Engineers rate valve train parts in number of cycles. The better the grade of material the higher the number of cycles. Valve springs are the highest wear part in a four-stroke engine. When the spring sacks out the valve is allowed to bounce off the valve seat and accelerate wear of the valve and seat. Stock valve springs last about the same as a piston, 20 to 50 engine hours.

Periodic maintenance like adjusting the clearance between the tappets or rocker arms and the cams is critical to finding fatigued parts and replacing them before an engine failure occurs. Normally the wear pattern makes the clearance tighter as the valve wears against the seat. The rule of thumb is if it takes three or more shim sizes to loosen up the valve clearance then the valve face is worn too far and must be replaced. Shimming the valve with a lot of clearance might buy some riding time but will also cause other valve train parts to be over stressed and break.

Another common problem is damage from dirt seeping past the air filter. The abrasive dirt wears off the protective oxide coating on valves made of titanium. The soft titanium wears fast and typically a cupped shape forms on the valve face, making it impossible to seal. Rust forms on the valve seat, on engines that are stored for long periods of time in warm, humid climates like the southeastern US. When the titanium valve strikes the rusty valve seat the coating wears off the valve. Cupped and leaking valves will usually make the engine hard to start. If the worn valves are left in the engine too long, they can break at the head and stem, causing expensive engine damage.

Running low on oil can cause a variety of problems in the engine, but the most vulnerable area is the cam bearing surface. Yamaha, Kawasaki, and Suzuki machine the head and cam cap as the finished bearing surface. If the engine runs low on oil, the cam can seize in the bearing and in most cases it can damage the cylinder head requiring complete replacement.

The most unusual and catastrophic valve train failures are caused from over-revving the engine to the point where the valve springs can’t react quickly enough and allow the valves to float and collide with the piston. The valves break causing the severe engine seizure and stick you with a repair bill of about $2,000. Riding circumstances that cause valve train failures involve downshifting for turns or as an engine brake on steep down hills. In motocross, landing hard from a big jump in too low of a gear can cause the engine to over rev and float the valves. Although all modern four-stroke dirt bikes have rev limiters built into the ignition system, those devices can’t prevent the engine from over-revving from gearbox loads like downshifting on jumps.

LOOKING FOR TROUBLE!
Worn valve train parts give you a fair bit of warning in advance. Funny noises, vibration, oil leaks, hard starting, popping, and just general loss of power are all red flags signaling potential engine problems. If you remove the valve cover every four rides and perform a quick check of the valve clearance, you’ll have a better chance of detecting worn valve train parts before they cause an expensive engine failure.

When the valves get cupped and lose seal, the first indication will be hard starting. The next sign is popping on idle and at high rpm. A grinding and slapping noise at idle might be worn cam chain guides or the tensioner. An unusual sign of trouble is the carb blowing off of its mounting spigot. That is caused by combustion gasses burning past the valve seat and through the carb, causing fresh gasses to ignite and blow the carb off of the rubber-mounting spigot.

When rocker arms and tappets start to crack the engine will make a loud clattering noise moments before it stops abruptly. Well before a failure occurs, the tappets or rockers will show wear patterns on the rolling surfaces. Tiny nicks or fracture lines on tappets or groove marks on rocker rollers are a sure sign of immanent failure.

Cams can seize from lack of oil and stall an engine. When the engine cools down and is possible to start, a seized cam bearing will make a shrieking sound or noticeable clicking sound at idle.

Most bikes have automatic cam chain tensioners. As the chain and guides wear, a spring puts pressure on a plunger that rides against the rear chain guide. The greater the wear on the chain and guides the less the spring tension applied by the tensioner. Sometimes the tensioners fail abruptly allowing the chain to jump one tooth on the cam and crankshaft sprockets. The engine may even continue to start but the power will be sluggish and make a clicking sound at idle.
 

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