Almost any dirt biker you meet today will agree that four-stroke dirt bikes are expensive when it comes to rebuilding them, especially if you have a shop do it. They have a lot more moving parts in the engine than a two-stroke engine does. This means Butterfly Valve that there are more things that can go wrong, and when something does break, it results in a domino effect that often grenades most, if not the entire engine.
There are ways to prevent these expensive repairs from happening. So instead of having your bike in the shop or sitting in the garage for weeks, or even months because you don't have the money, time, or parts, you could still be riding because you did some "preventative maintenance".
If you do a little research, you'll see that valves can get fairly expensive, considering you usually have to buy four or five of them. What will cost even more is when one of them breaks and puts a hole in your piston, resulting in a destroyed top-end. As far as timing chains, this is probably the most disregarded part in the top-end when it comes to maintenance. Many people think that timing chains will last hundreds of hours without failure because they're just spinning on the crank and cam(s). Well with the crank spinning from 1,500 rpm up to 12,000 rpm, and constantly accelerating and decelerating, the cam/timing is going to wear out eventually. When it does have to snap or break, it will seize the engine and more than likely destroy most of the parts.
Now I'm not trying to scare you, or to try to convert you to a two-stroke. I just want to inform you so that you know how to prevent something like this from happening. Now that you know why to replace these things, you should know when to replace them, otherwise it will do you no good.
Are You Racing Your High-Tech Four Stroke?
If you race and actually ride your 250/450F hard, then it's going to cause more wear on parts a lot quicker. Some bikes are different, and some valves can be defective, but I'm not going to say any names (Hon...da.....cough...cough). When racing, I recommend that you check the valves every 15-20 hours. Doing so will give you an idea about how long they may last. If everything seems good and they are still in-spec then you can screw everything back together and ride some more. If they are slightly out-of spec then you know they're starting to wear out, but should still be good. HotCams makes a Valve Shim Kit (VERY useful if you deal with a lot of 4-strokes) which makes adjusting easy.
When to actually replace valves is often the harder question. Some bikes can go a couple hundred hours with the valves still in spec, but that is very unlikely if you are racing. I suggest you replace the valves around 40-60 hours. Hard starting is a common indicator of at least a valve adjustment because they are tight and out-of spec. If you are racing then it would be safe to just replace them when this happens. It may seem like a lot of work, but trust me when I say it's cheaper to adjust or replace your valves every so often then letting them go until you drop one and grenade the engine.
What If I Trail/Play Ride?
Like I said earlier, I've heard of many people going 150+ hours on their four-stroke motocross bikes and still be good, especially on 450F's because you don't have to ride them as hard. I recommend, as long as you aren't riding every single day, checking your valves about once a year. If they are in-spec then you should be go to good for some more riding. Adjust them if they are out of spec.
If you've adjusted your valves 2-3 times, then it's probably time for some new ones. If you wait another time to adjust them it could be catastrophic.... If you have never had to adjust the valves on your four-stroke dirt bike, it's kinda' up to you on how long you want to go. I would replace them every couple seasons if I'm just trail riding, or after 100-200 hours.
What Is THE MOST Disregarded Part In Four-Stroke Engines?
Timing chains are the most forgotten part when doing maintenance on a four-stroke dirt bike engine. Why is this little chain so important? The big deal is that it can break and blow up your engine, 'nuff said! Now if you replace it every so often it will prevent that from happening. I explained earlier how timing chains get worn over time because of the abuse they get from the high-rpm engines. Not only can they break from fatigue, but after so much wear and tear from spinning on the crank and cam(s) it will get loose.
When the cam/timing chain gets loose it can slip and cause the teeth on the cam sprockets to jump. The timing is then changed, resulting in poor performance. If it jumps too much then the valve(s) can eventually hit the piston because the timing is so far off. And when that happens, it's over for good.
Now do you get the idea about why it is so important to replace the timing chain? I recommend replacing it every 100 hours of ride time. That's usually every year, unless you ride a lot more. It's a cheap part, and fairly easy to replace. It can save you hundreds in the long run if you just replace it after so long. Just call it 'cheap insurance'.
Now you may disagree with me, but it's pretty much a proven fact that Yamaha has been making the most reliable 250 and 450cc four-stroke motocross bikes. The other colored bikes would not last quite as long. But, within the past few years they have all stepped it up and got A LOT better. So keep this in mind when you're in the market for a used dirt bike. If you are looking to buy a new 250F or 450F, then it's pretty much pick a color and ride.
Have fun, and don't forget to do "Preventative Maintenance"!!!
-Tom Stark
Hydration Backpack
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