For many years, four-stroke engines were mainly found in trail machines designed only for casual play riding. Most were air-cooled and rather simple in design, and they were known more for their reliability than their tire shredding performance. Not anymore. When the factories began to experiment with high performance four-stroke engines featuring liquid cooling, multiple valve combinations, and intricate designs, maximum performance was achieved, but it was often the long-term reliability that suffered. Despite that fact, once the floodgates were open, the public became enamored with the new four-stroke technology, and ATVs, like Yamaha’s ground-breaking YFZ 450 were huge hits in the showroom and for good reason. The new breed of high performance four-strokes came with motor performance that had formerly only been available to racers through after-market shops, and they also arrived with a chassis to match the power.

One company that has been at the pinnacle of ATV performance practically since the beginning of the sport is CT Racing. As a pro racer, Alan Knowles knew what worked and what didn't, and it was a natural progression to found CT Racing and to bring the winning engine and chassis components he had already been building to the rapidly growing ATV market. With high quality parts and potent engine packages, success followed almost immediately, and since that time, Knowles' CT Racing has won practically every championship in ATV racing. From the motocross track to the desert, CT has won everything at one time or another, and recently they claimed another big win at the Baja 500. Initially, like many engine builders, CT was focused on two-stroke engines, but with the wave of four-stroke power plants, they were able to apply their considerable design, development, and never-ending testing regimen to the new breed of engines. It is the continued testing, development, and the application of that learned knowledge that has made CT a winner for many years, and for that reason we recently stopped by their shop looking for information on how to maintain today’s high performance engines, and we asked Alan Knowles a few questions.

J.Arens: What’s the most important part of maintaining a modern, hi-performance four-stroke?

CT: Keep your filter clean! Dirt eats valves, and it is absolutely critical to keep a clean filter. Buy a couple of extra filters, swap them around, and don't keep riding on a dirty filter. Changing the oil is a little less critical, but changing it often does get contaminants out of the clutch. While we're talking about the life of your motor, when you fire the motor up, don't blip the throttle. Get some heat to your motor. A lot of your wear comes in the first couple minutes before it's fully warmed
up. Blipping the throttle accomplishes nothing, other than making it
go “whoopa, whoopa” but that's it. A high idle is what it needs, and a gradual warm up is  good, and when it's fully warmed up, then you're ready to go out and pull the trigger!

J.Arens: Is building a four-stroke motor like doing a two-stroke, where you would polish the ports or the intake?

CT: We don't polish the intake. We give it a cutter finish, but sometimes we have a couple different finishes on the intake depending on the cutter angle. A smooth finish on the intake will form droplets and stick, and that's not what you want. Polishing the exhaust only keeps stuff from building up, but it doesn't make more power. It's insignificant really. It's the shape of the ports and what you do that makes more power. You have to know what you're doing, and have a plan for the motor as a package, and not as a collection of holes to be ground out. Simply cutting and hogging out the material won't help, and will probably hurt performance.

J.Arens: Is motor technology ahead of most people, and can local shops properly maintain the new engines? Everybody is running hi-performance four-strokes these days. Why isn't everyone on the same page?

CT: The bottom line is, doing valve jobs by hand on a high rpm, high-performance four-stroke motor is a disaster, because you can't hold the tolerances needed to do it right. It's OK on a motor running 7 or 8000 rpms, but when you're turning 11000 rpms on a motor with titanium valves that flex, it's going to pop the heads off the valves. We're repairing a lot of motors where that has happened, and the guy who did the motor is never going to take the blame, but he will say you over-revved the motor and blew it up. You want to know who's doing your work, and what they're using for equipment. A lot of shops talk about using NASCAR technology, but even much of that is using old stuff from the 1980s, and that won't cut it.

J.Arens: Any valve job today needs to be cut on CNC machinery if you're running a hi-performance four-stroke?

CT: There is a lot to be gained by using CNC equipment, because you can do like we do, and reshape the contour all the way down to the port work, reshape the circle diameter for bigger volume, and match it to the cylinder casting. You can gain horsepower from that. You're not going to get that out of a standard three-angle valve job. 

J.Arens: So the old three-angle valve tool doesn't quite get the job done in today’s hi-performance four-stroke motors, because they simply can't match the valve perfectly enough?

CT: Correct. What happens is, the new valve job is not perfectly concentric to the valve stem, which means it hits harder on one side, causing a vibration in the valve stem, which creates a harmonic imbalance and fatigue, eventually breaking the head off the stem. Then the valve floats around inside the motor for a couple turns until the quad makes a quick stop. Then you buy a new head.

J.Arens: Please explain a common mistake many riders make when installing a cam or rebuilding their top end.

CT: A lot of riders will get the timing incorrect. It's a common mistake when putting in a new cam. They'll time it to the firing mark on the flywheel, not the Top Dead Centre mark, and then they've got a real problem. There are two marks on a lot of flywheels, and it's critical you know which is top dead centre, and almost very bike has double sets of marks. Get it wrong, and you're probably going to buy a new top end. Some guys have tried to mark the cam chain, and then put it back where it was before, but that won't work either. Make sure you know what the marks on the flywheel mean, put it top dead center, line the marks on the cam up, and you're done. It's not that hard!

J.Arens: What about shimming the valves?

CT: You used to adjust valve lash with adjuster nuts, but now you use shims. It's more consistent because it can't back off like a nut and change. However, if it keeps tightening up, don't keep trying to adjust it, get a valve job before it gets too tight and breaks a valve.

J.Arens: So what kind of life span does a modern four-stroke have? Are they good, old, reliable run forever four-strokes?

CT: The maintenance interval of a modern high performance four-stroke is almost identical to a two-stroke. You have to stay on top of it. The difference is, the two-strokes are much easier to rebuild. (Read cheaper!) Today’s high performance four-strokes are not like the old 250X or 350X motors where you could run them into the ground. We call the pistons in the new four-strokes balls, because they are so short, when they wear they want to roll over, and then they break parts. You have to keep them tight. They're too high performance a motor to neglect. Ignore them and they will come unwound.

J.Arens: Come unwound?

CT: Then it gets very expensive. A rebuild costing $2,200 is not uncommon on some of these motors when they wad up. That's going to hurt your feelings. You know, a 250R two-stroke wads up, and it costs you a piston, a bore job, and a crank, and the whole job costs maybe $900. It hurts your feelings, but you move on. When your four-stroke wads up, and you've got a $2,500 bill sitting at the Honda shop, you're done riding for the year. You're fried, and that's why we're seeing a lot of bikes repossessed. People want a high performance motor, but they come with a high performance maintenance schedule.

J.Arens: OK. Let’s talk about a massive failure. Can you weld it up and rebuild it, or do you just start ordering parts?

CT: Depends on how many seats you tore out! Some of the heads are fairly inexpensive. A Honda is a little over $200, so if you've ruined all four, you're better off buying a new head than fixing the old one. However, if we do fix it, we'll put in better seat material, and it will be custom machined, so you'll get a better head. With us, one of the reasons we went to a CNC machine is so we could do repairs, and actually build better heads than come from the factory. We're setup, and we don't mind doing them because with the CNC machine, we can do them efficiently and turn them around fast, but a lot of shops don't have the equipment, and don't want to do them, so they send them out. We also do a lot of new motors for the super cross teams. We pull out the old seats, install better seat material, sometimes port them, and then CNC them out until they're perfect.

J.Arens: What about springs?

CT: They're wear items. They go through a lot of heat cycles, and lose their tension over time, then they float the valves. It's another item you have to maintain on these motors. It's critical to keep your air intake clean and stay on top of the maintenance schedule.

J.Arens: Thanks for all the advice, Alan!

CT: Thank you, and good riding!

We hope that helps you understand what it takes to keep a modern four-stroke on the track or trail. Today’s performance four-stroke motors offer great power, but they're not as forgiving as the four-strokes of a decade ago, and hopefully now you have a better idea of how to keep yours running. www.ctracing.com