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| four stroke maintenance; Mr. Knowles It All | |
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| Topic Started: 21 Jul 2010, 21:17 (266 Views) | |
| rookie | 21 Jul 2010, 21:17 Post #1 |
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 The world of four stokes has changed dramatically over the last 12 years. Yamaha dropped a bomb on the four stroke mar¬ket with the introduction of the YZ400 back in 1998, changing the face of four strokes forever. Until that time four strokes had been a little heavy, a little slow, but with lots of torque. While not exactly low main¬tenance, they were a popular choice for off road recreation. The YZ was light and fast, leading the path that nearly every manufacturer has emulated and advanced. The current crop of MX motorcycles and ATVs have gone to high tech, high RPM, short stroke performance. With the performance gains, cost and maintenance schedule have also increased dramatically PREVENTING STROKES With short strokes, high rpm, and ever lighter valves, and shorter and lighter pistons, the current crop of four strokes need signifi¬cantly more maintenance. We got together with one of the premier Southern California race shops to give you some tips on four stroke maintenance, CT Racing did a re-build on a TRX 450R that we use as a recreation quad. The things we picked up with the re-build should prove worthwhile for you to know. MX bikes and Performance ATVs are so similar now days many part numbers are interchangeable, so this really applies to bikes and quads. We’ll start with pistons. Most current per¬formance four strokes use slipper pistons, meaning they have no side support, and they are very short, sometimes referred to as pancake pistons. With very little support these pistons need to be serviced on a very regular basis. You can pull it down and have it measured, it’s just math, if the piston clear¬ance is in spec, hang rings on it and re-use it. If neglected and the pistons get too loose, it’ll wallow out the Nikosil, and you’ll be buy¬ing a cylinder or having it re-chromed or re-sleeved. Virtually none of the current motors have a cylinder that can be bored. So protect the chrome by keeping it nice and tight. ME SO HONE-NY Honing, is a bit trickier than most think. If a cylinder hone has ever been used on a steel bore, it should never be used on Nikosil. The steel will come off of the hone and get impregnated into the Nikosil and then it rubs out as you run the motor, and your cylinder will look like you’ve run dirt through it. There are a lot of shops around that don’t know that, so do a little questioning. The top per¬formance shops actually hone Nikosil with a diamond hone. Flex hones should not be used on Nikosil. When changing pistons, do a very thor¬ough inspection of the wrist pin and small end of the rod. If you see some checking, galling or any marks on the pin or the rod, the rod needs to be changed. Not doing so often leads to catastrophic failure. If it looks good it is good. VALVE TRAIN LANE Most of the current crop of four strokes have titanium valves, some have stainless, and Hondas tend to run titanium intake and stainless exhaust. Start with regular check¬ing of valve clearance. If you’re the guy that takes your bike to the shop for repairs you should learn to at least check valve clear¬ance. Then if you don’t want to adjust or re-shim it, take it to the shop, but at least do your own checking on a regular basis. Every 20 hours is a good rule of thumb for most motors. If you’ve had an issue with an air fil¬ter and think you may have passed some dirt or water, check your valve clearance. Valve stem size has gotten smaller to enhance HP. This creates issues with valve jobs, especially since a lot of the equipment that has been used to do valve jobs, was designed around large diameter automotive valves. This worked great through the 90’s, but it doesn’t cut it today. The valve job is guided by a pilot that runs through the valve guide. With the smaller 4.5 to 5.5 millimeter valve stems, the pilots can flex and lead to valve jobs that are not concentric to the valve guide. This leads to poor performance or broken valves. Make sure that you know who is doing your valve work, that it’s being done right, and that they use current, competent equipment. Another tip. Titanium valves should not be lapped in. That’s another reason that the valve job needs to be extremely precise. Titanium valves are usually coated, and lap¬ping takes the coating off. Titanium valves can be refaced, but it needs to be done by a good performance shop, that is switched on, and has the trick stuff to do it right. Valve springs that were engineered for titanium will be too light if stainless steel valves are installed. The added weight of stainless will lead to valve float. If you are doing a top-end service, that is more than a year old, replace the valve springs. Most of the OEM springs are very cheap, so just consider them a consumable. SHIM & BUCKET Sounds like the old folk group I used to play guitar with? Anyway, if you have a shim bucket style of motor, and the intake valves have tighten up more than .003, you can plan on doing a valve job. Shim bucket style valves don’t loosen up, they just tighten as the valves wear. If you find you have a lash adjustment, and you find them tightening up, it is the same deal. Intakes are where you will find the most significant wear, so this is what you want to check on a regular basis. Don’t bother taking your quad to a shop, to have your valve clear¬ance checked, it’s something you should learn to do yourself. It’s simple, just put the motor at Top Dead Center (TDC). Slide the feeler gauge in, taking the time to find the size that slides back and fourth, without much resistance, but you do want some drag on the feeler gauge. Once a valve drops off a couple of thou¬sandths, the surface of t he valve, and valve seat, are no longer straight, so they wear extremely quickly. That’s why you need a valve job after you have found wear here. If it’s a race motor, and you get even .002 wear, you should have a valve job done to keep everything at peak performance. PREVENTATIVE MAINTENANCE Most of the current four-stroke top ends have a maintenance schedule that is on par with a two-stroke performance engine. So, don’t neglect it. These engines are fragile and expensive to repair, so stay on top of regular maintenance procedures for your machine. Air filter maintenance is the key to long motor life. Try to spend a little extra money on a spare filter or two. Have them cleaned and oiled, tucked into a zip lock baggy, ready to slip on. The cleaner the air you feed the motor, the longer it will live. If you don’t have a clean filter ready to go, you are more apt to not check the filter, or even look at it and go, “it’s not that bad, I’ll just go for one more ride,” This ultimately wears out your nice expensive valves, and sacks your top-end. The golden rule? Change your engine oil on a very regular basis. Most motors, with the exception of Hondas, pump the clutch debris through the entire engine, so changing the oil and filter more often is a good thing, and that in turn, leads to longer motor life. EDITORS NOTE: All you readers out there with questions about your engine on your ATV, Dirt Bike, or UTV, feel free to send Mr. ‘Knowles it All’ your most intimate technical questions. If they’re mechanical, or the least bit entertaining, he promises he will take the time to read them and possibly even answer a few in the magazine. Be sure and send your diatribes and queries to: www.editor@ offroadill.com Be sure and include your real name, real city and possible state-of-mind, or we will assign a representative of the U.S. Government to track you down and fill in the blanks…. That is all |
 
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4:22 PM Feb 4