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Every thing you need to know about thumper's. Part 3

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COMMON PROBLEMS WITH LATE MODEL THUMPERS

Patterns of problems with certain year/model dirt bikes are starting to develop. These problems are more rider usage, maintenance, and materials related rather than being design flaws. Keep in mind that the manufacturers are cost limited on certain valve train components and intend for the owner to replace parts as they become fatigued.

Honda CRF models

2004 CRF250

Although no specific service limit is advised in the Honda service manual, you should replace all the valves and springs at the same time as the piston. Honda recommends changing the piston every 15 hours, and in practice I estimate the valve train and piston to have a useable service life of about 30 hours. In fact the valve train usually wears out before the piston. The dangers of leaving the valves in place until the engine wonÂ’t start anymore, is that the valve seats will become damaged and require an expensive service of valve seat reconditioning. In a case like that, considering that new head is so inexpensive, it may be cheaper to buy a new head rather than fix the old one.

Another problem that is rare for the CRF250 is a failure of the left side cam bearing. If the engine becomes particularly noisy at idle, or if there is a thick black residue of rubber present under the valve cover, then the cam bearing is worn and placing a greater side load on the cam chain guide. The rear tensioner chain guide is coated with rubber, which accounts for the thick black residue.

2002-04 CRF450

The two most common problems with this model include intake valve wear on all model years and cam chain tensioners that go slack on the 2002 and 03 models. At first the intake valves wear was attributed to dirt bypassing the air filter, causing wear to the protective oxide coating on the titanium intake valves. Some of the other theories touted on the Internet included valve seats that are too hard, too soft of a valve spring, and too steep of a closing ramp on the camshaft. The most reliable solution for the intake valve problem is to install a Kibblewhite spring kit and Black Diamond stainless steel intake valves. The parts cost about $300. Ferrea also makes stainless steel valves and their product features lighter weight and are designed to work with RD brand single coil spring kits. Ron Hamp Cycle offers a choice of lightened Ferrea stainless valves or solid titanium valves with DLC (diamond-like coatings) in standard and oversized along with custom machine work.

There are two options for the cam chain tensioner. The least expensive choice is to buy a tensioner from a 2004 CRF450, which sells for about $52. The 2004 and later Honda part is greatly improved but it should be replaced every 100hrs. Factory Racing of Italy chose a different approach to the problem; they make a manual cam chain tensioner. However manual cam chain tensioners require frequent adjustment and a careful touch. Another area of concern on the Honda is a worn exhaust rocker arm roller. Look for a deep groove to appear in the center of the roller. That indicates that the bearing is worn. You canÂ’t replace the roller itself because it isnÂ’t available from Honda separately. You need to replace the entire rocker assembly, which sells for about $120. If you are a fervent Internet news group reader there are two things that are recommended that should never be attempted. They include grinding down the valve shim pads to give valve to tappet clearance, and installing Honda ATV steel intake valves with the stock springs. When a valve is worn so far that Honda doesnÂ’t offer a small enough shim, its junk and needs to be replaced. With regards to changing valve materials, when you switch from a lightweight titanium valve to a heavier steel valve, you must also install a stiffer spring. One last thing that contradicts the Honda manual and Internet myths, never attempt to use valve grinding compound to pre-finish a titanium valve prior to installation. The gritty compound will damage the oxide coating designed to protect the valve.

Kawasaki, Suzuki KXF/RMZ250

In the first year of production these models suffered growing pains. Some of the problems associated with the valve train parts include an aggressive cam profile that challenges the valve springs and tappets. Riders who constantly bang the engine up against the rev limiter aggravate the problem. This model uses a two-piece titanium valve with a relatively soft spring. If the valve starts to get worn and cup-shaped it will not contact the valve seat evenly which could lead to the valve head breaking away from the valve stem. When checking the valve clearance, if you have to install a shim pad that is two sizes smaller than the shim pad that youÂ’re replacing, then you should assume that the valve is worn out and needs replacement. As in the case with all modern 250cc 4-strokes, replace the piston kit, valves, and springs all at the same time if using OEM parts. Typical service limits range from 15hrs for experts to 40hrs for novice riders. There are two other parts of the valve train that should be examined every time that the valve cover is removed, the tappets and the camshaft lobes. If the tappets have a defined circle in the top center, the tappet should be replaced because it is in danger of breaking. The small circle is about the same size as the shim pad, because the worn valve spring is allowing the valve stem to slam the shim pad up against the underside of the tappet.

Pro Circuit sells a high performance package for these models that includes solid titanium valves, a stiffer spring kit, and the optional labor to perform a multi-angle valve job, porting, and installation of the valves. WMR offers a similar approach using the solid titanium valves, conical springs, and the option of installing bronze valve seats that are more compatible with the titanium valves.

If youÂ’re more interested in long-term reliability and lower operating costs then Kibblewhite has the solution. They offer stainless steel valves, a dual coil spring kit with titanium retainers, and hardened tappets. Hot Cams offers a cam profile with a less aggressive opening and closing ramp that puts less strain on the valves, guides, and tappets. However there is a slight loss of low end but more power from 8,000-11,000 rpm.

Yamaha YZF models

YZ250F

The YZ250F is the most reliable 250 4-stroke, but that tends to make owners lethargic about maintenance. Some of the issues with this bike include valve seats that can rust in humid climates, valves springs with a short lifespan, and a cam chain of relatively poor quality that tends to wear the sprocket that is part of the crankshaft. If the cam chain isnÂ’t replaced with every top end rebuild, it can cause wear dot to appear on the sprocket teeth. When the sprocket wears the crankshaft must be replaced since the sprocket isnÂ’t removable. There is no better alternative for the cam chain; stock OEM is the only choice available.

Riding situations that tend to stress the limits of the valve train and can cause valve floating and engine failure is downshifting in the air over big jumps and landing with the throttle on. The kinetic energy of the motorcycle contacting the ground serves to raise the engine rpm past the limits of the valve springs enabling the valves to float and contact the piston. Rev limiters donÂ’t offer any protection when you downshift and land from a big jump.

When checking the valve clearance, if the clearance tightens up so much that you need to install a shim pad 2-3 sizes smaller, the valve is in danger of breaking the head and the valves and springs should be replaced.

Kibblewhite offers a White Diamond stainless steel valve kit and dual coil springs as an endurance alternative to the stock titanium parts.

YZ400/426F

The steel valves used on the 1998-2000 models have an excellent reliability record. However there are some typical problems with the later model titanium valves. Cheap OEM valve springs have a short and violent life. When extended past the service limit they tend to crack, sending the pieces throughout the cylinder head, typically getting ground up by the camshaft and tappets. One unusual problem observed on YZFs is rusty valve seats. This problem seems to be more common on bikes used in humid climates like the Southern and Mid-Atlantic States. When the engine is shut off, kick the engine over until the piston comes to TDC. If any of the valves are open, moisture in the air will condensate on the valve seat causing corrosion. ThatÂ’s not a problem for steel valves but a rough valve-seating surface will damage the oxide coating of a titanium valve.

YZ450F

The new generation YZF had some minor problems in the first year of production 2003. The rubber cap used to plug the old manual decompressor passage tends to pop out, allowing a loss of oil from the front of the engine. If the rider doesnÂ’t immediately notice the leaking oil, the cams will seize to the head. Several companies make billet caps with fastening bolts. The cam chain tensioner can loose spring pressure and retract, which may cause the cam chain to jump off of the cam sprockets allowing the valves to contact the piston. The only aftermarket alternative is Factory RacingÂ’s manual tensioner.

The stock titanium valves have a good service record in comparison to the CRF450. If you’re looking for better top end power and longevity, consider a Kibblewhite oversize stainless steel valve and spring kit. :thumbsup:

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Thought so.

Care to fill us in on the minor inaccuracies, inconsistencies and such?

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So it's actually written by Eric Gorr - you should state that. I started reading kind of thinking you'd written it yourself and what a good egg you were for putting so much effort in. Didn't realise until Part 3 that it was just cut and pasted from something Eric Gorr wrote!

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I thought he had written it himself as well, but when I checked his other posts and saw that he had one questioning why his bike was popping on decel then I knew otherwise.

If he really had written this article then he wouldn't be asking questions like that. :thumbsup:

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The first giveaway was the circumflex "Â" that appears before each apostrophe. Whenever you see that, or some other out of place character, it's a sure sign it was a cut/paste from a web page, or an OCR scan that didn't quite work 100%.

One inconsistency in the article that comes to mind immediately is that where the author says that a CR250f can run the valve to the point where no more shims are available, and then states that when you run a YZ two steps below OEM, you're risking mass destruction. While I agree with the last, I doubt the Honda, nor any other engine with Ti valves, is less subject to this. The problem is that the reduction in shim size indicates wear, and wear will not take place with the uniformity that we'd all like it to. The valve face and seat will wear off center with the guide, and that will cause the valve to "scrub" or twist slightly each time it seats. Over time, that can cause the valve head to snap of at the stem, even with one piece valves.

Then there was the thing about valves bouncing as they seat. The camshaft is primarily responsible for preventing this through the proper inclusion of "quieting ramps" in the transition from base circle to lobe and back. I've heard the speculation about valves bouncing before, but I'd think that valve float would result from weak springs before that.

In the discussion regarding valve seat materials, the copper/bronze alloy materials mentioned are top of the line, but Yamaha, the one with the best OEM reliability, are just iron, as far as I know.

In all, the original article was typical of what's usually found in magazines these days; a skim-over discussion of a subject in general, without an in depth examination of much of anything in particular. Useful as far as it goes for people new to modern thumpers, but not really anything tremendously advanced.

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This book cuts through the BS on valve wear. http://www.scribd.com/doc/24488413/Automotive-Engine-Valve-Recession

Honda should have read this book. It's interesting as a lot of research shows that cylender combustion pressures cause an "oil canning" type of deformation which causes the valve face to wedge into the 45 degree face cut into the valve seats. This sliding wear is very damaging. It's a very good book and has actual scientific analysis behind it. BTW, by and large, Yamaha's previous 5 valve engine design has proven to be very bulletproof. So far my KTM 450 SXF has been really good too.

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"Oil canning" used to be referred to as "tuliping" after the fact that the valve begins to take on the wine glass shape of a tulip. Not at all common any more, and very uncommon with valves as small as those in a multi-valve head.

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Anyways, this thread is kind of off topic now.

Guys even the guy that started all this has realized this is off track. NONE of this belongs in this forum. Please quit rambling about what bike is more reliable. Use your heads, do your research, and make your own decision. But please quit filling the engineering forum full of....

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