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Honda CRF250r Valves inquiry.

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Hi Guys,

I am 15 years old, and am not an idiot or anything, but i've never had to deal with valves before on my xr200, i keep my it in mint condition regards to looking after the engine and keeping it tidy. However, I am looking at upgrading to a crf250r, and have been researching on the internet. Everything makes sense to me about maintenance etc, except the valves. Can someone please explain to me what these valves are all about? Some people call them just valves, others call them clearance valves? I am confused. So as I said above, can someone please explain to me all there is to know about them, and maybe some links that explain them?

I appreciate your help, hope to get some replies.

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In a nutshell

Them valves they talk about...open and close as fast as the motor goes around. The metal wears with time and use, and the small clearances need to be adjusted to maintain a good seal at the valve face.

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The four-stroke engine has usually 4 valves 2-intake valves where air and fuel enter the cilinder and 2-exhaust vales where the fumes go out.

If you put your piston in the top position (usually there is a mark on the engine somewhere where x marks the spot) you can messure the valve clearance. This clearance is the free play between the top of the valve and the camshaft. You messure it with a gauge feeler. Intake clearance is 0.12mm and exhaust is 0.28.

It is important that the clearance is correct other wise the engine will not performe at its maximum. e.g. what usually can happen is that you loose intake clearance (0mm) because of valve damage (burned). If you have 0mm clearance the valve could be pushed down a little bit by the camshaft at all times meaning the cilinder is always open which means you loose compression and it will start more difficult and it will not run properly etc.etc.

Have a look at this animation below (this engine has 2 camshafts). In the top you see the camshafts with the cam lobes which push the valve open and close them each cycle.

Animation:

Edited by fransn
typo

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Interesting, thanks. I know it will vary, but how often do the valves GENERALLY need to be adjusted. If they are not adjusted, what could result?

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Depends on LOTS of things, but if it's a Yamaha...prolly never :bonk:. Everything else, once a year if trail ridden lots. More often if racing every week.

Valve could break, and the ensuing badness will most likely require lots of $$ to fix your entire motor. Most likely though, as our dutch friend above says, they will tighten up, and your bike won't start. Lots of kickin for nothin.

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a fourstroke motor works like this suck push bang blow right? so it has to find time to put fuel in and expel exhaust, intake valves open when the cam lobe pushes down on it (in a over head cam motor) and the downstroke of the motor pulls in the gas/air mix it pushes it up compressing and then vaporizing spark hits it and it goes boom pushing the piston down at this time the exhaust lobe opens the exhaust valves and then with the motion of the bang it blows the exhaust out.

and what was said above, yamaha's never do seem to have valve problems..

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I think the newer Honda's are better but on my 2004 CRF250R I had to replace one intake valve for 2 times in 2 years and the last time also after 1 year (about 20hrs on the bike) I had to replace both intake valves.

I don't know if its an air-filter problem or just a problem with these titanium intake valves. I now have the steel valves installed and hope this will fix the problem... and I'm going to clean my air filter more often.

About adjusting valves. Its really a very simple job to measure / readjust the valve clearance but once they go bad they are burning up very fast... in general it should not happen. But with the history of my bike I measure them every 2 months because I don't want to ride with bad valves...it might damage more than only your valve.

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About adjusting valves. Its really a very simple job to measure / readjust the valve clearance but once they go bad they are burning up very fast... in general it should not happen. But with the history of my bike I measure them every 2 months because I don't want to ride with bad valves...it might damage more than only your valve.

valve_in_piston.jpg

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Thank you very much guys, I fully understand about valves now. So based on my research, Yamaha looks to be the most reliable 250f all over? Valves and everything else. I would be using it for trail riding, and the odd motocross fun day as I do now. If I keep up entirely with regular maintenance, I should be able to get around 70ish hours on the top end, I wouldn't go over that 70 mark. Seeing as I would get the top end done professionally I figured that getting the valves checked at the same number of hours would make sense and all would be sweet? If someone (That knows what they are talking about :smirk: ) could just confirm that with me then all my research will be done! :bonk: Cheers guys. Oh and also! how am I to know if I need a bottom end rebuild?

Edited by James_Y_H

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