Valve cleareance and why it goes out of spec?

[flynbryan]

Guys I know this maybe common knowledge for alot of you, but I'm going to show my lack of knowledge here and ask a question. I just readjusted my valves on my Crf, and was asking myself a question I had no answer for: What parts wear to require valve cleareance to decrease/increase? And also having an 02' Crf I've read just about all the articles about valve issues w/the Crf and know that the intake valves are suspect to loosing cleareance and tightening up. What exactly really causes this. What part is it that is actually wearing? The valve seat, valve springs, or the valves themselves? I tried a search and found a few articles, but didn't really feel like they exactly answered the questions I had. Thanks guys. :thumb:

 

[biglou]

I'm interested in this one, too. I think the CRF's are bucket and shim, just like the YZF's, but with the single cam and rocker arm config instead of a dual overhead cam. The visual I have in my walnut-sized brain for valves tightening up is the valve seats wearing down (or maybe the backside of the valve itself) allowing the end of the valve stem to close, or "tighten", the gap between the bucket and the cam lobe/rocker arm. On the other side, if getting looser, I envision the shim and/or the bucket and/or the cam lobe wearing down ever so slightly, opening the gap a bit.

Here's one that always puzzled me: Let's say the valves are just outside of proper adjustment-one direction or the other, doesn't matter for this hypothetical scenerio. The difference is so minimal in comparison to how far the valve travels when it is in full open position, as long as the valve seats fully, ie: not so tight that the stem/shim/bucket is resting on the backside of the cam lobe/rocker arm that the valve is not held into the seat with the full force of the spring, how could this effect starting the way it does? I've gotten pretty good at thinking "you're valves culd be too tight" when I read about hard starting issues, but realize I don't understand why "tight valves" would cause this hard starting situation (I've fallen victim to parroting something, even if just to myself, that I've read enough times to have commited it to memory). Seems to me that the only two real issues here are that the cam lobe doesn't "slap" the bucket if OOS loose, or that there isn't constant pressure on the lobe if OOS tight (ie: Loss of compression due to a valve not fully seated). Am I correct, or is there some aspect of 4smoke design theory involved here that I'm not aware of?

Rich? Bill? Any other 4t professors out there?

ps: Sorry for jumping onto your thread, Bryan! ;)

 

[flynbryan]

Originally posted by BigLou
ps: Sorry for jumping onto your thread, Bryan! ;)

No problem Lou. If we're both curious on these questions we can both learn something. :thumb: One answer I have for the starting question you had; the only thing Dave and I had come to was that the lack of cleareance on the intake valves was creating a rich condition, and I can validate that claim after looking at my spark plug. I went to a hare scramble w/ Ocechap 2 weeks ago, and kick, and kicked, and kicked to no avail. (O ask Ocechap about the poor soul that came over to help, and the frustrated tongue lashing I gave him. :o ) Finally in exhaustion I pulled the plug and found that the plug was wet fouled. After reading the same articles as yourself Lou I pulled the valve cover off and discovered my left intake valve had NO cleareance. So that is the only cause and effect scenario I'v come up w/so far, and as usual if my assumptions are incorrect you guys fell free to correct me.

 

[jmics19067]

I have just read in the 4stroke forum<I beleive it was Rich stating> about titanium valves cupping. The heads deform a bit because the valve spring is trying to pull the valve up thru the seat. This would lessen your clearance.

I have very little knowledge of titanium valves and high performance 4 strokes. My guesses would be if the valves are tightening up to look at these things for the reasons. valve seat recesion<the seat is actually getting pounded into the head> valve seat and valve face wear , also the valve head cupping or valve stem stretching. With high spring pressures recquired to keep the valve from floating at high rpm only aggravates the problem

if your clearances are getting larger start looking for cam, cam follower/lifter and rocker arm wear< what I am accustomed to look for on high mileage non high performance engines>

If you are supposed to do valve clearance checks on a cold aluminum engine and there is not enough clearance you run the great possiblity that when everything gets hot and the alumimun head grows more than the steel/titanium valve train. That can easily cause the valve to be super tight and open eventually burning the valve. The hot combustion gases trying to sneak thru when it isn't supposed to will erode the valve/valve seat very quickly.

if the valve clearance is too great you run the possibilty of the cam follower slapping the cam<or other parts of the valve train slapping around> causing premature wear or breakage < again what I am used to looking at>

If the clearances are not correct you are basicly changing the cam profile. is too loose the valve "thinks" the cam is smaller with less duration too tight taller with more duration. If cam overlap gets too great or if the intake valve stays open you can have a problem with "standoff". This is when air goes thru carb picks up its fuel does not make it all the way to the engine and gets bounced back thru the carb picking up more fuel , then gets sucked back thru the carb sucking up even more fuel at low speeds. A very common problem with the lumpy idling car cams that people try and put on the street and wonder why they can't get thru an emission test.

this is probably just enough knowledge to start getting dangerous< a trait most people say I am blessed with greatly :eek: >