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Valve clearance ?

  • Thread starter Thread starter dgeorge
  • Start date Start date
D

dgeorge

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Ok let me see if I can explain what has me a little confused. It has been said that not enough clearance on the valve's causes a lean condition, correct? Why, if there is less clearance between the cam shaft and the valve shim would this not cause the valve to be opened more? causing a rich condition? Where if you had more clearance wouldn't this cause the valve to not open as much causing the lean condition? I'm confused,(but then I usuall stay that way :? )if someone could please explain my mistakes in reasoning I would greatly appriciate it.
 
Don't think that is correct. If the valve is open enough to let a half cup of mixture into the cyl., or a full cup of mixture into the cyl., it will only be rich, if the mixture is too rich when it comes out of the carbs.
 
I've never heard that valve clearance affects mixture. Within the extremely small range of acceptable clearances in the shim-adjusted GSs (.03 to .08 mm), valve clearance doesn't make the slightest difference to mixture.

The problem is when valve clearances get below the acceptable range or become zero. The hot valve does not spend enough time in tight contact with the valve seat to transfer its excess heat to the cylinder head. Temps build, then part of the valve melts or breaks and you're suddenly a lot poorer.

An excessively lean mixture burns too hot, which may cause the valves to overheat, erode, etc. A minor side effect of this erosion is that when the valve clearances become zero (at the top), the bottom end of the valve no longer fits tightly against its seat. Hot exhaust gases shooting by the valve quickly start cutting canyons and slots in the valves.

I had a leaky intake o-ring problem that lead to a lean mixture, and eventually led to overheating and this kind of exhaust valve erosion. I'll have to post pictures of the exhaust valves sometime.

So in other words, an excessively lean mixture (caused by intake leaks or installing pods and mufflers without rejetting) can cause all kinds of heat-related valve failure and erosion. A minor side effect of this valve failure is that clearances become smaller and eventually zero. The tight valve clearances are an effect, not a cause in most cases.

On the other hand, if you have a neglected GS where the valve clearances have been allowed to get so tight that the valve does not hold compression, it will have the same effect as any other compression leak. That cylinder will pull less vacuum and thus the carb dispenses less fuel for any given throttle setting. That cylinder runs lean, the valve gets hot, and metallurgical disaster ensues as above.
 
Re: Valve clearance ?

dgeorge said:
Ok let me see if I can explain what has me a little confused. It has been said that not enough clearance on the valve's causes a lean condition, correct? Why, if there is less clearance between the cam shaft and the valve shim would this not cause the valve to be opened more? causing a rich condition? Where if you had more clearance wouldn't this cause the valve to not open as much causing the lean condition? I'm confused,(but then I usuall stay that way :? )if someone could please explain my mistakes in reasoning I would greatly appriciate it.
Click to expand...
I think I know what you mean. If a valve clearance is out of spec' either way, then compression and combustion will be compromised. If combustion is compromised, you'll see this on the sparkplug.
 
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