# Valve clearance



## foggysail (Feb 21, 2015)

Why have guys here in the forums sometimes suggested checking for valve clearances to others trying to get a snowblower running? I understand how cam lobe wear along with valve stem wear might cause inadequate valve opening but certainly not an closing problem. Gees, why this suggestion/recommendation to anybody trying to get a small engine running? 

I have used many types of small engine powered machinery ............snow blowers, generators, pumps and so forth for years. Never ever had I knowingly had a valve problem. OK, I have had a few burned valves in cars but not many.


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## tabora (Mar 1, 2017)

foggysail said:


> Why have guys here in the forums sometimes suggested checking for valve clearances to others trying to get a snowblower running?


I think the "checking the valves" suggestions have mostly been a last-ditch effort where compression issues have been detected or all other likely causes have been eliminated. I too have very rarely had to deal with OPE valve issues, but I do have a Generac generator that had an intake valve keeper break in half, followed by the valve stem itself breaking. This was likely caused by a low oil situation when the genset was loaned out, but I'll never know for sure.


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## Grunt (Nov 11, 2013)

foggysail said:


> might cause inadequate valve opening but certainly not an closing problem.


 
It is quite common for flat head valves (usually exhaust) to loose all clearance and remain open a few thousandths causing a no or hard to start condition from weak compression. The valve seat gets heated and pounded into the block (head), the valve stem expands from the heat and the valve will not seat and seal.


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## tadawson (Jan 3, 2018)

If the seats wear, then yes, you *CAN* get a case of incomplete closure - fairly common on L heads, and a common reason for pops/backfire through the muffler.


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## crazzywolfie (Jun 2, 2014)

yup when your muffler starts glowing red on a flat head you usually have an exhaust valve clearance issue. the ohv engines will sometimes not run right if valves are not adjusted right or within spec. had a lawn mower that ran poorly this summer and cleaning carb did nothing. adjusted valve clearance a tiny bit to get it within spec and ran like brand new again.


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## foggysail (Feb 21, 2015)

THANKS GUYS!!!!

Nice to participate in a forum where so many knowledgeable people contribute/share their valuable experience!!! :3tens:


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## 2.137269 (Nov 15, 2019)

valve face and seat wear with time. when so the gap closes making motor run run among other issues. generac there i have had to replace heads as the pressed in to the casting seat became loose moving up and down banging in the head, only way it was known was that the gap would close up in a very short time and the stem be way higher than the other


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## 2.137269 (Nov 15, 2019)

tabora said:


> I think the "checking the valves" suggestions have mostly been a last-ditch effort where compression issues have been detected or all other likely causes have been eliminated. I too have very rarely had to deal with OPE valve issues, but I do have a Generac generator that had an intake valve keeper break in half, followed by the valve stem itself breaking. This was likely caused by a low oil situation when the genset was loaned out, but I'll never know for sure.


here is from a generac forum as to what has been found many times my own unit included


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## Mike C. (Jan 28, 2016)

crazzywolfie said:


> ....the ohv engines will sometimes not run right if valves are not adjusted right or within spec.... QUOTE]
> 
> 
> Common problem on OHV lawn tractor engines is a situation where the valve lash is so excessive the engine can't roll over because the compression release won't actuate.I can't count how many people I know that have bought new electric starters and batteries thinking that was the problem.


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## 2.137269 (Nov 15, 2019)

how true mike c

i can't count how many times a machine came in with the owner saying it won't turn over,i have a new battery and starter . adjust the valves and send them home.


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## badbmwbrad (Jul 30, 2019)

Exhaust valve seat recession may be caused by no-lead fuel. Lead coats the exhaust valve seats and "cushions" the valve disc during closure. Old engines needed the lead due to the metallurgy of the exhaust valve seat material. New engines have improved exhaust valve seat material that tolerates no-lead fuel.

Absent the lead coating on valve seat, the disc makes hard, point contact on the valve seat at high spots. The disc momentarily micro-welds to the seat. When valve opens, disc tears away a small (microscopic) piece of the valve seat. 

Over the passage of time, the seat slowly wears away; causing the disc to seat deeper and deeper. Clearance between the valve stem and the lifter becomes smaller and smaller. When the engine is hot, the valve stem is longer (thermal expansion); further reducing the valve stem clearance until the exhaust valve does not fully close. The engine loses compression when hot. It will stall out and not restart until it has cooled.

www.flowspeed.com


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