When does a valve "burn" ?

[Jkb4c]

Hello all.

I recently did a valve clearance check on my suzuki motorcycle. This 650cc V-twin uses a shim-under-bucket design, where the camshaft actuates the valves directly.
All my valves were in-spec, but at the absolute tight side of spec. I understand that the valves get tighter over time, so this makes sense.

I know that never adjusting the valves can eventually lead to a "burnt valve" condition, where the exhaust valve never fully seats, and therefore never dissipates heat.

My question is this; When would this condition occur? Is there generally a "margin of safety" built in to this sort of thing, or does barely out-of-spec mean that the valve isn't seating? Am I tempting fate leaving them on the tight limit?

 

[zrxkawboy]

The mfr gives a range, and as long as you are in the range, you're fine. However, if you really are right on the edge of the range, keep in mind that you'll probably soon be out of the range. Keep an eye on the clearance.

 

[Bluestream]

The valve clearance on most engines gets more clearance over time, not less. When they start to chatter, you need to set the lash, and it is always loose...

 

[soldierman]

MOST will loosen up.... But during my time wrenching on powersports stuff I found more often than not during initial break in that most Japanese bike and ATV's intake valves will tighten up. Usually due to the face of the valve cupping where it contacts the seat. It allows the valve drop into the seat a bit more and thus the stem rides a bit higher in the guide and lessens clearance. The exhaust valves are usually stellite and don't do it near as much, I'm guessing due to its hardness. I often had single cylinder ATV's come in that had never had the first service done when scheduled and had become "hard" to start. First thing to check on those was the intake valve.

I'm guessing you didn't adjust them because you didn't have any shims. Pull the buckets and see whats in them. Draw out a diagram and mark whats in each bucket and find out what shims are available and what it will take in swapping each around to come up with the needed play.

 

[Shannow]

Our local bike shop offers shim swaps for only a couple of dollars.

 

[XCELERATIONRULES]

Correct term= valve check.
The clearance on the tight side is a good sign,they usually loosen up with wear,not get tighter.
If they aren't making noise,leave them alone.
If It ain't broke,don't fix it.

 

[willix]

If the adjustments are neglected, to burn a valve is the least of your worries. Taking out the cams is the issue. The original shim will live a long life before it will wear out of spec. The hard coating on the shim will deteriorate and can start to flake off if the cam is allowed to bang on the bucket getting in the oil system then other damage can occur.

 

[440Magnum]

Originally Posted By: XCELERATIONRULES
Correct term= valve check.
The clearance on the tight side is a good sign,they usually loosen up with wear,not get tighter.



That's not true on all engines. On some engines- particularly air-cooled engines- the valves stretch faster than wear creates more clearance, so the valves DO tighten with time.

As for when the valve will burn, that depends a lot on how hard the engine is run. The harder its run with valves not seating good and hard, the faster the burn.

 

[Trav]

With OHC using bucket/shim cam wear is normally very minimal,initial valve seat recession no so much stretch is usually the cause of valves going tight and is pretty common in high RPM bike engines.Adjust them to as close as possible mid range of the spec.Burning is be a real issue with tight valves.
After the first adjustment they tend to stay in spec for a long time.

 

[SteveSRT8]

yeah, that's the case in my stable. Typically after the 2nd interval the setting seems to stabilize.

I've had a Zook go over 10,000 street miles w/o needing lash set.

 

[mechtech2]

First, you are in spec, so leave it alone.
Unless something goes wrong, it should change very little, and very slowly.
See what she is at next time.
Remember that hot/cold makes a difference.
I use dead cold, because it is the only way to get consistent results.
Because how hot is hot? By the time you are finish, it is way cooler than when you start the first check.