[WayneS]

My 81 GW has had what sounds like valve tappet noise for years. I've checked and set the rocker arm clearance twice over the last two years, probably within a 1000 miles since the last time. Still the noise persisted. No exhaust leaks, couldn't nail down any spark leakage (even changed to automobile coils, which works nicely by the way). The tapping was slowly getting worse, and I found some tiny shiny stuff in the oil yesterday. I figured it was soon done for given the price of parts to overhaul.

I wanted one last check so I removed the right valve cover and timing belt cover. Cut off the top 1/3 of an old valve cover and put that on so I could see it run without dumping oil everywhere - I was thinking maybe a worn rocker shaft moving around or something. The cam belts ran smoothly, the valves all seemed normal enough. Pushed one of the tappets against the valve - the noise changed. Turns out the end of the valve stem was seriously dished. Using a flat feeler gauge gave a false reading. It has actually chewed down the end of the valve over 1/16." I screwed in the tappet, the tap tap tap went away and just left a nice purrrr. Gotta admit I felt a little dumb. It sounded just like a collapsed lifter on a car I used to have, but I had checked the clearance! I've worked on engines for 35 years and this was a new one to me.

Wayne

 

[Ansimp]

Science or sound? There are a lot of arguments that say the best way to set valve clearance is by a trained ear, very much of school.

 

[glhonda]

This was always a dilemma after a valve adjust. You'd adjust the lash, then get a noise that of course "wasn't there before you worked on it".
We would readjust, and if the noise was still there, sorry. Fix it, or live with it.
I don't own a dished feeler gauge, so I have no way to measure the proper clearance.
The valves are cooled by being in contact with the seat. The longer they're open, the hotter they'll get.
The adage is, better .005 too loose, than .001 too tight.
Be careful about how snug you get that valve to make the noise go away.

 

[littlebeaver]

So it would be a bit costly to fix it proper but in the end probably worth it as you would need to get new valves and gaskets and such I would imagine... I have a light tapping and if freaking drives me insane, I also have a chrome cover for the fuel pump that only makes the tapping a bit louder... Thanks for the information .......

 

[Wingnoob]

One would think that if the end of the valve is dished, then that valve is bad.

 

[gumbyred]

Seems that the problem with noisy rockers on the 1100 was the rocker arms moved sideways back and forth on the rocker shaft. Too much side play. At one time someone was making needle bearings to use on the rocker arm that quieted things down.
gumbyred

 

[luvmyhotrod]

Try getting the gage between the cam and the fallower?

 

[Ken Bergen]

One would think that if the end of the valve is dished, then that valve is bad.

+1
The adjuster is likely Fubared too.
Possibly/likely guide also.

 

[DaveO430]

Try getting the gage between the cam and the fallower?

That would be fine except the clearance will be different, the rocker arm is not 1 to 1. One could check the clearance at the cam end on a known good one and use that measurement. Or adjust one that is good and observe the amount of turn it takes for the screw to contact the valve head from there and adjust the rest the same.

 

[WayneS]

Yes, it needs a new valve - but it will wait until winter with no trouble. The tappet is fine, but running out of adjusting space - the end of the valve has worn that much. There are only about 3 threads engaged in the lock nut. The stickout of the worn valve stem from the keys is only about 1/2 of the others. I set the lash by feel - but a dial indicator on the end of the tappet would work too - just hard to clamp down on an aluminum engine. It was a pretty significant tap tap tap - the tappet needed over a full round to get to where it needed to be. The valve stem tip has been wearing for a long time as the tappet threads are half way down the nut, whereas the others have several threads showing. There's enough threads to lock it securely now, but I wouldn't want to screw it in much more.

Too tight is a problem, but not because it sticks into the hot chamber more. It's because it won't fully close when things heat up - which then vents the hot gases through it and burns the valve.

 

[glhonda]

It's because it won't fully close when things heat up - which then vents the hot gases through it and burns the valve.

It will always "fully" close (unless it's bent). It burns valves because the valve is not it contact with the seat long enough to dissipate the heat.

 

[JerryH]

I wonder what caused that, because I have never seen it either. I have always adjusted valves to the loose end of spec, because I have never seen anything but the valve and valve seat wear, which always results in the valves getting tighter as they wear. Usually wear on the other end, or on the cam, is galling caused by a lack of lubrication. I would also check the valve and valve seats for wear. And definitely don't adjust them too tight. That will result in the valves not closing all the way, and they will burn quickly. I would never adjust valves by sound, always use a feeler gauge. I can see where that didn't work in your situation, but that is a highly improbable occurrence. I believe manually adjustable valves should always have a slight ticking, especially when cold. It should become less noticeable as the engine warms up.

 

[Chromo]

If you tighten a valve to tight it will not fully close, and the combustion does rush by and burn it.
The more you screw in that adjuster the further open the valve is held. It's like putting a spacer above the stem. If you have a 3" valve stem and put in a 1/2" spacer, how is that valve supposed to close that extra 1/2" ? It never will of course! Adjusting too tight is the same thing as adding a spacer.

When adjusted to tight by enough it will even backfire through the carb or exhaust depending which valve is too tight because the valve will be open still when the cyl. fires even though it should be totally closed at that time.

Actually the only thing that determines how long a valve stays fully closed is the cam lobes, and they do not change any at all based on adjustment of a valve. Once off the lobe to the fully closed position the valve remains fully closed till beginning back on to the lobe to open again. Adjustments do not effect the duration timing on the came any. Either the valve is fully closed or it is not.

You could loosen a valve too much so that it closes while still slightly on the lobe, and it will not open as soon when slightly going back onto the lobe thus forcing valve to stay closed longer than designed, but that is not how cam lobes are designed to work.

 

[glhonda]

Hmmmm. All this time I thought if the valve wasn't closed, you lose compression.
I also thought that combustion gases go past the exhaust valves as a point of operation.
Silly me.

 

[littlebeaver]

Well, I have decided to live with a slight tapping noise until I get new gaskets and new valves....a simple adjustment has not done away with the noise as it will in many cases but not on my this time.....thanks again for this post..

 

[WayneS]

Glhonda, yes if the valve is left fully off the seat by much, you would not have sufficient compression for ignition. But when it closes enough that you still have sufficient compression but too much leakage, it will burn the valve. The exhaust valve does vent the gases, but not until after expansion, which significantly cools the gases.

The shorter duration on the seat is not a problem - just consider all those high profile longer duration cams on hot-rodded engines. Hotter temps than normal, valve is off the seat longer, but it doesn't cause the valves to burn.

 

[glhonda]

I guess I wasted all that money on my ATA degree in auto mechanics then.
Maybe you'll believe someone else.
http://www.ehow.com/how_8112651_burn-exhaust-valve.html

 

[Chromo]

never mind

 

[glhonda]

Well you are correct there, but having the valve too loose does not mean it closes any tighter nor does it mean it cools any more

I know I'm correct there. I'm correct throughout the entire post.
"The valve is cooled by being in contact with the seat. The longer it is off the seat, the hotter it will be. Conversely, the longer it is on the seat, the more it will cool."
I've backed up my position with education, thirty years in the field and external sources, while others fill the post with conjecture based on how they "think" it "should" work.

 

[WayneS]

GLhonda, I stand corrected on the importance of the valve / seat contact for cooling the valve. My problem valve (#1 intake) is set as near as I can tell to the same valve lash as the #3 intake, which is set with a feeler gauge to .005".

Wayne