Why do we adjust valves??


15 replies to this topic
  • brentn

Posted November 08, 2010 - 08:20 PM

#1

I was thinking that we should have a thread about why we adjust our valves, might give some people some understanding as to why it's so important.

4 Strokes are still new to me and I'm trying to understand valve wear and why it happens so maybe some of you guys can answer some questions?

-What causes excessive valve clearance?
Why is it that we are always adding shims to the valves, why is the clearance getting larger between the cam and the tappet? Is the spring shrinking?
If this is the case then why does shimming make starting easier?

I really don't understand the whole valve situation. I understand how they work, how valve timing works. I just don't understand what exactly is wearing in a 4 stroke dirt bike engine that causes a valve to drop in regards to shimming valves. If anything a stretched cam chain will cause a valve to hit the piston, but why is it that if you ignore valve clearances with the cam your engine will explode?


thanks for the help

  • grayracer513

Posted November 08, 2010 - 09:17 PM

#2

It's a little bit debatable whether anyone actually adjusts valves on a YZ450. I mean, I've heard of it....

But the first misconception you have is the idea that the clearance increases. It doesn't, in fact it's normally just the opposite. The wear, when it occurs, is at the valve face and/or seat. Loss of material here causes the valve to move up into the head, closer to the cams, closing the clearance.

  • rmcr250r

Posted November 08, 2010 - 09:54 PM

#3

Dirt getting into the motor is a big contributor to valve wear. I know from first hand experience. Had an issue with the filter screen holder sealing right with the intake boot.(was unaware of it) I kept finding dirt in the intake boot and eventually my valves clearance started tightening up.

  • Steel Panther

Posted November 08, 2010 - 10:11 PM

#4

You can imagine a valve opening and closing a bajillion times is going to wear down the surface causing the clearance to close up, like rmcr said dirt can play a substantial role in increased wear as well.

  • KJ790

Posted November 08, 2010 - 10:20 PM

#5

Yup, as the others said, the valve and seat wear from the contact between the two as the valve opens and closes each time. Obviously when the valve hits the seat billions of times, wear will begin to occur. If dirt gets through the air filter, then this dirt acts as an abrasive, drastically speeding up the process. This is the biggest cause of short valve life. As the valve and seat wear, the head of the valve gets thinner where it contacts the seat, thus making the rest of the valve sit farther up in the head closer to the cam, which means your clearance gets smaller. Shims are used between the valve and the lifter to set the proper clearance. As the clearance gets smaller, you need to replace the original shim with a thinner one to get the clearance back into tolerance. If you ride with too small of a clearance it can cause big problems down the road.

The thing with titanium valves (which most MX bikes come with now) is that titanium is really too soft of a metal for a valve. To account for this, titanium valves have a very hard, but very thin coating. As long as this coating is in tact the valve may not need to be adjusted at all. It is common to get 100 hours or more without the valves going out of adjustment (as long as the air filter is always maintained properly to keep dirt out). However, once the coating wears through and the valve itself starts to wear, it will wear quickly. This is why it is common for people to go a long time before the valves first go out of spec. After reshimming them into spec, they will often find that they have worn out of spec again in only a few hours. This is because the coating has worn through and the valve itself is now wearing. At this point it is best to change the valves before they get so thin that they break and destroy your engine.

  • brentn

Posted November 09, 2010 - 12:26 AM

#6

So if you end up with no clearance then you def have valve seat problems and you should get the head re-done pronto?

When clearance does increase from say .1 which is spec to .2 or something, then is it the tappet that is wearing?

  • tpars121

Posted November 09, 2010 - 05:04 AM

#7

However, once the coating wears through and the valve itself starts to wear, it will wear quickly. This is why it is common for people to go a long time before the valves first go out of spec. After reshimming them into spec, they will often find that they have worn out of spec again in only a few hours. This is because the coating has worn through and the valve itself is now wearing. At this point it is best to change the valves before they get so thin that they break and destroy your engine.


This wearing of the valve and valve seat also prevents the valve from dissipating heat and causes it to run hotter which compounds the problem.

  • fishkiller

Posted November 09, 2010 - 06:33 AM

#8

Great question!
I too was curiuos about this. Two of my intake valves have incresed in clearance, not decreased. What is happening to make my surface contact from shim to tappet increase?

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  • grayracer513

Posted November 09, 2010 - 07:41 AM

#9

So if you end up with no clearance then you def have valve seat problems and you should get the head re-done pronto?

When clearance does increase from say .1 which is spec to .2 or something, then is it the tappet that is wearing?

Yes to the first question.

On the second, maybe, but not likely. It's more likely to be wear in the cam cap, a bent valve, or something holding the valve off the seat.

  • KJ790

Posted November 09, 2010 - 07:59 AM

#10

It is rare that the clearance increases on these bikes, but it does happen sometimes. Generally it happens when aftermarket shims are used, as some of them seem to be softer than stock ones. These engines are designed so that the cam lobe hits the lifter (a.k.a. bucket) off center so that the bucket will rotate as the engine runs. This keeps the cam lobe from always hitting the same point and wearing a "dent" in the top of the bucket. This will sometimes cause wear to occur over time to the bottom of the bucket, the shim, and/or the tip of the valve stem. More often than not it is to the shim, as that seems to be the weakest link of the three (and cheapest to replace). It's rare, but if it happens the clearance to grow larger if it occurs while the valve/seat do not experience any substantial wear to counteract it.

  • fishkiller

Posted November 09, 2010 - 08:45 AM

#11

I say it has increased but on after thought did not check when bike was new. Could very well have been this way the whole time then. Think I will go ahead and shim it back to spec (using larger shim) and then keep and eye on her from there. Thanks.

  • brentn

Posted November 09, 2010 - 11:33 AM

#12

Alright, so if clearance decreases and you shim it, you are actually putting in a smaller shim instead of a larger one?
If clearance increases then you use a larger shim, both ideas are to get the valve back to .1 for example?

Bad air filter maint would be a good example of why a valve decreases in clearance, as the valve seat and valve edge get worn and cause the valve to "sink back" into the head. Dirt gets trapped and acts as an abrasive to the valve and seat.
Bad oil maint would be a good example of a valve increasing in clearance cause the cam lobe is wearing? Also excessive clearance could be the result of a crappy aftermarket shim that's compressing in size???

thanks for the help!

  • Justin Pearson

Posted November 09, 2010 - 02:43 PM

#13

My YZ "outer" intake valves usually loosen up a little over time as carbon builds up. The center valve never does which I attribute to the fact that the accelerator pump squirts directly on it cleaning it.

  • brentn

Posted November 09, 2010 - 02:54 PM

#14

Back when I had an RX7 and I used to frequent the rx7club forum, a popular way to decarbonize an engine would be to dump a little ATF into the combustion chamber. Let it sit over night and then start the car the next morning.
Tons of white smoke but when it all cleared you had a decarboned combustion chamber which was a big deal for a rotary engine.

I wonder if it would be safe to do on a dirt bike, say if you had carbon build up that affected the valves???

  • Justin Pearson

Posted November 09, 2010 - 03:47 PM

#15

ATF is a famous snake oil, almost as effective as "Tussin"... :p

I do use some ATF in motor oil on occasion as a "flush" if the motor as been submerged or if the oil changes were badly neglected.

The old school method is to use distilled water. Same effect as a blown headgasket, it "steams" the carbon off, however doing this also risks damaging other things like rings, cylinder walls, valves, etc.

I would recommend trying Sea Foam as I have seen minor improvements using it. But I have not yet had the time to do a "scientific" study to see how effective it really is...

  • grayracer513

Posted November 09, 2010 - 03:55 PM

#16

ATF can be effective to a degree. Water works, but water/coolant works better. The best thing for the purpose is an actual top engine cleaner, or a fuel system cleaner. This last item is the same basic product that is already in all pump gas anyway ("Techroline" is Chevron's name for it). Know that running ANY fluid through the engine is risky (hydro-lock), and needs to be done carefully.

It also needs to be stressed that there is a very limited benefit to any of this sort of thing.





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