Better to adjust valves for max clearance?

loudhvx wrote:

Nessism wrote: I disagree that more clearance will cause the valve to hammer into the seat. The cam profile determines the acceleration rate of the valve, not the lash. More lash means the valve just sits on the seal slightly longer is all.

The cam profile, at the heel, is round. This means zero lift per degree. As you enter the lift area, the profile gradually changes from zero to more lift per degree. If you skip over this change area, because the lash is too large, the lift rate, as seen by the valve, suddenly jumps from 0 to some positive rate. The more lash, the greater the jump. The greater jump, the harder the impact.

Just take the extreme cases to see it. Imagine an extreme lash of 1/8". As the cam nose swings around, it will whack the bucket., and conversely, as the valve closes, it will be moving very quickly, then suddenly stop as it hits the seat. Whereas, with exactly zero lash, the cam profile slows the valve down gradually to a complete stop, thus the valve will be closed without ever actually hitting the seat, but it will be closed and just barely resting on the seat.

Looking at it another, simpler way: the fact that valves are noisier when the lash is greater, tells you they are hitting harder. That's what generates the noise.

Hard to believe opening the clearance to the upper end of the specified range would cause any appreciable increase in wear. You're only changing it a couple of thousandths.

BTW: on the 750 twins the original spec was .002 - .004" but later opened up to .002 - .006" by Kawi so I guess that range is safe to run.

loudhvx wrote: Looking at it another, simpler way: the fact that valves are noisier when the lash is greater, tells you they are hitting harder. That's what generates the noise.

I'm not sure that's true. I have always noticed it's a little clickier when cold because as it heats up, all parts expand and close down the clearance. But without some clearance (hot) you will be burning the valve and seat.

I think the point is that the whole system is designed to handle the forces of the valve being closed down by the spring. In fact, at higher RPM you need the spring to be strong enough to prevent valve float where it doesn't close fast enough (ie, follow the cam back down).

bountyhunter wrote:

loudhvx wrote: Looking at it another, simpler way: the fact that valves are noisier when the lash is greater, tells you they are hitting harder. That's what generates the noise.

I'm not sure that's true. I have always noticed it's a little clickier when cold because as it heats up, all parts expand and close down the clearance.

You are supporting my point. The lash is greater when cold. When it heats up, the valves get longer and the lash reduces.

bountyhunter wrote:

loudhvx wrote:

Nessism wrote: I disagree that more clearance will cause the valve to hammer into the seat. The cam profile determines the acceleration rate of the valve, not the lash. More lash means the valve just sits on the seal slightly longer is all.

The cam profile, at the heel, is round. This means zero lift per degree. As you enter the lift area, the profile gradually changes from zero to more lift per degree. If you skip over this change area, because the lash is too large, the lift rate, as seen by the valve, suddenly jumps from 0 to some positive rate. The more lash, the greater the jump. The greater jump, the harder the impact.

Just take the extreme cases to see it. Imagine an extreme lash of 1/8". As the cam nose swings around, it will whack the bucket., and conversely, as the valve closes, it will be moving very quickly, then suddenly stop as it hits the seat. Whereas, with exactly zero lash, the cam profile slows the valve down gradually to a complete stop, thus the valve will be closed without ever actually hitting the seat, but it will be closed and just barely resting on the seat.

Looking at it another, simpler way: the fact that valves are noisier when the lash is greater, tells you they are hitting harder. That's what generates the noise.

Hard to believe opening the clearance to the upper end of the specified range would cause any appreciable increase in wear. You're only changing it a couple of thousandths.

BTW: on the 750 twins the original spec was .002 - .004" but later opened up to .002 - .006" by Kawi so I guess that range is safe to run.

When you degreed a cam, didn't you notice the lift-to-degree rate increases?
The further you go into the initial lift area of the cam, the faster the cam approached the bucket as it traverses the lash distance. So if the lash distance is greater, the cam makes first contact with the bucket at a higher velocity... hence impact. The only way you would have no impact is if the cam was always in contact with the bucket (or shim in shim-over).

Obviously, in order for valves to work, there must be a certain amount of lash, or the valves won't close after warmup. More lash cause more impact. So there is a compromise. Kawasaki determined what is the acceptable range for lash as the compromise for acceptable wear. That's what should be followed on a stock motor.

As long as the valve clearance is between the factory minimum and maximum you will be fine. If you set them so that they are right at the maximum there will be no additional ware at all. The opening and closing ramps on the cam lobes are very gentle. Set them and go ride.

I've never thought about it like that but I support Loudhvx's analysis, It's why valves have to be measured (and adjusted) when stone cold......so that when they heat up to operating temperature that lash must become close to zero to reduce the impact of the lobe on the shim.....hence the clicking when cold but not when hot etc...

Thts why I say to set the exhaust valves just slightly loose to allow for the expansion as they heat up,intakes don't grow nearly as much as exhaust valves in my simple little world anyway.

loudhvx wrote:

bountyhunter wrote:

loudhvx wrote:

Nessism wrote: I disagree that more clearance will cause the valve to hammer into the seat. The cam profile determines the acceleration rate of the valve, not the lash. More lash means the valve just sits on the seal slightly longer is all.

The cam profile, at the heel, is round. This means zero lift per degree. As you enter the lift area, the profile gradually changes from zero to more lift per degree. If you skip over this change area, because the lash is too large, the lift rate, as seen by the valve, suddenly jumps from 0 to some positive rate. The more lash, the greater the jump. The greater jump, the harder the impact.

Just take the extreme cases to see it. Imagine an extreme lash of 1/8". As the cam nose swings around, it will whack the bucket., and conversely, as the valve closes, it will be moving very quickly, then suddenly stop as it hits the seat. Whereas, with exactly zero lash, the cam profile slows the valve down gradually to a complete stop, thus the valve will be closed without ever actually hitting the seat, but it will be closed and just barely resting on the seat.

Looking at it another, simpler way: the fact that valves are noisier when the lash is greater, tells you they are hitting harder. That's what generates the noise.

Hard to believe opening the clearance to the upper end of the specified range would cause any appreciable increase in wear. You're only changing it a couple of thousandths.

BTW: on the 750 twins the original spec was .002 - .004" but later opened up to .002 - .006" by Kawi so I guess that range is safe to run.

So if the lash distance is greater, the cam makes first contact with the bucket at a higher velocity... hence impact. The only way you would have no impact is if the cam was always in contact with the bucket (or shim in shim-over).

I understand the concept, I am just having trouble believing that a change of a couple of thou will have any significant affect.

PLUMMEN wrote: Thts why I say to set the exhaust valves just slightly loose to allow for the expansion as they heat up,intakes don't grow nearly as much as exhaust valves in my simple little world anyway.

That's what I always thought which is why I was surprised when I saw the factory manual's clearances on my 750 twin were the same for intake and exhaust.... which is .002" - .004". For a long time, that's the range I ran.

However, on the net I read that Kawi opened up the limit to .006" so I set the intakes to .003" and the exhaust to .005" because (like you) I think exhaust valves get hotter and need more room to grow.

bountyhunter wrote:

loudhvx wrote:

bountyhunter wrote:

loudhvx wrote:

Nessism wrote: I disagree that more clearance will cause the valve to hammer into the seat. The cam profile determines the acceleration rate of the valve, not the lash. More lash means the valve just sits on the seal slightly longer is all.

The cam profile, at the heel, is round. This means zero lift per degree. As you enter the lift area, the profile gradually changes from zero to more lift per degree. If you skip over this change area, because the lash is too large, the lift rate, as seen by the valve, suddenly jumps from 0 to some positive rate. The more lash, the greater the jump. The greater jump, the harder the impact.

Just take the extreme cases to see it. Imagine an extreme lash of 1/8". As the cam nose swings around, it will whack the bucket., and conversely, as the valve closes, it will be moving very quickly, then suddenly stop as it hits the seat. Whereas, with exactly zero lash, the cam profile slows the valve down gradually to a complete stop, thus the valve will be closed without ever actually hitting the seat, but it will be closed and just barely resting on the seat.

Looking at it another, simpler way: the fact that valves are noisier when the lash is greater, tells you they are hitting harder. That's what generates the noise.

Hard to believe opening the clearance to the upper end of the specified range would cause any appreciable increase in wear. You're only changing it a couple of thousandths.

BTW: on the 750 twins the original spec was .002 - .004" but later opened up to .002 - .006" by Kawi so I guess that range is safe to run.

So if the lash distance is greater, the cam makes first contact with the bucket at a higher velocity... hence impact. The only way you would have no impact is if the cam was always in contact with the bucket (or shim in shim-over).

I understand the concept, I am just having trouble believing that a change of a couple of thou will have any significant affect.

"Significant" is subjective. The idea that there is no affect, is what I dispute.
Obviously valves wear, at least in some part, due to the impact of opening and closing. When two pieces of metal contact each other, there can never be zero wear. In addition, If you increase the speed at which they impact, the increase in the wear cannot be zero. So the idea of increasing the lash and seeing zero change in the wear rate is not possible.

My point, as stated earlier, and to the original poster's question, is don't try to out-think Kawasaki. Just do what they recommend. That is, not trying to hit the larger end of the range, or the shorter end of the range, but simply follow the lookup chart for new shim sizes based on old lash measurement and old shim size.

I'm all for people modifying their bike and doing re-engineering, but for a stock motor, there is no upside to varying from Kawasaki's shim chart, and only potential downside. The only reason I can even think of for not following it, is in the case of the 750 twin, where many shim sizes are harder to find than Jack links at an Ed Begley Jr. dinner party.

Great discussion, thanks everyone! I ended up playing it safe and just adjusting the exhaust valve that was under spec and a couple others that were at the margin so that everything is about midway in the suggested range. Boring, I know, but seems like the compromise solution.

Nessism wrote: I do agree with following the Kawasaki procedure. This is particularly important with regards to positioning the cams the way they specify during the adjustment. The common simplified method of simply pointing the cam lobe away from the bucket and checking the clearance that way is incorrect and will result in false measurements. Follow the factory specified method to assure the valves are set correctly.

I agree...many neglect the method though.