Better to adjust valves for max clearance?

Hi all,

I'm doing my first valve check/adjustment and am wondering what clearance to aim for. Most of the valves are at the mid to bottom end of the allowable range, with one exhaust valve below spec by 0.02mm . If I look at the service manual it says "no change required" for the valves that are within the allowable range, but I'm wondering if I should change shims to bring their clearance closer to the highest allowed value, to allow for future wear. Anybody have any thoughts on this?

Also, just out of curiosity I'm wondering if the Zephyr 550 and KZ550 share the same specs. These are the specs for the Zephyr

Exhaust: 0.15-0.25mm
Intake: 0.10-0.20mm

Thanks!
Mark

This sounds promising -

"Valves may be adjusted to increase the length of time of combustion. The idea being that the fuel burns more completely and produces more power. The Inlet valve clearance is adjusted to minimum spec so that the intake happens as soon as possible. The exhaust valves are adjusted to maximum spec so that they open as late as possible. In this way, the amount of time for the fuel to burn completely is optimized. Also, pressure from expansion of combustion gases drives the piston down for the maximal length of time before the gases are exhausted. The length of time that the fuel burns is of course increased by a very small fraction of a second but if power output needs to be optimized even to the smallest degree, this may be an appropriate part of engine tuning."

I keep valves on the higher gap. Valves tend to close down over time and a tight valve that doesn't close can burn a valve seat.

Set the gaps towards the widest specified.
Trying to set the gaps for maximum burn efficiency will be a waste of time for a road bike and will only create a requirement for very frequent service intervals which will cost more than you would ever gain.
The principal outlined in your second post would only be beneficial in a pure balls out performance engine like a drag bike where the engine will be stripped, inspected and re-adjusted after very short running periods.

I know lots of folks have seen their valve clearances get tighter over time. My experience has been different. When I bought my bike in April, 1977 it had one valve that was near the max clearance and the other valves were near the center of the range. I adjusted the one so it was close to the center of the range. Since then I have measured the valves periodically. What I have found is the clearances of some valves may get very slightly tighter or looser over time, but then those same valves may go in the opposite direction given more time. Not a single valve clearance has ever moved to the edge of the specified range, either tighter or looser, in more than 56,000 miles, and I have not needed to make any more adjustments. I have always been very pleased with the fact that they stay within the specified range. I don't know if my experience is different than others because my bike is a KZ650 with shim under bucket and theirs may be the larger engines with shim over bucket. Also, I don't know if this has any bearing whatsoever on the valve clearances, but I have often ridden my bike hard, and I always used premium fuel.

So, from my experience I would set them as close to the center of the range as possible and then check them every 5,000 miles or so to see if they change, and if so, which direction they change whether they change enough to care about. Ed

loose valves are happy valves.

I'd venture to guess that burned valves are one of the most common reasons a lot of old bikes hit the scrap heap. Adjusting valves intimidates many owners, and neglected valves results in the clearance going away followed by burning of valves.

Photo below shows some neglected valves pulled out of a shim and bucket Suzuki GS head...

Just from a logic standpoint,
The wider you set the gap, the harder the valves hit the seats. It also increases how hard the cams hit the buckets (or shims in the case of shim over). This increases wear on the valves and seats. This shortens the life of both before you have to replace or cut them, and increases wear on the cams and buckets/shims.

Because of the increased wear rate, it's a false economy to set the gap at the widest to get a longer maintenance interval. The increased wear rate will quickly reduce the gap initially to the more stable gap range, then the maintenance interval will only be slightly longer than normal. But, you've also greatly increased the wear in a short time, so the overall life span of the valves and seats is reduced dramatically.

The initial and final cam lift is very gradual to allow for soft impact on the cams and valves. Once you go outside of those ramps, it becomes a hammer.

But regardless, it's best to let Kawasaki do the thinking in this case, since they've done far more research on it than anyone else. The answer is simple: Just use their shim replacement chart, as spelled out in the factory service manual. (... after you confirm the manual has no misprint, of course)

loudhvx wrote: Just from a logic standpoint,
The wider you set the gap, the harder the valves hit the seats. It also increases how hard the cams hit the buckets (or shims in the case of shim over). This increases wear on the valves and seats. This shortens the life of both before you have to replace or cut them, and increases wear on the cams and buckets/shims.

But regardless, it's best to let Kawasaki do the thinking in this case, since they've done far more research on it than anyone else. The answer is simple: Just use their shim replacement chart, as spelled out in the factory service manual. (... after you confirm the manual has no misprint, of course)

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.

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.

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.

I always set the intake valves towards the middle and the exhaust valves just slightly towards the loose side of scale for heat/expansion issues on exhaust valves.

What a timely topic. Just completed my valve inspection last night. Had avoided it as mentioned above. That being said I was looking at this same question. Think following the manual is probably the best way to go. Good hunting.