Effects of incorrectly shimed valves?

[motoman]

Have to agree with Crazcnuk. As an ole' high-performance builder and racer, not an opinion, just the facts were stated there. Aside from that, the 81 and later DO have wider clearance settings on the inner two cyls. Figure that falls in line with the 80 and later having the inner two cyls. jetted two steps richer to account for the extra heat created in that location. Guess Yamaha figured that was an issue headed towards the lean-burn lower emmission requirements.

[crazy steve]

...If your valve(s) are too tight, you will experience compression loss at all engine speeds once the engine is up to temperature because the valves will open sooner and close later than they are supposed to...

I'll have to disagree with this statement. Running the valves 'tight' will slightly decrease low-end performance, but can increase power up high; this has the same effect as increasing lift and duration. This has been a racer trick in the past and is still used in classes where camshaft specs are limited; the valves open sooner, open more and close later, allowing more air into the motor, for a small but measurable increase in power.

Would I recommend doing this on a daily-driven XS11? Nope... the possible consequences outweigh the benefits unless you like checking/adjusting your valves every 500 miles or so, but if you're going racing and checking the valves before every race, this can be free power...

'78E original owner

[Crazcnuk]

"This has been a racer trick in the past and is still used in classes where camshaft specs are limited"

Yes, but the 'trick' is to set your valves to the minimum clearance, not less.

We aren't talking about properly set tight valves, but valves set too tight.

[crazy steve]

...Yes, but the 'trick' is to set your valves to the minimum clearance, not less...

No, the 'trick' is going below the 'recommended' clearance. Back in the day, a popular SBC cam was the solid-lifter 'Duntov 30-30', which got it's name from the recommended valve clearances of .030" on both intake and exhaust. Many drag racers routinely reduced this to only .010", with some braver souls going as low as .005". An increase of .020" to .025" in net valve lift is nothing to sneeze at, with a gain of 3-5 hp reported at WOT. But this didn't transfer well to the street at all; guys that tried generally ended up with burned valves, and occasionally someone would suck a valve.

Again, I wouldn't advocate that anyone do this on a street-driven vehicle, as recommended clearances allow operation in the broader conditions encountered in 'normal' use. This was/is a trick that's generally only used by drag racers and some short-track guys; it's not something you do for distance. Particularly on a DOHC shim/bucket assembly, where the minimum can quickly disappear. Personally, I adjust my XS valves to the 'max' as it's just that much longer before I have to in there again, and the slight reduction in duration helps with low-end power.

If I were trying to wring every last bit of power out of a XS11, I'd adjust the valves to the minimum spec cold (as per the manual), then check it again hot; at this point, adjust all the valves to the minimum number you find. But this would require constant checking/adjusting of the valves (every few hundred miles? Even less?) and who wants to do that?

'78E original owner

[Crazcnuk]

Well, your confusing mfg recommended versus mine. I consider 'minimum' clearance to be bordering on burning valves.

If you adjust too tight, by my measure, your valves will not close. You can try and get more performance from that, but I don't think it will work.

MFGs use a different 'recommended' range to try and minimize possibility of failure while not creating too much noise, or requiring excessive maintenance.

[Drumagician]

This is some good information going on here. After reading the last few posts, I'm almost afraid to even ride my bike at all until the clearances are set right; being that my clearances are about 0.07mm cold. How long does it take to burn valves up with this condition?

[crazy steve]

Well, your confusing mfg recommended versus mine. I consider 'minimum' clearance to be bordering on burning valves.

If you adjust too tight, by my measure, your valves will not close. You can try and get more performance from that, but I don't think it will work.

MFGs use a different 'recommended' range to try and minimize possibility of failure while not creating too much noise, or requiring excessive maintenance.

That's the crunch; what is the 'minimum' clearance? Theoretically, clearance could be zero as long as the valve fully closes, but achieving that in the real world isn't likely. The way I would define 'minimum' would be the clearance needed to allow the valve to fully close and contact the seat well enough that the valve could dissipate excess heat through the seat and into the head. Go under that amount, and burned valves will be the result, sooner or later. Go tight enough that the valve doesn't fully close, and it'll be sooner.

I'd be curious to know just what the 'hot' clearance on these motors is...

'78E original owner

[Crazcnuk]

You bet.

As for hot clearance, set the cold clearance, where you want it, put the valve cover on and warm the engine to operating temp, then quickly measure a couple valves.

[crazy steve]

...As for hot clearance, set the cold clearance, where you want it, put the valve cover on and warm the engine to operating temp, then quickly measure a couple valves.

I don't want to know badly enough to get the inevitable burns that would result...

'78E original owner

[Crazcnuk]

Yes, that would be a problem.

[Drumagician]

I'm not to familiar with the term burnt valves. What exactly is this? I'm guessing it is when your seat area on the valve becomes distorted due to excessive heat and will not seat properly therefor loosing compression. If this is true, I can then recheck my compression when cold and know if I am destroying my valves. ????

[crazy steve]

I'm not to familiar with the term burnt valves. What exactly is this?

Burned valves is the condition you'll find when the valve isn't able to properly dissipate heat. This almost always only happens to exhaust valves, because when an intake valve seal fails the compressed mixture is blown back into the intake/carb; if this happens, you'll know it!

Causes can be maladjustment (this can burn intake valves), very heavy long continuous loads, fuel quality, not enough backpressure (open pipes), no or too short of an exhaust pipe, too-lean mixtures, sometimes just plain wear, or combinations of any of the above. Once a valve starts to 'burn', it will quite literally erode away; I've seen some that had about 1/3 of the head missing when removed. Generally, a burned valve will have to be replaced unless you catch it very quickly.

If you only have .07mm clearance, yes, you very definitely need to adjust your valves.

'78E original owner

[Crazcnuk]

What he said, plus, the valves are cooled when they contact the valve seat. if they don't contact the valve seat, you get a hot spot. That hot spot, over time, can mlet or burn through the valve.

In extreme cases, the valve seat can also burn.

[petejw]

heres a pic of a burnt exhaust valve,



came out of the number 1 cylinder, it occured a few
thousand k's after i lapped the valves.

the thickness of the edge of the valve should be no
smaller than 1mm, its a bit hard to tell from the pic
buts its smaller than that.

[Steptoe]

hot or cold

as far as I was taught the range in the clearances {set cold} is to make it as close as practical when hot,,, for an every day rider

as for checking clearances when hot,{a hot set},, in the trucks that's exactly what we do,,
drive them into the w/shop straight off the road, rip the top off and check them,, as long as an eyelash can slide thru, they're left alone,,
it's a bit more involved but as for valves that's pretty much it

not a job for the light hearted, but the mechanics know what they're doing