Effects of valve clearances

[old_chopper]

I'm working on my XS1100 which has a similar design overhead cam/valve setup. The valve clearances are larger than my GS, nominal is .007" and .009" are nominal for the intake and exhaust respectively. I found 2 intakes and 2 exhausts were .001" tighter than spec (.005" and .007"). I'm chasing shims today to correct this but I wondered what kind of change could I expect to see when corrected. The other 4 were spot on. Oh yeah, the zip tie method works like a dream on these too!

 

[Guest]

I'm working on my XS1100 which has a similar design overhead cam/valve setup. The valve clearances are larger than my GS, nominal is .007" and .009" are nominal for the intake and exhaust respectively. I found 2 intakes and 2 exhausts were .001" tighter than spec (.005" and .007"). I'm chasing shims today to correct this but I wondered what kind of change could I expect to see when corrected. The other 4 were spot on. Oh yeah, the zip tie method works like a dream on these too!

It could make a difference, isn't the .005" thight by .002"?

 

[TheCafeKid]

Tight valves will cause a number of issues. Hard cold starting is a common symptom. Rough running is another. The valves are subjected to a massive amount of heat, and when they close that heat is allowed to dissipate throight the head as the valve contacts it's seat. This obviously keeps the valves cooler. When they can't close all the way, the heat cant dissipate, and builds up to the point where it can burn the valve, which obviously isn't good. The valve not being able to close completely will also have an effect on your compression numbers, intake or exhaust circulation, combustion chamber temps and air/fuel mixture. Basically it's simply not good to have tight valves.
They tighten generally because as the valve opens and closes, over time, the valve face "tulips" a little, causing the valve to recede into the head. This takes up some of the clearance between the cam lobe and the shim installed currently. If left alone, the current shim would then have no clearance between it and the cam lobe and the lobe would be constantly depressing the shim bucket to some degree, not allowing the valve to shut.

 

[old_chopper]

It could make a difference, isn't the .005" thight by .002"?

Bottom tolerance on the intake is .006 so .005 is .001 out, bottom on the exhaust is .008 so the .007 is also .001 out. They both are .002 from nominal. Just ordered shims, they were $22 EACH at the local Yammie dealer, I can wait a couple days on Z1.
I guess you have to consider even though .002" isn't much when you're used to working on pushrod motors that in this case it's 29% of the specified clearance. That puts it in perspective. The bike has required some "choke" cold starting since I got it running even when it's 100 degrees out. Hopefully this helps that.

 

[tkent02]

The valves are subjected to a massive amount of heat, and when they close that heat is allowed to dissipate throight the head as the valve contacts it's seat. This obviously keeps the valves cooler. When they can't close all the way, the heat cant dissipate, and builds up to the point where it can burn the valve, which obviously isn't good.

One more thing, if the valve is tight, but still contacting the seat enough to not overheat itself, then if you wail on the engine for a bit, the valve gets hotter. Hot metal expands, so the valve gets a little bit longer and now it can't touch the seat, it can't cool itself at all! Vicious circle, it gets hotter and hotter, gets less and less cooling, and suddenly it gets so hot one side of the valve burns away, your neglect has caused burnt valves, your smooth running engine is done.

 

[Nessism]

With the clearance as specified, and only being slightly out of spec, you won't notice any change in the engine running by installing the proper shims. It's only when you get drastically out of spec that running problems will creep in the case of an engine using a base clearance spec that wide.