Originally Posted by katana

Well If I'm setting an engines valve clearances, I'll set then to the most open cold settings, especially as the engines I've worked on have been either locknut and screw or low number of shims.

But was wondering if there was any point as long as the minimum clearance is there? Would larger clearances give a little bounce at higher revs so bit more power and torque.

With bigger clearances the valves wouldn't open as far in the first place so I doubt it. Also with the valve bouncing the valve clearance is irrelevant as the extra movement is only manifest when the valve is on its seat.

With larger clearances, you're making the valves open later and close earlier. That can give you more power at low revs (which may what some people call more torque), particularly because of the earlier closing of the inlet.

However, if the effect was sufficient to be noticeable, it would almost certainly upset the carb mixture at all engine speeds as well as rob power at higher revs.

Also, setting them wider than the book says can cause a hammering effect which might disagree with the valve train components.

Originally Posted by TheEnglishman

I read on ADVRider that at our level (avg joes, not GP Racers) that it's much more important to set the valves the same, over a bit tight or a bit loose....

Valve Adjust- set'um tight or loose? - ADVrider

Why, I have no idea.

On the BMW air and oil heads they have two valves per inlet and exhaust so they should be set the same also as its screw and nut so easy to do.

edit - can see the logic of having both valves of the inlet or exhaust being set the same.

Originally Posted by Velocette

With bigger clearances the valves wouldn't open as far in the first place so I doubt it. Also with the valve bouncing the valve clearance is irrelevant as the extra movement is only manifest when the valve is on its seat.

Doesn't valve bounce occur at full open not closed?

Originally Posted by ink ink

With larger clearances, you're making the valves open later and close earlier. That can give you more power at low revs (which may what some people call more torque), particularly because of the earlier closing of the inlet.

However, if the effect was sufficient to be noticeable, it would almost certainly upset the carb mixture at all engine speeds as well as rob power at higher revs.

Also, setting them wider than the book says can cause a hammering effect which might disagree with the valve train components.

Short time open means cooler valve as its spent a longer time on the valve seat? Why would it affect carb mixture, or do you mean cylinder charge ie contents in the cylinder.

Originally Posted by katana

Short time open means cooler valve as its spent a longer time on the valve seat? Why would it affect carb mixture, or do you mean cylinder charge ie contents in the cylinder.

Longer on the seat might mean a cooler exhaust valve yes, but there isn't a burnt valve problem to be addressed, is there?

Mixture:
The inlet valve does not close until well after bottom dead centre (~60 degrees after on my bike). At high rpm and associated high gas velocities, inlet gas inertia causes inlet charging to continue after BTDC, while the valve is open. Low rpm is different: The piston pushes some of the inlet charge back out through the inlet and carburettor until the valve closes. That gas picks up more fuel as it passes through the carb in the wrong direction. Then some of it hangs around the carb intake as a petrolly cloud and gets sucked back into the engine on the next intake stroke, picking up more fuel on the way. Your idle jetting and some other carb settings have to allow for this, so if you change things so the valve closes earlier the mixture will not be right.

It's probably a bit irrelevant in the case of a roadgoing modern bike.

Originally Posted by katana

On the BMW air and oil heads they have two valves per inlet and exhaust so they should be set the same also as its screw and nut so easy to do.



Doesn't valve bounce occur at full open not closed?



Short time open means cooler valve as its spent a longer time on the valve seat? Why would it affect carb mixture, or do you mean cylinder charge ie contents in the cylinder.

Well it occurs between fully open and partly closed and may become continuous (valve float)but that was my whole point, if there is clearance it can't be bouncing the valve must be closed. Excessive clearance will reduce the movement of the valve and increase tappet noise.

Years ago I knew people who set their tappets to half a thou hot but if you get it hotter than when you set it that way the seats will burn so I wouldn't recomend it.

The extra movement I referred to was the extra free movement of the cam due to the propsed wider clearance.

Edit

I am beginning to see what you mean now but I think it is a bit like increasing the gap between the pads and the disc. The brake is already off so it won't speed the bike up

We always shim our race engines to the middle of the scale as closely as possible.