xs650 valve back pressure

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htown16

htown16

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I have an xs650 valve on my 72 commando. I have the head off to fix a stripped spark plug hole. I notice when I turn it over by hand there is almost like a back compression against the piston and its a little hard to turn. Nothing is binding but you can feel resistance on the down stroke and then the sound of air escaping. I'm assuming the xs650 valve has a reed in it that has to be forced open by a certain amount of air pressure. No complaints about the performance of the bike and just some minor leakes around the exhaust rocker covers. Just seems different than my Triumph and BSA which are easy to spin with the plugs out.
 
That's something I will check out. I've certainly done dumber things! Here's what it looks like. Late 72 with crankcase vent at the bottom rear connected to bigger opening in center. Smaller output line on side goes to oil tank. Tank vent comes back to small filter.
xs650 valve back pressure
 
I'd expect it would be more difficult to turn when the piston is on the up-stroke, when there is some amount of vacuum under the piston (which is a good thing).
Any gas that leaked past the rings on the up-stroke (or into the crankcase from anywhere), would be blown out the valve without any effort on the down-stroke.
 
Hi,

when you blow with your mouth into the valve you feel a little resistance before it opens. This resistance increases by the longer way of the tube and it needs a bit more time until it opens. When the engine is running a vaccum will built up and I assume the valve will hardly open anymore.

Ralf
 
htown16 said:
I have an xs650 valve on my 72 commando. I have the head off to fix a stripped spark plug hole. I notice when I turn it over by hand there is almost like a back compression against the piston and its a little hard to turn. Nothing is binding but you can feel resistance on the down stroke and then the sound of air escaping. I'm assuming the xs650 valve has a reed in it that has to be forced open by a certain amount of air pressure. No complaints about the performance of the bike and just some minor leakes around the exhaust rocker covers. Just seems different than my Triumph and BSA which are easy to spin with the plugs out.
Click to expand...

Ive been known to read things wrong, but what i read here is that you have the head off and turning over (inspecting bores maybe) and you hear a "hiss"..

With a yami reed valve fitted, IMO i would expect to hear sump pressure (air) being evacuate from sump via the reed valve, even with head off....

Thats what its there for, being to create a minimal "vaccum" in the c/case to reduce oil leaks..

2 pistons pushing @~750cc of air when it goes to bottom of stroke the air has to go somewhere, eg reed valve in your case, non reed valve goes to oil tank...

That what i think!

I just checked my xs valve and you have it connect the correct way...........
 
Ok, to feel a resistance while the pistons are moving upwards is usual with valve or understandable at least. By the way, to set the pistons at OT after shutting off to reduce wet-sumping could cause the opposite because the vaccum sucks the hot oil into the case.
 
Towner said:
Ok, to feel a resistance while the pistons are moving upwards is usual with valve or understandable at least. By the way, to set the pistons at OT after shutting off to reduce wet-sumping could cause the opposite because the vaccum sucks the hot oil into the case.
Click to expand...

Wow, Are we reading the same book............ It probably me again so i will buttout....
 
olChris said:
Towner said:
Ok, to feel a resistance while the pistons are moving upwards is usual with valve or understandable at least. By the way, to set the pistons at OT after shutting off to reduce wet-sumping could cause the opposite because the vaccum sucks the hot oil into the case.
Click to expand...

Wow, Are we reading the same book............ It probably me again so i will buttout....
Click to expand...

I experienced that it wet-sumps more with piston at TDC (=OT in German, sorry :oops: ) with the valve installed.
 
Towner said:
olChris said:
Towner said:
Ok, to feel a resistance while the pistons are moving upwards is usual with valve or understandable at least. By the way, to set the pistons at OT after shutting off to reduce wet-sumping could cause the opposite because the vaccum sucks the hot oil into the case.
Click to expand...

Wow, Are we reading the same book............ It probably me again so i will buttout....
Click to expand...

I experienced that it wet-sumps more with piston at TDC (=OT in German, sorry :oops: ) with the valve installed.
Click to expand...

That's an interesting point Towner.

If these valves create a negative pressure in the crank case, and if the cases are under negative pressure when the engine is switched off, then I guess it WILL make wet sumping worse as the negative pressure will draw in oil. If the pistons are at TDC, the negative pressure will be at its greatest.
 
Fast Eddie said:
If these valves create a negative pressure in the crank case, and if the cases are under negative pressure when the engine is switched off, then I guess it WILL make wet sumping worse as the negative pressure will draw in oil. If the pistons are at TDC, the negative pressure will be at its greatest.
Click to expand...

Piston rings are not airtight due to the ring gaps, etc., so I can't see how any negative crankcase pressure could last more than a few seconds? :?
 
L.A.B. said:
Fast Eddie said:
If these valves create a negative pressure in the crank case, and if the cases are under negative pressure when the engine is switched off, then I guess it WILL make wet sumping worse as the negative pressure will draw in oil. If the pistons are at TDC, the negative pressure will be at its greatest.
Click to expand...

Piston rings are not airtight due to the ring gaps, etc., so I can't see how any negative crankcase pressure could last more than a few seconds? :?
Click to expand...

I agree LAB. Unless of course his rings are particularly good... and his pump is particularly not!

And, of course, if the engine has just been running, the oil will be hot and thin...

The question is, how much oil will be sucked into the crankcase in that time? And does that initial 'boost' lead to more rapid follow on leakage?

I don't know the answer to these questions, it was Towner who wrote that he did indeed see an increase in wet sumping. I was simply trying to explore what the possible link is between the two (the breather and the wet sumping).
 
Fast Eddie said:
And, of course, if the engine has just been running, the oil will be hot and thin...

The question is, how much oil will be sucked into the crankcase in that time? And does that initial 'boost' lead to more rapid follow on leakage?
Click to expand...

Rings are good and pump as well. Reconditioning of the pump decreases wet-sumping about 50%. Most amount was always within the first hours after shutting off - before and after the pump reconditioning. More than cold oil within 2-3 days. But with the valve and pistons at TDC it is noticeably more within the first hours. Now I omit the TDC procedure, less effort anyway.

Edit: With the pistons at TDC combustion chamber is closed, so venting through the rings could last longer.

Ralf
 
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