Case Pressure

[pd]

I seem to recall there being an amount of case pressure needed for OEM Simplex motors to run . I guess I'm using the wrong search terms , but , I can't find where I read that .
If someone can point me in the right direction , I'd appreciate it .

Thanks ,
Pete .

[Ricks]

Pete, not sure what you are talking about, but it seems to me if you have case pressure, you will not have vacuum.

[pd]

Hi Rick ,

Case pressure would be created by the down stroke , as little as that pressure might be .
Just like vacuum is created by the up stroke .

Maybe I'm recalling seeing a vacuum measurement .

Still , there has to be some pressure in the case in order to move the fuel mix to the combustion chamber .
I have to think that pressure should be measurable at the carb or the inlet .

In my mind , both pressures have to be present in order for these motors to run .

But , maybe I'm all wet .

Pete .

[pd]

I must have been confused as to what I was looking for , as a quick search found this thread on vacuum :

http://www.simplexservi-cycle.com/index.php?topic=1600.msg14972#msg14972

My confusion brings up the question 'why couldn't either measurement be used ?'
Probably as simple as vacuum can be easier to get a decent reading on than case pressure .

Still , it would be interesting to see what ever difference there is .

Maybe the same plate could be used to measure both case pressure and vacuum , using different gauges .

Pete .

[kartjockey]

Simplex engines are of the third port design.  To run they require base pressure in order to push the fuel charge up in to the cylinder.  This happens on the down stroke of the piston. On the up stroke of the piston the third port is closed off creating a lack of pressure(vacuum) which draws the air/fuel mixture into the crankcase.  I think I got this right! 

[pd]

Simplex engines are of the third port design.  To run they require base pressure in order to push the fuel charge up in to the cylinder.  This happens on the down stroke of the piston. On the up stroke of the piston the third port is closed off creating a lack of pressure(vacuum) which draws the air/fuel mixture into the crankcase.  I think I got this right! 

That's how I understand it , Tom .

Pete .

[Ricks]

I don’t see how you could measure case pressure.  I’m not even sure what kind of number you would see.

[pd]

I don’t see how you could measure case pressure.  I’m not even sure what kind of number you would see.

I would think it could be measured in pounds , similar to measuring compression , only from the other end .
As for numbers , I have no idea , really .

Like I said , measuring vacuum would likely be easier ...
Seemingly , both measurements would tell the same end story .

Pete .

[Ricks]

Measuring vacuum is easy, just make a plate for the carb opening.  That would not work for case pressure, as the rotary valve port would be closed. One could probably make a compression plate with a port in it to take a pressure reading from, but I don’t really see the point.

If you think you have a vacuum problem caused by rotary valve to case clearance, send it to Wayne and have it sleeved.  A couple of the old timers at Portland say these motors run better when that clearance is tightened up.

[kartjockey]

you are right about the clearance thing but....who are you calling old?

[pd]

Rick ,

Case pressure could be measured via the intake plate used for measuring vacuum . I can't imagine any of our rotary valves being so tight that it wouldn't allow some pressure to pass . It would be nice if it were , but ...
Again , there really is no need to measure case pressure , other than to insure proper sealing . But , that can be done from the spark plug hole and would be a different pressure source .
Not that it proves anything , but , I still think a case pressure reading would be interesting , if for no other reason than because it could be done and maybe comparison .

Agreed on the rotary clearance . That's akin to valves in a 4 stroke . 4 strokes don't run so good , if at all , when the valves aren't tight to their seats .

As for old , well , that's just a perceived condition , sometimes 'reinforced' by reality . LOL

Pete .

[Ricks]

If you measure at the spark plug hole, you would get a compression reading, not a case pressure reading, as I see it.

Jim Lokai showed me how he checks for a good seal at Portland this year.  He puts the crank in, closes off the rotary hole, then sprays carb cleaner into the case.  If it runs out, he says no good, if it holds, he calls it good!  LOL

[pd]

If you measure at the spark plug hole, you would get a compression reading, not a case pressure reading, as I see it.

Jim Lokai showed me how he checks for a good seal at Portland this year.  He puts the crank in, closes off the rotary hole, then sprays carb cleaner into the case.  If it runs out, he says no good, if it holds, he calls it good!  LOL

You are right about measuring at the plug hole , Rick . That's why I said case pressure could be measured at the intake , just like vacuum , but with a different gauge .

And , if the rotary hole gets plugged , wouldn't that still leave the intake to leak pressure ?
Maybe I'm missing something there ...

If the intake and exhaust are sealed on an otherwise complete motor , air pressure applied through the plug hole should expose any leaks , which become very obvious when the motor is submerged in water . Just like checking a tube or mounted tire for leaks .
 Carb cleaner has a tendency to eat rubber and rubber type things . It's why I never use it and always use brake cleaner instead . Brake cleaner does a better job of cleaning anyway .

Pete .