Venturi piston not moving on one carb?

what could be the matter? sides just fine if i move it with my finger but it won't move on it's own when it rev it up.this is on a 500. this is on the left rear. all others move just fine.

 

I would suspect a bad diaphragm.

-Jake

 

Is the diaphragm tore?

 

it's not torn but it is a little more wrinkled than the others. it's difficult to get it to fit down in the grove so it can seal. the vacuum comes from the little port on side correct? this port is clear. I'll try messing with the gasket on the outside of the diagram and maybe it just wasn't seated correctly.

 

Is that cylinder running (exhaust pipe hot?) idle smooth? Butterfly for that carb opening? All good the diaphram must be leaking unless the vac port is plugged.

 

i've been messing with it for the last hour in a half. this is what i noticed:
while cranking this cylinder has significantly less vacuum on the top of the carb than others. (i can tell just by putting my hand over top of the carb)
while running if i push the piston up with my finger the piston pushes back down against me?
exhaust is no where near as hot as all other exhaust.
spark plug looks good and i get a big fat spark.
my next thought was a compression test but i don't have the right adapter for this motor.
tinkerinWstuff: good idea on the vacuum. i should of tried capping that port off. that but i just took the carb off and took it apart. to make sure everything is still clean. all orifices flow with ease.

 

It pushes back because there is a spring in there. It sounds like you have a pretty significant vacuum leak. If you've been doing all this testing with the petcock vacuum port open I would cap that off and start over.

-Jake

 

If the intake manifold (rubber boot) is good you need to check compression or pull that valve cover and check valve springs and clearance.

 

lawl yea it was just the petcock vacuum, i plugged that off temporally and now it's fine. yea i do need to check valve clearances soon though i'm sure she's more than due.

 

That's much easier than valve trouble.