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Display Mesage #114366

vacuum piston not functioning
Written by mikeyb_houtex on 6/25/2012 at 05:03 am

Well, it's really quite elegant, and actually somewhat simple.

First, though, lemme say that I find it quite interesting that you say the
engine is running correctly AND the left carb's piston doesn't move. These two
'facts' simply cannot coexist, I don't think, as the carbs don't work that way?
Surely I'm not that far off on these carburetor's functions that I'm mistaken in
that they can't run good without proper piston movement... Am I?

I can see the confusion. Odd indeed.


How the pistons work:

The carb gets air moving through it from the air cleaner to the engine. The
engine sucks in what it needs. Coupled with the allowance of how much air is
allowed to be sucked in by the butterfly valve, among other variables, and you
get an allowance of air, and a ratio to that of about 1/14 of fuel required.
Hence the piston/needle system to determine that fuel delivery, and when it's
right, it's really a thing of engineering beauty, you ask me.

The piston gets into the act by using differential pressure between the venturi
of the carburetor and the tube that's coming in from the back of the gas fill
box. There's a little filter there too.

This goes to one side of the diapragm of the piston in the pressure chamber.
It's the side that's *away* from the chamber's top. This is what 'pushes'
(semantics on that) the piston out.

However, this can't happen unless there's a vaccum on the other side of the
chamber/diaphragm, and that's supplied by a small hole on the side of the
chamber, and is partially unblocked by the chamber's top. See this webpage:

The inset graphic on the right middle shows the top has to be put on JUST SO, to
let the vaccuum supplied by the main venturi of the carburetor to be let into
that side of the chamber. Without this, there's no way for the system to 'read'
that it needs to move the piston, and therefore the jet needle. Any problem
that causes these two 'sides' of the chamber to not be able to 'see' the
differential pressure... and the piston stays down, at 'idle' so to speak.

Assuming the spring is in there, and it's not broken, of course. :)

This is why I say the two 'facts' cannot coexist. If your engine is running
well at all speeds/demands, all this is working correctly on both carbs.
Without the piston moving, the needle doesn't move, and the engine cylinder of
that carb starves of fuel.

Again... odd.


The easiest way to tell if the thing is working it so take off the tube that
supplies 'ambient' atmosphere air to the chamber and blow in it gently. You
don't need a whole lot, just enough to make it move. You can use a pressure
regulated air blowing tool or make a tube that you can blow into and hook that
up. Just enough to see it move. It does NOT take much, and if you use too
much, you can blow out the diaphragm. Be careful with this.

I dunno if that helps or not, but I think it's working, or you wouldn't have a
good running PC, as it sounds like you do. I'd be interested to hear how you
did the previous testing though.

That's not saying that I disbelieve you, I just... well, having gone through the
amount of material to figure out what was wrong with my carbs/engine... I don't
think it's possible they're not both moving, honestly, and it not give you
*really* bad problems... Namely, how does an engine get fuel if the needle is

That's why it's odd to me. I'll be waiting to see what you wind up with on

'95 PC800, "Ride"

--- In, "jdoyle_53" <jdoyle_53@...> wrote:
> Can someone point me to a discussion or link describing the workings of the
carburetor vacuum piston on a PC800? My left carb piston doesn't move ? right
carb seems OK even after swapping diaphragms between sides. I left gas in it
last winter expecting to ride but did not. Not much of a rider and no mechanic
at all, I relied on posts here for direction to clean up the mess and have
finally had some success. I replaced low speed jets since I could not get them
clean (a wee little light peaks thru the length of the new ones) and installed a
new fuel filter to get smooth running. New plugs fixed
starting/acceleration/power problems. I rode the misses across county to visit
friends this afternoon ? will ride to work some next week. Now I am baffled by
the non functioning piston on a machine that seems to run just fine and would
like to see if I can get this straight.
> Thanks in advance. I live sort of off the grid so I will catch up with any
replies when I venture into civilization.

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