How stuff works: Carb. ABC and white plastic thingies

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How stuff works: Carb. ABC and white plastic thingies


Post by octane »

What happens in those carbs of ours, airflow/fuel wise?

Let's see
starting from engine-end we look into the carb
and first we see the pilot outlet
witch lets in fuel regulated by the pilot screw

right behind it (not visible on photo)
we find the bypass ports

Next are (left) the outlet controlled by the main nozzle
and (right) the outlet controlled by the jetneedle attached to the vacuum piston.
So what causes the fuel to flow into this space and on into the
Well, because the diameter of the passage through the carburetor
lessens right here, constrictting the passage of air; the venturi effect comes into play
and causes fuel to be sucked into the venturi.

Now how is this vacuum thing working for the vacuum piston/needle.
Same thing:
Take a look at the piston/(and needle)


or seen from 'inside' the vacuum chamber, through the piston anddown into the venturi (blue light inside venturi):


so here we have tree holes to be sucked at !
yeah, but no fuel or anything in there to be venturi-sucked out through those holes, you might say?
Oh yes; air will be sucked out, in effect creating a vacuum inside
the (surprise) vacuum chamber

Take a look here:


DARK blue is air being sucked out through the holes in the piston
Yellow is the vacuum thus created.
Note the small air passage right under the space
to witch the white plastic thingie forms the top 'lid'.

This drawing shows the situation with the trottle valve (right side)
1/4 to 1/2 open.
(PS: LIGHT blue is fuel)

What happens is that when you open the trottle more and more
---> more and more air sucks--->more vacuum--->
piston is moved upwards because of the vacuum--->
more air AND fuel is lead into engine (jet needle, controlling
fuel rate is attached to piston..remember)

(for more on what happens when, please look
at this, and the pages following)

Now back to that tiny air passage at the top of the
vacuum chamber (here back-lit through the pastic thingie


as you can deduct from the drawing shown above this passage is
needed to 'vent' the space created by the hollow piston 'rod'
and the tube in witch it slides (also shown here back lit through
the plastic)


so yes; you are in trouble if your plastic thingies are not intact.
Not only will the air passing through a hole in the plastic,
lower the vacuum in the chamber, and thereby in effect
lower the position of the piston (lowering the intake of fuel/air)
but 'false' air will enter through
hole--> vacuum chamber--> holes in piston--> venturi-->engine

Not good.

Still don't believe me ?!

For the sake of investegating this facinating subject
I put my health at risk and lit up a cigaret
..grabbed a vacuum chamber with a holed plastic top
..inserted a piston/needle (a bit of tape to make up for
slack in piston/side of chamber)
..and blew smoke into the hole


yes indeed; the smoke passed

hole in plastic--> vacuum chamber--> holes in piston-->
(and would pass on to; venturi-->engine)

Conclusion: replace chambers with defect plastic tops.

BTW another thing came up:

The plastic thingie does in fact act as a
upper-level-stop for the pistons.

Not only are traces of the piston rod/tube clearly visible at the underside of the plastic:


but I did measure vacuum chamber-tube/piston-rod/
piston etc. to confirm this.

Facinating stuff...right?
Da' Supercharged Bulldog


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not when there is nothing left to add
but when there is nothing left to take away"

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Post by Neil »

Glad my 1100 does not have plastic tops, think that's why Honda changed them?

Nicely done ... again.
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Post by heraldhamster »

ah. aah-hah. hmm.

now if, say, I've removed the white, plasticky button thinga-ma-jiggies, and covered the tops of the CV piston housings with something else... [insert round object here] does the volume of the vacuum chamber have an effect?

following a Randakk rebuild, meticulously adhered to, even weight balanced the slides on a gram scale. I might have installed my [round objects] too low on the CV piston housings to allow total upward movement of said piston.

2 reasons I ask, I'm having "some" issues, mostly at idle which is likely not relevant to this, but I'm getting some "chuffing" on hard throttle at higher RPMs.

and also 'cause I like to know more stuff about GoldWings.
sorta bulldogged custom 1978 GL1000 - "geekster"
full Vetter dress 1979 GL1000 - "Barge" (currently down)
1986 1200 Aspencade - "Heart of Gold" - daily rider
1990 1500 Aspencade - It's ALIVE! but very, very naked. not in a good way.
1978 for $100 - project in worx
1978 from a previous member here - taking up space
my original '79 bought in '91 - replacing engine (eventually, maybe someday)


A competent and self-confident person is incapable of jealousy in anything. Jealousy is invariably a symptom of neurotic insecurity. ~ Robert A. Heinlein

The good thing about science is that it's true whether or not you believe in it. ~ Neil deGrasse Tyson

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