bendix carb question

My old C-85 has a Bendix Stromberg NA-S3A1, which is leaking for a time
after shutdown.

Previous owner of the plane had tied off the mixture control so shutdown is
by reduced throttle and magnetos turned off.
Casual conversations are leaning towards a needle seat or float level
problem. Any other possibilities?

Not knowing how the mixture control works inside the carb body and not
finding any carb parts breakdown pictures or float setting information
around in the chapter library today and having carb accessable with the
cowling off, can I just remove the airbox and then what appears to be the
float bowl for some kind of inspection??

I did see that A.S.S. has an overhaul manual and some pretty pricey needles
but am hoping to get back into the air sooner. Can anyone
help? Float setting procedure and needle inspection.

Thanks, Dick

On Thu, 16 Jun 2005 21:46:39 GMT, "Dick" wrote:

My old C-85 has a Bendix Stromberg NA-S3A1, which is leaking for a time
after shutdown.

Ahhh...the "Stromberg Drool." I've got it, too, though it gets a lot better
when I run avgas instead of autofuel. I live with it by shutting the engine
down with the fuel valve instead of the magneto switch.

More than you ever wanted to know can be found at:

http://www.bowersflybaby.com/tech/engines.htm

Scroll down to "Neal Wright on the Stromberg Carburetor."

Ron Wanttaja

Note! The mixture controls on that carb is NOT an idle cut-off type.
You must still either shut off the fuel supply and let the carb run dry
or use the mag switch(es) to shut off the engine. Minor detail, but
probably a good thing to be aware of. A lot of those mixture controls
were wired full rich. They were rather touchy.

It probably has the steel needle and seat installed so that it can run
on auto fuel. Those things seldom ever seat tightly. It might be best
to just keep the fuel shut off when parked. However, if you are unsure
what kind of needle/seat is installed and you plan to run auto fuel, you
should pull the carb off the engine and install the steel needle and
seat. Fresno Airparts has the conversion kit for $99.50. (559) 237-4863.

That carb has service instructions and parts list in my Maintenance and
Overhaul manual for the A50/A65/A75/A80. It is also available at Fresno
for $17.50. It MAY also be in the manual for the C-85, etc. and that
manual is $15.00, so you might call Fresno to check on that.

Scott
http://corbenflyer.tripod.com


Dick wrote:
My old C-85 has a Bendix Stromberg NA-S3A1, which is leaking for a time
after shutdown.

Previous owner of the plane had tied off the mixture control so shutdown is
by reduced throttle and magnetos turned off.
Casual conversations are leaning towards a needle seat or float level
problem. Any other possibilities?

Not knowing how the mixture control works inside the carb body and not
finding any carb parts breakdown pictures or float setting information
around in the chapter library today and having carb accessable with the
cowling off, can I just remove the airbox and then what appears to be the
float bowl for some kind of inspection??

I did see that A.S.S. has an overhaul manual and some pretty pricey needles
but am hoping to get back into the air sooner. Can anyone
help? Float setting procedure and needle inspection.

Thanks, Dick

Might have the needle with the neoprene tip, and autogas will
attack it and cause it to leak, or worse, maybe stick open or closed
and kill the engine. It's worth looking at. There was a Delrin or
similar polymer needle available at one time for that carb; sealed
better than the steel and resistant to autogas, IIRC.

Dan

I have the same carb and a delrin needle and took the carb apart
recently to check for trash because a friend told me he found a
dirt-dauber in his float chamber. I was running a little lean. I
replaced the big gasket after finding nothing inside.

This carb has always worked well and not dripped even though the fuel
run through it may vary.

I have rebuilt several of these very simple carburetors and think a lot
of them, but they won't give you the oomph or diversity of a Marvel
Schebler.

The mix control is working on mine but it doesn't kick in until you get
to something like 1500 rpm, making it almost impossible to kill the
engine by starving the mixture, although it CAN be done by completely
leaning and firewalling the throttle.

The last Stromberg NAS3A1 I rebuilt we tested on the bench and the
steel needle would not seat completely. It dripped a little, so the
A&P/IA who signed it off whacked the needle against the seat with a
hammer. Which stopped the leak. Don't ask me how hard to hit it.
It must be done gingerly, deftly, prayerfully.

BTW, the ultimate authority on Strombergs is Neal Wright (Cougar) of
Sacramento.

Check out http://www.cub-club.com/PrintedTechlist12.2.04.doc and you
can find float level setting instructions for this carb. I don't know
this carb specifically, but most lean the mixture by applying
differential against the float bowl changing its level. I'm guessing
the float level is too high to begin with.

Float level is set with washers under the float valve seat.

Dan

I should have been a little more specific. The float level is set
with shimming washers under the float valve seat; they're fiber washers
that come in a couple of thicknesses to get the level right. If the
carb is a bad dripper the float might have a pinhole leak in it and be
partly filled with fuel, making it sit lower and allow the level to
rise enough to drip out of the main nozzle.
The mixture control doesn't change the float or fuel level. It
applies a bit of venturi suction to the bowl, reducing the atmospheric
pressure in the bowl and holding the fuel back. The bowl is vented to
the venturi wall, and another port passes through the mixture control
valve to a dead-air space behind the venturi that has normal
atmospheric pressure. The venturi suction is constantly drawing air
from behind the venturi through the bowl's airspace, and pulling the
mixture to lean closes off the atmospheric pressure supply and lowers
the bowl pressure. At idle or low power settings, there's little or no
venturi suction and the mixture control has no effect.

Dan

The last Stromberg NAS3A1 I rebuilt we tested on the bench and the
steel needle would not seat completely. It dripped a little, so the
A&P/IA who signed it off whacked the needle against the seat with a
hammer. Which stopped the leak. Don't ask me how hard to hit it.
It must be done gingerly, deftly, prayerfully.


Wouldn't it be safer to lap it in with some very fine lapping compound?

LCT Paintball wrote:
The last Stromberg NAS3A1 I rebuilt we tested on the bench and the
steel needle would not seat completely. It dripped a little, so the
A&P/IA who signed it off whacked the needle against the seat with a
hammer. Which stopped the leak. Don't ask me how hard to hit it.
It must be done gingerly, deftly, prayerfully.


Wouldn't it be safer to lap it in with some very fine lapping compound?

You know what? We tried lapping it in with valve grinding compound,
with toothpaste, and jeweler's rouge (not in that order), and it didn't
work. But the whack with the hammer stopped it from dripping.

One of our gurus with A-65's in Alliance, OH also uses a hammer to
"seat" a steel needle.