hmmmm ... I get the same symptom with the cap completely removed. So
it can't be a problem with the cap, venting/non-venting/loose
I can verify that the choke cable is connected and working correctly.
I did replace the gasket that contains the reed valves as part of the
carb-rebuild. I made sure to install the spring against the carb inself
pressing outward against the gasket towards the tiny plate that bolts
onto the carb - this is the way the spring was oriented when I first
opened it up. Sure looked correct - the inside of the tiny plate did
not have a machined area for a spring to reside whereas the
corrosponding area within the carb itself was machined. And yet - no
fuel pumping would surely explain the symptoms I'm experiencing. How
does this pumping occur in a carb with no moving parts?
Tomorrow I'll try filling the gas tank -extremely- full i.e. enough to
make sure some gas is sloughing into the shallow pan.
Thanx for everyone help so far!
The suction to draw gas from the bowl into the carb venturi nozzle is
the Bernoulli effect.
Perhaps you're asking how the fuel pump works? Part of that thing you
called a "the gasket that contains the reed valves" is actually a pump
diaphragm. Those little flaps are the check valves, and they do act
like reed valves. A pulse of intake vacuum pulls the diaphragm one
direction while compressing the spring, and this draws the liquid
through the foot valve flap, up via the pick-up tube. Then the spring
ejects the liquid through the other valve and into the bowl.
A similar type of pump is used on 2-stroke outboard motors, chainsaws
etc. In that case, the crankcase supplies pulses of both vacuum and
pressure to operate the diaphragm both ways, so the spring is typically
The design of the tank may make this difficult. You might be able to
snake a tube into the bowl through the filler opening and squirt some
gas in that way. If the engine can be revived by filling the bowl
directly, suspect the pump. Confirm your suspicion by checking the
level in the bowl after a nuisance shutdown. The bowl should be full to
the point of overflowing back into the tank proper. If this requires
tank removal to confirm, be careful not to affect your reading by
sloshing fuel out of the bowl as you do it.
A weak pump might be able to supply enough fuel when the engine is not
loaded, but as the throttle opens and consumption increases, the level
in the bowl drops and leans the mixture, or just plain starves the
engine if the level drops sufficiently. Running lean under load can
cause overheating, which may cause the engine to seize or shutdown
after a few minutes.
A weak pump might work OK when the tank is full but not so well as the
tank level drops. If the pump cover doesn't mate with the carb casting
(almost) perfectly, air can leak into the wrong part of the pump and
decrease its effectiveness.
If the diaphram has a pinhole, it can play heck with the mixture,
making it very throttle-dependent and hard to set reliably with the
needle. Did the pump spring have a little "ring" that fit over it to
protect the diaphragm from the sharp end of the spring wire?
Smaller engine (chainsaw), but similar symptoms - cleared when I cleaned
the carbon off the screen over the exhaust outlet.
On 22 Sep 2006 02:32:08 -0700, email@example.com wrote:
This was in my hotmail (below) but worth the readers and the engine
owner to see. I learned a bit, thank you.
That's some seriously interesting information. I got out my old, dusty
Briggs and Stratton book. turns out that it says that suction from the
intake stroke powers the diaphragm which moves fuel. Essentially what
And you're right that I got pulsa and vacu backwards. Ah, well.
Thanks for the advice, it' someone else's engine, I'm just a
From: Husky <
To: Stormin Mormon <>
Subject: Re: B&S 5HP engine runs for a minute then stalls
Date: Mon, 25 Sep 2006 23:32:00 -0400
Stormin Mormon wrote:
Wrong! The vacu-jet only had ONE pick-up tube, and is a "suction"
system where the entire fuel tank ACTS as the carburetor bowl. Two
tubes indicate the carb is a PULSA-JET, which uses the short tube as
the venturi nozzle and the long tube as the fuel pump pick-up. The
3/8" aluminum tube connecting the crankcase breather to the carburetor
is the CLOSED CRANKCASE VENTILATION "hose", and the breather and tube
combined are the equivalent to your car's POSITIVE CRANKCASE
VENTILATION (PCV) where the breater itself is the valve. The fuel pump
is operated entirely by the intake vacuum pulses directly from the
inside of the carburetor throat. On horizontal shaft engines, the
diaphragm is open directly to the throat between the throttle plate
and the mounting flange; on verticle shaft engines it is ported to the
same area by a drilled passage. Some verticle shaft engines also
include a vacuum choke control ("pull off") on the same diaphragm, and
vacu-jet carbs only use a diaphragm to provide the choke pull-off. A
newer version of the Pulsa-Jet, called the Pulsa-Prime, replaces the
choke and choke pull-off with a bulb operated primer, which is
basically just an extra fuel pump jetted directly into the throat of
the carburetor. This should not be confused with the primer installed
on B&S/Walbro carburetors, which pressurize the bowl with air to force
fuel up the nozzle.
The reason for the Pulsa-Jet carbs is that the fuel level in the bowl
affects the mixture at which the carburetor operates. The higher the
fuel level, the richer the mix it provides. To more closely control
the mixture, the Pulsa-Jet maintains the inner bowl at a full to
overflowing level by the operation of the fuel pump and the opening at
the top of the inner cup which spills back into the fuel tank. The
Vacu-Jet carbs change their mixture as the fuel level drops, requiring
the specific instruction in the service manual that they are only to
be adjusted at the half-tank level.
Five horsepower engines can be equiped with EITHER an air-vane
governor OR a mechanical flyweight internal governor. Both will have
the spring and linkage however, and should move such that the throttle
plate can travel from fully closed to fully open.
One thing I "missed" the first time is the "Jet" staked into the end
of the short tube. This indicates the engine is a late model, and is
probably pollution "controlled". There should have also been a fine
metal screen sock over that short tube. Everything I posted earlier
still applies, but due to the leaner operating environment of an
E.P.A. or C.A.R.B. engine, it may have to be started two or three
times before it will stay running, or it may have to be "warmed up"
for as much as a minute on part choke. The other thing to suspect is
the "jet" may be partially blocked. The shallow pan can only get fuel
from the fuel pump, if it isn't working, you would never start the
engine in the first place, as the only way to get fuel into the "bowl"
would be to completely "overfill" the tank, which, I suspect is
impossible due to the angled fuel fill cap the fits into the cut off
corner of the tank. The older tanks had a raised neck, onto which the
cap threaded, but the "jet" you describe in the short tube was
introduced about the same time as the new tank with no neck and a
quarter turn, non-vented cap like the old cars. The vent is through
the carburetor on these models. It is possible, if your problem with
stall occured only after the carb had been taken apart, that the
gasket may be installed incorrectly, or be the wrong part altogether,
and is blocking the vent. That would make the engine "run out of gas",
as the pump would be unable to suck fuel up. But, IIRC, you said you
ran it without the cap and it did the same thing?
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