B&S 5HP engine runs for a minute then stalls

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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 threads/etc
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!
Steve
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On 22 Sep 2006 23:20:06 -0700, snipped-for-privacy@telecom-digest.zzn.com wrote:

The cylinder piston pulls a vacuum and the pressures as the air flows past the jets suck the fuel from the jets into the air stream.
Might be time to put a pressure gauge on the spark plug.
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A relatively unusual possibility that can also have symptons like this is a leaking head gasket. The engine can start and run fine and then as it heats up, the gasket begins to leak.
Gary
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snipped-for-privacy@telecom-digest.zzn.com wrote:

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 not necessary.

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?
%MOD%
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Hello again,
I'm back from a brief vacation and ready to tackle this issue again, thx for keeping the thread alive.

Ah, most interesting. The engine doesn't seem overly hot once it stalls.

Yes.
Someone suggested I remove the load from the engine (remove fanbelt connecting engine to compressor) to see what difference that makes - good idea, I'll try that as well.
Steve

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Smaller engine (chainsaw), but similar symptoms - cleared when I cleaned the carbon off the screen over the exhaust outlet.
George
On 22 Sep 2006 02:32:08 -0700, snipped-for-privacy@telecom-digest.zzn.com wrote:

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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 you said.
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 spectator.
From: Husky < To: Stormin Mormon <> CC: 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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