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

[George]

Smaller engine (chainsaw), but similar symptoms - cleared when I cleaned the carbon off the screen over the exhaust outlet.

George

[Husky]

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 A newer version of the Pulsa-Jet, called the Pulsa-Prime, replaces the choke and choke pull-off with a bulb operated primer, which is basically just an extra fuel pump jetted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?

[Stormin Mormon]

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&#39;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&#39;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?

[modervador]

The suction to draw gas from the bowl into the carb venturi nozzle is the Bernoulli effect.

Perhaps you&#39;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&#39;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%

[modervador]

I have/had some horizontal shaft B&S engines from the 1960s with PulsaJet (pump and bowl) carbs and screwcap tanks. Friggin stamped caps were not knurled very well and not so easy to grip with a gloved hand, as I recall. I don&#39;t remember the 1/4 turn plastic caps showing up till the &#39;80s, but there&#39;s a lot I don&#39;t remember.

I just now remembered that I have a 5 hp horizontal B&S (screwcap pulsajet) in the far corner of the shed, in an old dishpan, on a bottom shelf, hidden behind some outboard motor gas cans. Haven&#39;t used it since I switched to a 12V water pump. I may have to make a go-kart just so it doesn&#39;t go to waste :-)

%MOD%

[canoe414]

Hello again,

I&#39;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&#39;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&#39;ll try that as well.

Steve

[kent.mccarthy]

I had a B&S 5 hp generator that had similar symptoms. Discovered that the suction tube from carb to tank had developed a split, preventing gas from being drawn up into the carb. A little JB Weld on the tube cured it.

[vanderbentgary]

[vanderbentgary]

Have observed very little fuel in fuel filter when engine shuts down. Should it be full.

[Diesel Dan]

Don't try this at home but...

when it starts to slow down, I'd try feeding the carb air inlet with some propane from an unlit torch.

[trader_4]

Probably. Disconnect the fuel line at the carb, or take the bowl off and see if fuel flows freely. Make sure you're prepared to deal with whatever amount is in the tank.

[Amanda Szabela]

Steve, did you find the solution? If so, would you mind sharing it on this forum? I have a similar phenomena occurring on a 3hp B&S engine that powers a leaf sucker. Model 80212, 2357-01. It is truly being a pain so far. I've ready most of the posts up above. I just didn't see a final conclusion. might have overlooked it. I'm planning to swap carburators altogether. I've tried removing muffler, changing diaphram, replaced the fuel inlet tube, i don't see any cracks or issues with the brass tube, getting spark. I noticed as the engine starts to slow before it dies, the exhaust outlet port is sending out blue flame. thoughts??

[Clare Snyder]

Blue flame is running excessively lean

[Amanda Szabela]

thank you for the feedback.

[Amanda Szabela]

put a new carb on the 3hp briggs today. engine fired right up, however, ran into the same problem we had before where it would only run for 20 seconds and then die. Just for kicks, I checked the spark plug and it was functioning fine immediately after the engine died. So I think I'm back to square one. If you have any ideas or know of how Steve got his engine to run let me know. by the way, just for the sake of it, we changed the engine oil prior to running it today.

[Amanda Szabela]

final update: pulled the cylinder head cover off, discovered a hole in the head gasket. had a small engine mechanic inspect the piston rings for wear. Rings were totally worn down. Which means blowby past the rings. and as the engine block heats up the air inside the combustion chamber is heating up it becomes less dense and is getting past the rings, losing compression. explains the initial startup just fine but after about 15 seconds the rpm's start to lower until it quits. Hope this helps others out in the future. Going to invest in a new motor. approximate cost for new rings and seals ($150-$200). Found a new engine going for $200 at Harbor Freight.