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

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Hi all,
Recently acquired an air compressor equipped with a B&S 5HP motor. When cold motor starts on 2nd pull and runs fine for the first minute then gradually slows down over the next 30 seconds till it quits.
Googling revealed people with similar problems. So I've: --loosened cap on fuel tank ---------- no difference --replaced spark plug with new plug ---------- no difference --replaced air filter ---------- no difference --rebuilt carb, complete disassembly + soaking in carb cleaner overnight, resent needle to 1-1/2 turns. ---------- no difference --double-checked there is no gunk in bottom of gas tank clogging the tiny screen at the tip of the fuel inlet ---------- no difference
OOoooooOOOOooo this make me SO frustrated!!! Any more ideas to try? Help!!
TIA,
Steve
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Two things come to mind:
If you use the choke to start you should take out the choke within one minute. A hot engine will just not run on full or even half choke.
You might have some restrictions in the fuel supply. The most reliable small engines have a gravity feed to the fuel bowl. It doesn't take much to clog the needle valve in the float.
Other small engines have a "fuel pump" which is a diaphram that's part of the carb. If you truly rebuilt the carb this should have all new sealing and moving parts.
There is an oddball possibility that when the engine starts to heat up the ignition system fails.

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I know this probably obvious, but beyond loosening the gas cap, do you still have a shipping plug in the gas tank, under the cap.
I got a great deal on a leaf vacuum that a guy returned to the store with you exact symptoms and one I removed that blue gas plug, it ran great.
But generally you definitely have a fuel problem, having a partial vacuum in the tank would do it, a constriction preventing the fuel bowl from filling properly would do it. As it sits over a minute or too enough air/fuel seeps through to allow a re-start .
Good luck.

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Use starting fluid or squirt fuel in the carb after it dies to see if its the fuel system, if nothing see if you get spark after it dies, it could be the ignition system,
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snipped-for-privacy@telecom-digest.zzn.com wrote:

The choke cable may have come loose, and the choke is closed. Pull the air filter off and check it out.
There's a small tool that you attach between the spark plug and the spark plug wire that allows you to see the spark as the engine is running. Briggs tool #19368 or similar.
You could hook this up, start the engine, and watch as it stalls to see if there is still a strong spark.
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Ether Jones wrote:

Another problem can be overheating because of leaves and debris in the heat fins. Last Spring that happened to a small mower we had -- B&S. I was mystified and paid $65 for a "tune up" that didn't do anything. It took 3 more trips to the shop and screaming at the owner to get him to fix it. The high school kids working for him would need help falling off a ladder.
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Thx so far to all who've replied...
can't be a vacuum problem, I've tried it with the gascap completely off. can't be the choke, I move it to "run" right after cold-starting can't be any sort of exhaust restriction can't be clogged cooling fins
carb is a suction-feed. There's a long stem that goes to the bottom of the fuel tank. This has a fine-mesh metal screen for a filter, which is clean. There's another stem, shorter with what in a Holley automotive carb would be called a 'jet' staked into the end - this goes into a shallow pan tack-welded to the inside of the gas tank. There's no moving parts in any of this except for the throttle and choke plates. I can't figure out what the purpose of this short stem/shallow pan is or how fuel gets into the pan to cover the jet, but it'd make sense that once the shallow pan ran dry the engine would quit.
I don't think the engine runs long enough to get hot enough to cause the ignition system to fail. Where it the system located? I don't think it's the RPM governor but could check it - where is it located and how can I check it?
Grrrrrrr. I'll make a call to the local small-engine repair shop, maybe they can give me a straight answer.
Steve
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snipped-for-privacy@telecom-digest.zzn.com wrote:

Yes, but did you actually look at the choke plate in the carb to make sure it is opening?
If the choke cable got loose, you could have full choke even with the choke lever in the "off" position.
Does this engine have a primer bulb? What happens if you pump that a couple of times when the engine starts to stall?
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wrote:

Yes the "butterfly" valve could just be sticking.

Like you said, you gotta look at the "choke plate". -- Oren
"Well, it doesn't happen all the time, but when it happens, it happens constantly."
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On 22 Sep 2006 12:24:59 -0700, snipped-for-privacy@telecom-digest.zzn.com wrote:

A vacuum problem can be as simple as a misaligned or a damaged gasket.

Before you re-mounted the carb, did you check for easy operation, like moving the parts....is something "binding", resulting in poor operation? -- Oren
"Well, it doesn't happen all the time, but when it happens, it happens constantly."
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snipped-for-privacy@telecom-digest.zzn.com wrote:

The shallow pan welded into the top of the tank is the carburetor bowl. The long tube is the fuel pump pick up, and the short one is the carburetor venturi tube. The fuel pump is under the side cover, the rubber gasket with the spring and protector ring. The two little flaps are the "reeds" for the check valves.

Try taking the belt off the engine and compressor pulleys and see if it runs okay. It could be a siezing compressor head.
Husky
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Here's a trick. When the engine starts to slow down, take a propane torch and turn on the propane. Dont light the torch, just hold the torch over the intake on the carburetor (remove the air cleaner first). If you are not getting enough gas, the propane will keep the engine running (not the smoothest, but it will run). You can vary the valve on the torch and move closer or further from the carb to adjust the engine speed. If this keeps it running, you have a fuel problem. If not, it's ignition. Do be sure the choke butterfly is staying open. Look at it, not the lever for it.
If it's ignition, replace the entire magneto. That IS your ignition system all built into one. (unless you got an old engine with points and condenser).
Mark
On 22 Sep 2006 12:24:59 -0700, snipped-for-privacy@telecom-digest.zzn.com wrote:

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carb is a suction-feed. There's a long stem that goes to the bottom of the fuel tank. This has a fine-mesh metal screen for a filter, which is clean. There's another stem, shorter with what in a Holley automotive carb would be called a 'jet' staked into the end - this goes into a shallow pan tack-welded to the inside of the gas tank. There's no moving parts in any of this except for the throttle and choke plates. I can't figure out what the purpose of this short stem/shallow pan is or how fuel gets into the pan to cover the jet, but it'd make sense that once the shallow pan ran dry the engine would quit. CY: Briggs had two carbs, well, more than two. I think what you're describing is a Vacu-Jet. When the piston goes towards the spark plug, it creates a slight vacuum in the crankcase. This slight vacuum goes to the carb in a silver tube about 3/8 diameter. Which moves a diaphragm, and pumps a bit of gas into the small pan. The air flow through the carb body creates a slight vacuum (venturi effect) which lifts the gas into the combustion air. On the other side of the motor is a cover over the bottom of the valves. The cover is abotu 1 inch wide, and 3 inches tall. You'll need a 5/16 nut driver, it is held on with two screws. This cover has a gasket, and contains a check valve. When you get the cover off, you'll see a hole in the inside surface of the cover. It is possible this cover is dirty, and not allowing the crankcase vacuum. Gently remove the gasket, and clean out the valve cover with brake cleaner, soak in gasoline, etc.
I don't think the engine runs long enough to get hot enough to cause the ignition system to fail. Where it the system located? CY: Under the motor cover. Next to the flywheel. If it's an older motor, it has a points and condensor setup under the flywheel.
I don't think it's the RPM governor but could check it - where is it located and how can I check it? CY: On top of the carb, there is a spring. And a short rod. the rod goes to what looks like the rudder of a small plane. This rudder (can't remember he name of it) is under the motor cover, and is blown by the airflow coming off the flywheel). These parts should move gently and easily when you try to move them.
Grrrrrrr. I'll make a call to the local small-engine repair shop, maybe they can give me a straight answer. CY: Please keep us posted.
Steve
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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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Husky wrote:

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't remember the 1/4 turn plastic caps showing up till the '80s, but there's a lot I don'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't used it since I switched to a 12V water pump. I may have to make a go-kart just so it doesn't go to waste :-)
%MOD%
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snipped-for-privacy@telecom-digest.zzn.com wrote:

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.
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snipped-for-privacy@telecom-digest.zzn.com wrote:

Check that the RPM governor is free to move.
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snipped-for-privacy@telecom-digest.zzn.com wrote:

I've seen this happen on a Stihl two stroke weed wacker.
Turned out that the spark arrester screen was fouled and was retricting exhaust gasses thru the muffler.
If the engine easily restarts after quitting on you, that may be the cause.
Check the muffler for restrictions.
--
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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On 22 Sep 2006 02:32:08 -0700, snipped-for-privacy@telecom-digest.zzn.com wrote:

It is a bad fuel cap.
new caps have a tiny air vent that lets the tank breath and the hole prevents vacuums.
Yours is defective and the tank is creating a vacuum.
.
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DK wrote:

Depending on how old the engine is, it may not have any vent in the cap at all. Most of the newer B&S engines vent the tank through the threads themselves, to reduce splash spilling and evaporative emissions... Won't be long and they'll have to do away with that and add a vapor canister...
Husky
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