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

[canoe414]

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

[John Gilmer]

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.

[nobody]

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 .

[m Ransley]

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,

[Ether Jones]

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.

[Stubby]

Check that the RPM governor is free to move.

[Chris Lewis]

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.

[Stubby]

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.

[canoe414]

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

[DK]

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.

.

[Ether Jones]

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?

[Oren]

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."

[Oren]

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."

[Husky]

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

[Husky]

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

[maradcliff]

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

[canoe414]

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

[Stormin Mormon]

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

[DK]

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.

[Gary Dyrkacz]

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