Small engine repair question

The unit under test is a generator powered by an 8HP Briggs. I cleaned the fuel tank, replaced the fuel lines and filter, and installed a
carburetor rebuild kit.
Engine starts and runs well for five or ten minutes, then stumbles and stops. Starts up again with a single pull.
What should I be looking for?
Kevin Gallimore
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Clogged vent, in the fuel cap. Try running with the cap unscrewed, a half turn or so. If it runs with the cap loose, look inside the cap for one or more tiny holes. Clear the holes with a straight pin.
Christopher A. Young Learn more about Jesus www.lds.org .
The unit under test is a generator powered by an 8HP Briggs. I cleaned the fuel tank, replaced the fuel lines and filter, and installed a carburetor rebuild kit.
Engine starts and runs well for five or ten minutes, then stumbles and stops. Starts up again with a single pull.
What should I be looking for?
Kevin Gallimore
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On Sat, 24 Nov 2012 09:41:58 -0500, axolotl

Fuel restriction somewhere? Could be plugged fuel cap vent or a filter.
Pete Keillor
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axolotl wrote:

There is a torrent of BS manuals out there, if anyone is interested. I ran across the link when I was looking for a part to fix my mower a while back. I didn't bother to copy the link.
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Be prepared to choke the thing the next time it stops. If choking it or squirting ether at it keeps it running , it's a fuel problem. If not, it's either ignition or a valve temporarily hanging up.
I like the plugged gap cap idea.
Do the symptoms change under load?
Pete Stanaitis ---------------

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On Sat, 24 Nov 2012 09:41:58 -0500, axolotl

Loosen the gas cap and see if it still poops out. I'm thinking clogged port in the gas cap. A stuck float would cause it to die quicker.
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Stuck or misadjusted float.
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PrecisionmachinisT wrote:

I don't believe these recent engines use floats. They have a bowl above the fuel tank that has fuel pumped by a vaccum-operated diaphragm, and the excess fuel spills over back into the tank.
My guess is the magneto has gone bad, and fails when hot. If you restart immediately, how long does it run the 2nd time? If it gets progressively worse, each restart runs shorter than the last one, that is a good indication it may be the magneto. If you let it sit for 5-10 minutes before restarting, then it runs much longer?
I had this exact problem on a 5 Hp recent Briggs engine. I put a magnetic-coupled timing light on it, and saw the spark got weak while running, and then no spark (no timing light flashes) when it died. A new magneto coil fixed it.
Jon
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Depends on how old it is then I guess...I have a coleman generator and also a lawnmower with B&S engines having float type carbs, indeed, both are getting on in years.
The mower had a magneto went out on it a couple years ago...but in my case, some days it simply would not put out any spark....
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PrecisionmachinisT wrote:

I've got 3 things with Briggs engines, I think they call them "Quantum" series. They have the flat, pleated paper air filter under a swing-out plastic cover. So, I think this has been the standard engine from Briggs for about 20 years back, in the 5 and 8-HP size.
This carburetor design does seem to be pretty robust, the rest of the engine is awfully flimsy. The main crank bearing is on a cover that is so flimsy you can bend it with one hand. So, the thing that keeps the two main bearings aligned is the crank itself! If the crank is hit with a severe side load, like on a mower or wood chipper, there is no block support for the crank, and it breaks in the middle. You can guess how I know about that.
Jon
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On Sat, 24 Nov 2012 20:57:02 -0800, "PrecisionmachinisT"

Blocked filter ir fuel fitting at tank? Pull the hose off and do a fuel flow test.
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On 11/25/2012 7:46 PM, snipped-for-privacy@snyder.on.ca wrote:

Today's fooling around yields slightly different symptoms. Starts, runs with surging, about a 10 second rep rate.
Tank cap is not clogged, loosened the cap to make sure. Fuel filter shows fuel on both sides of the filter media.
The engine wants the choke on, indicating too lean a mixture.
The governor slams back and forth as the engine is surging, indicating that it is moving freely.
It is certainly possible that I screwed up the carb cleaning and rebuild.
What would be a good diagnostic test for the magneto? I replaced one in a different Briggs not long ago, but that engine simply would not go when hot.
Thanks,
Kevin Gallimore
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axolotl wrote:

The fuel hose may have an internal restriction. It is obviously getting some gas, but not enough.

Does the carb have mixture screw that you can adjust?

Sounds more likely that something is clogged in the fuel system. Either the fuel line, float valve or main jet may have some dirt restricting flow. Or it might be running lean because the intake is leaking air (bad gasket, loose bolts or cracked manifold).
You might also want to check that flywheel is timed correctly.
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On Sun, 25 Nov 2012 20:27:10 -0500, axolotl

Check for gasket air leaks, one of the most common problems. Don't overtorque the carb when putting it on, either. DAMHIKT.
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Some small engines run better, with a coating of Permatex IIb non hardening gasket sealer, on the carb to engine gasket. Messy stuff to work with, but comes off with alcohol.
Christopher A. Young Learn more about Jesus www.lds.org .
Check for gasket air leaks, one of the most common problems. Don't overtorque the carb when putting it on, either. DAMHIKT.
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I've seen engines, where the gasket between the carb and the engine leaks air. Usually, this means won't start, needs ether or gas poured into the air intake to start. Not starts on one pull.
Sometimes, water in the gasoline will make engine stall, but restarts on one pull. I'd want to pour out the gasoline into a glass jar, and see if water settles out.
Christopher A. Young Learn more about Jesus www.lds.org .
wrote in message
The engine wants the choke on, indicating too lean a mixture.
Thanks,
Kevin Gallimore
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I think magneto problems are typically that -- won't run when hot.
You're right, that's puzzling. I'm not getting any good answers.
Christopher A. Young Learn more about Jesus www.lds.org .
Today's fooling around yields slightly different symptoms. Starts, runs with surging, about a 10 second rep rate.
Tank cap is not clogged, loosened the cap to make sure. Fuel filter shows fuel on both sides of the filter media.
The engine wants the choke on, indicating too lean a mixture.
The governor slams back and forth as the engine is surging, indicating that it is moving freely.
It is certainly possible that I screwed up the carb cleaning and rebuild.
What would be a good diagnostic test for the magneto? I replaced one in a different Briggs not long ago, but that engine simply would not go when hot.
Thanks,
Kevin Gallimore
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Stormin Mormon wrote:

If you have an inductive timing light, that is a great tool. When the spark is weak, the light will not fire regularly. When the spark basically goes away, the light will not fire at all. if the light flickers and then quits just about the same time the engine quits, that is a really strong indication of weak spark. It the timing light continues to blink right down to the engine's last stroke, the problem is probably NOT ignition.
Jon
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I have a LawnBoy that drove us nuts. It was way too lean, requiring choke to stay running.
We did everything, including replacing the top & bottom seals on the crank. That helped some, but it's never been really fixed. It has begun running better of late, but we don't know why.... [mechanic's shrug]
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On Sun, 25 Nov 2012 20:27:10 -0500, axolotl

Feed propane or acetylene into the intake and see if it runs well. If so, it's not the mag.
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