Baffling engine issue - runs for 6 minutes, dies, but then starts right back up

[wesw]

Normally pretty good with mechanics, I've got one that's totally stumped me. My Briggs & Stratton VTwin 22HP will run for about 6-7 minutes, then the governor starts hunting and it eventually dies, even under no load. What's odd is that if you choke it, it'll start right back up (hunting still), then die again. Here's what I've tried already:

• Replaced fuel filter, spark plugs, and air filter, changed oil and oil filter
• Drained fuel and replaced to make sure nothing weird going on there
• Tried running it with air filter off after it's hunting, no difference
• Removed gas cap while hunting, no difference (it's not a vented cap, this design has a hose from the air intake back into the tank for pressure intake - so it's always clean air). Checked hose and it's clear.
• Replaced fuel pump and vacuum hose
• Replaced carb (Nikki carb o-ring seal and jets seem to be a common problem)
• Replaced both magneto armatures (ignition coils), which include attached spark plug wires. Set gap to the specs.
• Checked exhaust while hunting - plenty of air movement coming off

Engine only has about 30-40 hours on it, but it is a few years old and did sit outdoors covered for awhile.

At this point I'm at a loss. Not much has turned up on searches so hoping this forum might have some fresh views. Super frustrating because it starts up and mows beautifully... for 6 minutes, at which point it brings out the worst in me.

Any ideas?

 

[bertsmobile1]

This is exactly what happens when you get a build up of clippings in the fuel tank .
While running the debris gets sucked into the outlet then the instant the flow stops it floats away .
The cycle then continues .
If you want to confirm this get a small tank ( pinch one from a push mower ) and hook it up to the inlet side of the fuel pump

 

[wesw]

I can certainly give that a shot. Given the tank is completely sealed and the vent is through the filtered air I have no idea how anything would have built up. I also did drain the gas tank and nothing came out except gas.

 

[MParr]

One more thing. Have you drained the carburetor bowl? It’s highly likely that there is a good bit of crud in the bowl.

 

[bertsmobile1]

I can certainly give that a shot. Given the tank is completely sealed and the vent is through the filtered air I have no idea how anything would have built up. I also did drain the gas tank and nothing came out except gas.

Which is why I suggested the substitute tank to verify the problem.
In the past there have been recalls because a casting fin would bend over & block off fuel outlets or would catch debris in the tank.
Unless you bought it new in a crate and checked the tank with a mirror before you filled it the first time you can not be sure what was in there.
Over the year I have found all sorts of things inside tanks from condoms to lunch wrappers to price tags , old shop rags, all sorts of stuff.
Then there is the small child factor when some kids had been filling a can with water and tipping it into the fuel tank "just like daddy does "

After an obstruction the next best guess would be a fuel line running too close to the engine / exhaust heating the incoming fuel which causes it to expand & because a carb works on fixed volumes of fuel if the fuel is thinner then less enters the engine so you get a lean burn which makes the engine run hotter which heats the fuel more till the fuel is so hot there is no longer enough to maintain engine function.

 

[wesw]

One more thing. Have you drained the carburetor bowl? It’s highly likely that there is a good bit of crud in the bowl.

Brand new carb - bowl is clean.

 

[wesw]

Which is why I suggested the substitute tank to verify the problem.
In the past there have been recalls because a casting fin would bend over & block off fuel outlets or would catch debris in the tank.
Unless you bought it new in a crate and checked the tank with a mirror before you filled it the first time you can not be sure what was in there.
Over the year I have found all sorts of things inside tanks from condoms to lunch wrappers to price tags , old shop rags, all sorts of stuff.
Then there is the small child factor when some kids had been filling a can with water and tipping it into the fuel tank "just like daddy does "

I ordered a small tank to try it out. Be a few days before I can report back on that one, but fortunately at 6 min don't have to wait too long once I get the tank. I can rule out the small children factor though, haven't had those in awhile.

If that is it I'll have to figure out how to clean it out. It's mounted under a welded frame and I'm mildly concerned that to replace it you'd have to take the engine off.

After an obstruction the next best guess would be a fuel line running too close to the engine / exhaust heating the incoming fuel which causes it to expand & because a carb works on fixed volumes of fuel if the fuel is thinner then less enters the engine so you get a lean burn which makes the engine run hotter which heats the fuel more till the fuel is so hot there is no longer enough to maintain engine function.

I haven't replaced the fuel lines themselves. They're routed along a clipped channel on the side - I suppose I could also give that a shot if you think there's anything where swelling, etc... would be heat sensitive. I have blown them out when replacing the filter and fuel pump.

Appreciate the ideas. Will either fix it or rule yet another thing out.

 

[MParr]

Brand new carb - bowl is clean.

Is this a genuine Briggs and Stratton carburetor or an Amazon or Fleabay knockoff?
How is the air filter?

 

[sgkent]

sounds like a fuel starvation issue. Try pulling the air filter and cupping your hands closer and closer to the carb inlet as it starts to surge. See if that affects the behavior. If the engine smooths out as you move your cupped hands closer than it is running lean. If it gets worse then it is rich. I had something similar on a pressure washer and a new genuine carb solved the issue. Actually it took two because lots of old stock stuff has been used before. Someone told me that I should also look for cracks in the inlet manifold if it was plastic. Mine was aluminum but a small crack would be easy to over look. Also if one has a loose head it can cause similar behavior. The compression on the pressure washer went from about 30 psi to about 100 psi when I replaced the head gasket and adjusted the valves. Remember that valves grow as the heat up so if they are tight that can be an issue too.

 

[wesw]

Is this a genuine Briggs and Stratton carburetor or an Amazon or Fleabay knockoff?
How is the air filter?

The issue started with the original Nikki carb. It wasn't really excessively dirty but I decided to clean and rebuild it anyway. Also bought a Kohler replacement. Both behave the same way. Pretty sure the carb isn't the problem as the issue exists in both exactly the same. Air filter is also new, forgot to mention that I also replaced that in the original post.