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

[wesw]

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.

Choking it (either via choke or changing carb inlet) will make a minor difference for just a couple seconds but it'll hunt and stall either way. It's hunting so much that it's hard to tell if it's making it better or worse for the short duration.

When I had the intake manifold off (mine is plastic) I did check it over carefully for cracks. Didn't see anything, but I didn't have a good way of pressure checking it or anything either.

Haven't checked the head or compression on the valves. If the alternative gas source doesn't fix it, suppose I can check that. The mower only has about 30 hrs on it and it ran fine for probably the first 20 (mower is a few yrs old, so despite lower hrs the warranty is up). When I had the intake manifold there were no signs of a burnt valve or excessive buildup that would change the valve seating. I've never had to adjust valves on one of these B&S VTwin's before - including the 15 yr old John Deere I've got. It looks like a pain from the service manual - remove exhaust, governor, remove and mark push rods just to get to one of the cylinder heads. Gaskets need replaced so rebuild kit. Also appears as if the threads are all using Loctite 242 and torqued 100 ft-lb, so they shouldn't have been creeping to start out with. Is it common in the VTwin's to have some kind of misadjusted valve or head gasket leaks?

 

[sgkent]

hold the throttle butterfly steady with your hand and see if the engine speed stabilizes. If so it may be the governor.

 

[DaveTN]

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.

Agree w/ you that it sounds like some sort of fuel starvation even with all that he put on it. I was wondering if the fuel lines themselves might have some internal blockage. Was he using 100% Pure Gasoline? That ethanol will cause hoses to clog up over time as they deteriorate on the inside. But he said he blew out the line. Hmm... still a mystery. Welcome to the LMF!

 

[Bertrrr]

I've run across this before, bought an electric fuel pump and ran the supply line " new " to a 5 gallon can of gas, crank up and watch it run, no problems , then began replacing fuel lines on the supply side to the tank, ended up finding an air leak on the supply side. make sure your kill selonoid is not the problem as well.

 

[wesw]

Agree w/ you that it sounds like some sort of fuel starvation even with all that he put on it. I was wondering if the fuel lines themselves might have some internal blockage. Was he using 100% Pure Gasoline? That ethanol will cause hoses to clog up over time as they deteriorate on the inside. But he said he blew out the line. Hmm... still a mystery. Welcome to the LMF!

It's a commercial mower, so just using regular unleaded that's supposed to cap out at 10% ethanol; however I did hear that during some of the gas price/shortage issues there was a waiver to allow 15% for awhile... so certainly possible that may have some something. I always use Stabil in whatever I get though. That tank is some sort of metal.

My current plan is to try the new tank like @bertsmobile1 suggested early on. The behavior of this most closely matches his suggestion so far. It's got to either be a blockage that's inconsistent or something heat related. And if it's heat related, it doesn't affect the immediate startup after it stalls. Since the mower as a VTwin should be able to run on only one cylinder and I can see both are getting spark from testers and the spark plugs are new, seems to me unlikely that it's something in the cylinder too as whatever is happening seems to be affecting both cylinders. I also don't think it's the kill solenoid on the bowl as it behaves the same with two different assemblies.

So current plan is 1) check with new small tank, 2) replace fuel lines, 3) check valve/rocker clearance from TDC. Will report back on what if anything helps.

 

[sgkent]

also don't think it's the kill solenoid on the bowl

have you put a volt meter on the circuit to see if the voltage changes at the time it dies?

 

[wesw]

Well, I finally got the external tank (it was about 3 days late). Decided to replace the fuel line as well and try it with the new tank. Ran fine for about 20 minutes (had to fill the small tank twice. So I blew out the old tank with an air hose real good, stuck an inspection camera down in it and aside from some vapor it looked really clean. Hooked up the onboard tank and ran it for 30 minutes with no real issues.

So would appear that somewhere between the fuel lines and cleaning out the tank that the problem is solved.

Only other oddity I noticed is that when removing either one of the spark plugs the engine actually seemed to run smoother. Either way though, no stalls this time around. I am wondering though if there is something going on with the diodes and cross talk on the ignition coils. If it happens when warmer outside I might give that a look.

Calling this one solved. Appreciate all the help and ideas from the forum!

 

[wesw]

So I thought I had this sorted out. But I fired up the mower again today and while it lasted longer (ran about 30 min) it then started to hunt and stall. Video of problem

I've replaced pretty much everything. After it stalls, choke it, will start right back up again. B&S VTwin's 22. Replace

I'm honestly at a loss if anyone has any ideas.

 

[MParr]

Is your governor adjusted correctly? Seeing that video, that would be the first thing that I checked.

 

[Tbone0106]

Had a similar problem with an older Cub RZT50 that belongs to my church. Replaced all the fuel lines, the fuel filter, the fuel pump, went through the carb, and so on, and it didn't change a thing. Finally pulled off the fender that covers the fuel tank and discovered a brass 90 degree elbow fitting that fits in a rubber grommet on top of the fuel tank. The fuel line feeds from one side of that elbow, but the other side was fitted with a pickup tube made of rigid black plastic, like a big heavy drinking straw. There was no filter on that pickup tube, and the problem lay there. Something -- grass clippings, dead bugs, whatever -- was getting sucked up through that pickup tube from the bottom of the tank, and whatever it was was too large to fit through the brass elbow. The passage through that elbow is MUCH smaller than either the pickup tube or the fuel lines. The elbow was essentially a bottleneck.

The mower would stop at random times, but always start back up after sitting for a few minutes. Whatever was blocking that elbow would drift back down the pickup tube once the engine stopped, and it would be slurped up again sometime later. Lather, rinse, repeat.

What I did to fix things was remove that rigid plastic pickup tube and replace it with a length of standard 1/4" black rubber fuel line. I weighted the bottom end of it with a couple of steel nuts and installed a filter to keep out the inevitable tank-bottom crud.

That was four years ago. That old RZT is still cutting our grass, and it never misses a beat.

Also, FWIW, this fuel pickup design is very much a standard feature on MTD products. I have run into the same problem on other MTD mowers and I've been able to solve the problem in much the same way each time.