For anyone with expertise on small engines:

I have a Cub Cadet commercial walk-behind mower with a 16HP Vanguard (B&S) V-twin.  It is about 15 years old with maybe 350 hours.

The last four times I used it, the engine would cut out and bog down during a 50-60 minute mowing period, when under  load (i.e. PTO engaged and moving).

I stop and it recovers pretty quickly and I continue on.  It was happening after about 30-35 minutes.  Last time it did it within 10 minutes, and again at about 40 minutes.  I recently replaced the air filters, and since it started doing this I have replaced the fuel filter and spark plugs, but no change in behavior.

 

I know that is not a lot of information, but any ideas on what else might cause this or anything I might consider looking at?

And if I bring it into a shop, it takes some time to reproduce.  Do they have a means of putting it under load to reproduce the issue?

 

Thanks in advance!

Check the vent hole in the top of the gas cap.  If it's plugged it can cause all kinds of problems.

 

Other items:  valve lash, dirty carb, bad coil can all cause similar problems.  If you can explain more about what you mean by "bog" I might be able to narrow it down to either fuel or ignition... 

Thanks!  I will check the gas cap next time.  Any ideas on something that would happen intermittently?  I rally have not noticed much of a pattern with it yet.

 

It seems to lose power under load.  RPMs drop off sharply when cutting through normal grass as if I were cutting into high/thick grass.  As soon as I stop forward motion it recovers pretty quickly.  Once it recovers it seems like I can resume mowing.  However, it still seems like it is not generating as much power as before.   I monitor the RPM and It seems to be pretty high, but it sounds like it is slowing very slightly.

I typically run it at about 3600 RPM unloaded.  When I engage the PTO it will drop to 3450-3500.  When the issue occurs, hear and see the RPM drop off into the 3000 range, maybe even 2500.  When it sounds to me like it is slightly slowing, the RPM meter still reads about 3450-3500.  But it may be averaging many cycles and might not register that quickly.

Sounds like it's starving for fuel under load.  It's probably got a very simple carburetor.  If you can pull it off, just disassembling it and making sure there's no debris in bowl where the needle valve and float are, or in any of the jetting will probably resolve it.     Make sure choke and throttle plate move freely.   Also that the float can drop all the way down...if the float won't open all the way, it won't let enough fuel in.

 

Just for a lark, pull the air filter off entirely and see how it behaves.  It's possible that's packed full of dirt.

This all happens on flat land or with the mower tilted?

Sounds like like carb problem.

Yeah, I will start looking into the carb. My first thought was fuel, so I replaced the fuel filter, but that didn't help. Will try to find a rebuild kit.

 

This happens on flat land, and I just replaced both air filters.

If it runs smooth but loses RPM without missing, I'd check the valve lash next.  Loss of power in these motors is usually by loss of compression, and the biggest (easiest?) cause is the valve adjustment.  With the hours on the motor you mentioned it's reasonable to have to adjust the valves.   

 

That assumes there isn't any carb problem, but a carb problem (running lean or rich) usually shows up by a stumble or surge.  If it's lean it will surge at full throttle, if it's rich it will stumble or you might even see smoke.  The gas cap issue I mentioned causes a lean surge because, as gas drains from the tank it creates a slight vacuum, stopping gas from flowing into the carb.

Water in gas?

Drain the bowl and dump the gas can. Get new gas from a dif place.

 

Tight valves are typically indicated by hard starting.

Thanks all.

I checked the fuel cap vent today before I ran it.  I exercised the mechanism a few times and it seemed to be okay.

But about 15 minutes in it lost power and RPM fell off to about 2000 RPM.  As always, I immediately pulled back on the drive bar.  Was about to disengage PTO, but the motor came right back up to speed.  FInished the remainder of the lawn (about 35 more minutes) without incident--although it seems to slow a little more than usual when going into thick grass.

 

I will take a look at the valve lash next, just seems odd that it would cut out just one time, and then work fine for the rest of the mowing time.

The valve lash adjustment looks pretty straightforward.  But can I do this by rotating the engine (via the pull cord) until I see that each valve is fully open, then adjust the lash on the other.  Or do I need to use a gage or mark to get it to exactly TDC?

You can use pencil in the cylinder to find tdc. Just make sure it's on the stroke that neither valve is moving.

Have you checked the fuel pump and vacuum line? 

Sounds like like carb problem.

Ethanol in the fuel may be an issue.

When the mower sits in the off season, the fuel can cause corrosion of the aluminum, and little bits of it may be plugging or restricting the jetting.

Take the carb off and inspect and clean it.

The corrosion looks like fine sand.

B&S Vanguard engines are really Dihatsu engines made in Japan. The carbs on them are very sensitive to dirt and if it's 15 years old ethanol could also be an issue since they weren't putting it in fuel back then when the engine was made. Dirt can get stuck in the jets or as Fred mentioned the ethanol likes to eat away at things. I've seen ethanol swell the seat for the needle if it's made from neoprene. I've also seen ethanol eat away at rubber fuel lines and cause small pieces of rubber to clog up the carburetor. You can try taking off the carb and thoroughly cleaning it, spraying carb cleaner through all the orifices, but it only works some of the time, you  may need to replace the carburetor.

 

Carb problems can be erratic sometimes, but if it's only doing it after the engine gets hot it could also be an issue with one of the coils. Electronics can get squirrelly when they start to go bad.