Mower bogging problem

Hey Guys
Here's my problem. I have a Toro Time cutter zero turn mower that's got me stumped. The guy that brought it to me described the problem as the engine cutting out and stumbling under load. He stated that he replaced the carb and stated that the problem improved but he could not get the carb adjusted to the point to take the stumbling out of it. When I started checking the mower it wild cut out on flat ground even when the mower was not under load. He replaced the fuel filter at time of carb change. It is spitting back through the carb also but intermittently. It's not smoking. I pulled the new carb back down ( to assure that all was well with the float and needle and seat). The carb looked fine. Also, the muffler was broken at the exhaust neck and it had caused damage to the spark plug boot and sparkplug/coil wire. So I'm thinking that since it's an intermittent problem, maybe the coil wire was breaking down. I replaced it with a new coil since it needed to be repaired anyway. Still, same problem. I adjusted the valves thinking the spitting back problem could be the problem. The valves were out but not by much. The engine seems to have great compression. So I have attacked fuel, spark and compression. Now where do I go? The mower info is below. Any help I can get would be greatly appreciated.
Thanks
B&S 17.5 HP OHV
Model Type Code
31777 0148-E1 001019ZD

Addition : While cutting high grass the engine stumbles and boggs intermittently. If I stop and let the engine smooth out it runs fine for a minute or so even under load, then it repeats the symptoms.

First a new carb does not mean new gas. Could have used old gas and caused the same problem. I would clean the carb then check to see if springs where attached back where they belong.

Cleaned or replaced plugs?

Good point. I asked him about that and he assured me he had fresh gas in the tank when he replaced the carb. Being the ultimate pessimist, I tore down the carb, inspected in and cleaned it again just in case. I have also replaced the spark plug. I swear, it seems when I'm riding it that it is misfiring causing the bog and spitting back but it may just be me. I'm at a loss. Thanks for replying.

Did you fix the muffler? That would cause problems if not fixed.

I did. I had a guy weld the neck back on it but all the guts have rotted out of the muffler and had been removed when I got it. That is very interesting. How could the muffler missing cause the problem? I have always heard in mechanics tales that if you ran an engine (like starting it up for even a brief period after overhaul to check things) that cold air can rush in and bend the exhaust valve. Am I barking up the right tree? My problem is maybe I'm not thinking outside the box enough.

medic2575 said:

I did. I had a guy weld the neck back on it but all the guts have rotted out of the muffler and had been removed when I got it. That is very interesting. How could the muffler missing cause the problem? I have always heard in mechanics tales that if you ran an engine (like starting it up for even a brief period after overhaul to check things) that cold air can rush in and bend the exhaust valve. Am I barking up the right tree? My problem is maybe I'm not thinking outside the box enough.

Click to expand...

Insufficient back pressure allow fresh air to get up into the exhaust pipe and burn the exhaust valve

Hey Guys. I spoke with the owner of the above said mower and he stated to me that the muffler on this mower has been burnt out for the 2-3 years that he has had it and it had run fine until now. In thinking about what bertsmobile said about the possibility of cold air burning the exhaust valve I pulled the head off the machine and tore it down. After removing the valve springs and valves a noticed something that may be amiss. The valve guides appear to have moved in the head. The Exhaust valve guide is sticking through the head (on the rocker arm side) a good 7/8" and the Intake valve guide is sticking through the head (on the rocker arm side) a good 1/2". I checked the Briggs service manual and it didn't give me much help on the depths of these guides. Are these lengths normal or have the valve guides moved possibly from high heat? If so, could this be causing my spitting back and intermittent bogging problems. I have look at every OHV head picture that I can find and can't get a good angled picture of the depths of the valve guides.
Thanks

The 1/2 inch sticking out on the rocker side is closer to what they should be than 7/8. You have a valve that isn't staying open long enough to exhaust properly, so you are getting blowback into the carb when the intake opens. If the rocker is moving all they way and not hitting the guide and bending or breaking the pushrod, I suspect you also have a lobe on the camshaft. Remove the valve cover and the spark plug. Rotate the engine by hand watching how far the valves are opening. Are they opening the same distance or is one opening like 1/4 inch.

At this point you will need to change the head, and possibly the camshaft.

Thank you ILENGINE for responding. I set the valves on this engine when I first took it in. I always check to see if I have equal movement on both rockers and this one seemed to have equal movement. I roll tested both pushrods on a flat surface and they both seem straight. From what I understand, these heads are prone to the valve guides slipping in them from overheating. Guess it's new head time. If that doesn't work should I move to cracking the case and inspecting the lobes on the cam? Thanks