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Discussion Starter #1
I've got a Gravely with an 8HP Kohler which runs great , justs purrs right along, but if it's working hard or it's hot outside it starts sputtering, then slows down and dies. If I wait a while it will start right back up. I notice the muffler emits hot exhaust gas directly on the carburetor. I'ts too hot to touch when the engine dies. Is this the problem? Anyone else experienced this? Thanks in advance
 

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Is there a hole in the muffler blowing hot exhaust onto the carb? Or is the gasket blown out? Generally speaking, carburetors aren’t designed to function properly when heated with exhaust gasses.

How do you know it’s overheating? IR thermometer?

Could be crap in the float bowl, clogged vent in gas cap, or a myriad of other things.
 

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Actually some auto engines do use exhaust heat to keep the carb warm enough to prevent icing in cool damp weather and vaporize the fuel better in cold climates..like a small block Chevy,they have exhaust passages cast into the intake manifold that lets exhaust flow under the carb ..

But on an air cooled engine most don't like carbs getting too warm,especially when you use ethanol gas,which burns hotter than the old leaded gas did back when,and also has a lower "boiling point" than old leaded gas..it can lead to vapor lock or lean running conditions if the carb gets too hot,and lean mixtures burn hotter,aggravating the issue....
Some air cooled snowblowers had "heater boxes" around the exhaust & carb to keep the carb from icing up,but all the rest don't need extra warmth and will run better if the exhaust heat is kept away from the carb..

One test you could try is to make a sheet metal heat shield to keep the exhaust heat away from the carb..

If it still does the same thing,maybe your ignition coil is crapping out,they usually do lose spark after they get hot as a early symptom it is getting ready to fail..they can throw a good spark "cold",but heat increases the resistance of the windings and too much heat can melt the insulation inside on the windings,and when it gets hot enough they short out..
 

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Discussion Starter #4
Actually some auto engines do use exhaust heat to keep the carb warm enough to prevent icing in cool damp weather and vaporize the fuel better in cold climates..like a small block Chevy,they have exhaust passages cast into the intake manifold that lets exhaust flow under the carb ..

But on an air cooled engine most don't like carbs getting too warm,especially when you use ethanol gas,which burns hotter than the old leaded gas did back when,and also has a lower "boiling point" than old leaded gas..it can lead to vapor lock or lean running conditions if the carb gets too hot,and lean mixtures burn hotter,aggravating the issue....
Some air cooled snowblowers had "heater boxes" around the exhaust & carb to keep the carb from icing up,but all the rest don't need extra warmth and will run better if the exhaust heat is kept away from the carb..

One test you could try is to make a sheet metal heat shield to keep the exhaust heat away from the carb..

If it still does the same thing,maybe your ignition coil is crapping out,they usually do lose spark after they get hot as a early symptom it is getting ready to fail..they can throw a good spark "cold",but heat increases the resistance of the windings and too much heat can melt the insulation inside on the windings,and when it gets hot enough they short out..
 

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I had an old briggs vertical in a montgomery ward mower that would do that. It had some fancy large muffler that wrapped around the front of the engine but most of it was under the hood. It would suck all that hot air going over the muffler right up under the hood and down through the fan. It would run until it got too hot and would lose power. I just took the bolts out of the hood. When I needed to use it, I pulled the hood off and never had any more trouble with it. When the job was done, turned it off and put the hood back on it.
 

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Sure sounds to me that there is something wrong with the muffler. There should be no hot gases blowing on the carb or the intake manifold (if there is one). I would change the muffler and add the suggested baffle/heat isolation plate and go from there. Also, people do not understand how bad today's gas really is. If you are using 90/10 ethanol gas you must readjust your carb's high speed mixture jet to account of the difference in energy content (ethanol laced gas is lower than straight gas). Other wise the engine will be running in the lean zone and any added heat (like this muffler thing) will cause problems.
 

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Discussion Starter #8
I've got a Gravely with an 8HP Kohler which runs great , justs purrs right along, but if it's working hard or it's hot outside it starts sputtering, then slows down and dies. If I wait a while it will start right back up. I notice the muffler emits hot exhaust gas directly on the carburetor. I'ts too hot to touch when the engine dies. Is this the problem? Anyone else experienced this? Thanks in advance
 
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