26 hp intek belching after getting hot

Was mowing tall grass on a 90 degree day, with throttle nearly open, and afterwards noticed that this 26 hp intek started belching frequently. It doesn't exactly sound like a back fire, but on that order. It starts and mows fine, and when running at slow speeds, or idle, it is not as noticeable. I had just been bragging about this engine, which has over 1000 hours on it. It has been well maintained, uses no oil, and has always run perfectly. I will do some checking tomorrow, but wondered if others had experience this.

Sounds kinda like the valves need to be adjusted then again on that steep grade it might have been sucking some oil in the carb or back in the heads causing a misfire. Should always run an air cooled engine full throttle it's the only way it can stay cool is to draw in all the air that it can. Could also be some water in the gas as well causing your issues.

Thanks for the input. I can tell you are experienced about air cooled motors. I had just set the valves at .005 " a couple of weeks ago, because they were a little loose, and making it hard for the starter to crank sometimes. I was just bragging on how well it ran. Guess that doesn't pay, ha. I don't think there is any water in the tank, and it ran perfectly before I got it too hot. I will check valves and compression tomorrow and let you know what I find, thanks.

"Should always run an air cooled engine full throttle it's the only way it can stay cool is to draw in all the air that it can."

Have often seen this statement and puzzles me. I believe cooling required is dependent on the amount fuel being burned, faster the engine runs, the more fuel it is burning and the more cooling it requires. Seems to me the above quoted statement assume the same amount of cooling is required regardless of the amount of fuel being burned, speed, of the engine.

Walt Conner

It is quite true Walt.
The simple analogy is fans on cars
Before they all went electric a lot had an auxiliary fan that came on with the air con and also when the engine fan was not shifting enough air to cool the water at low engine speeds.
The volumetric efficiency of the fan is not linear and the intake and outlets are fixed sizes.
To work efficiently at all speeds the outlets around the cylinder would need to adjust.
Also the rate of heat exchange ( R value) is proportional to the difference in the temperatures
At lower volumes the air is in contact with the engine for longer so by the time the air gets from the cylinder to the heads it is already quite hot so the rate it can cool the head reduces.

To work efficiently at lower speeds the shape of the fan blades & their spacing would need to be different.
The blades would need to be deeper , closer & the spiral tighter.
However at full speed that shape becomes very inefficient because at high speeds you need fewer blades with a shallower spiral.
Thus the designers optimize for the speed the engine will be run at most 3600 rpm with some consideration for running at 1200 rpm.
Also after mowing the entire engine will be at full temperature and have a lot of stored heat in the crank.
Unless the exterior is cooling quickly the interior can not cool fast enough so you have a cooling piston & rod on a hotter crank.
Thus you thermal expansion operating clearances are going in the wrong direction

If you would have stopped at a gas station back in the days In a volts wagon and filled it up with gas. Then going down the interstate and got behind a semi truck and followed it for several miles I would almost bet your engine would over heat and blow up and your gas tank would almost be still full of fuel. Because gas is a hazardous product and is cold from the start with a low ignition point it aides in the cooling process but if you take away the flow of air it would over heat at a faster rate and cause failure over time.

Luffydog said:

If you would have stopped at a gas station back in the days In a volts wagon and filled it up with gas. Then going down the interstate and got behind a semi truck and followed it for several miles I would almost bet your engine would over heat and blow up and your gas tank would almost be still full of fuel. Because gas is a hazardous product and is cold from the start with a low ignition point it aides in the cooling process but if you take away the flow of air it would over heat at a faster rate and cause failure over time.

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Well the air cooled Volkswagen engine and air intake are in the back, doubt air pocket effect would keep the cooling blower from cooling engine but what does that have to do with running small engines at full GOVERNED speed, which is not full throttle by the way, to keep it from getting hot? Full governed speed of small engines typically vary from 3200 for LB to 4K for some generator engines. If you are saying there would be no air back behind the semi then the occupants would suffocate and really would not mater if engine was hot or not. : )

Walt Conner

Thanks to all 3 of you guys about the effects of heat dissipation, or lack of, in air cooled engines. I know they need plenty of air alright. I remember old VWs didn't hold up real well on our gravel roads after they had a few layers of dust and oil on them. I checked valve clearances on this B&S today, and all checked a snug .005. I had guessed that one was too tight, but wasn't. The compression was 150 and 160. The color of the old plugs looked fine, but put in new ones. There was some grass pieces in the fins and fan blade, but not too much. The air filter needs cleaning, but didn't look too bad. Guess I could run Sea foam through carb. and carb cleaner on head fins. Will try firing it up tomorrow and see how it sounds. Thanks.

Tinkerer200 said:

Well the air cooled Volkswagen engine and air intake are in the back, doubt air pocket effect would keep the cooling blower from cooling engine but what does that have to do with running small engines at full GOVERNED speed, which is not full throttle by the way, to keep it from getting hot? Full governed speed of small engines typically vary from 3200 for LB to 4K for some generator engines. If you are saying there would be no air back behind the semi then the occupants would suffocate and really would not mater if engine was hot or not. : )

Walt Conner

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Thanks Walt. Yes, I sure mower motors wouldn't last too long if they were governed, and the operators might not either. I probably keep mine around 3K. This one time, when it was over 90 degrees outside, and I was cutting tall grass, I was going about to the max. It started coughing, and I have been working on it since. Just checked the valves, air cleaner, and put in new spark plug. Just got back from testing it. Runs and cuts fine, and idles like nothing is wrong. But, when I rev it up, it starts to snort once in a while. Wonder if it is a little rich, but like I said, never any problem before that day it started. Not sure what to check next.

SlopeMan2 said:

Thanks to all 3 of you guys about the effects of heat dissipation, or lack of, in air cooled engines. I know they need plenty of air alright. I remember old VWs didn't hold up real well on our gravel roads after they had a few layers of dust and oil on them. I checked valve clearances on this B&S today, and all checked a snug .005. I had guessed that one was too tight, but wasn't. The compression was 150 and 160. The color of the old plugs looked fine, but put in new ones. There was some grass pieces in the fins and fan blade, but not too much. The air filter needs cleaning, but didn't look too bad. Guess I could run Sea foam through carb. and carb cleaner on head fins. Will try firing it up tomorrow and see how it sounds. Thanks.

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Just now got through mowing for about an hour with this machine. When riding on it, and mowing at higher rpm, you don't notice any problem. It starts instantly, and idles smoothly. But, when it is sitting still, and you get behind it, while increasing rpms, it starts with frequent hic up, and even a subtle back fire now and then. After having checked the valve clearances, compression, and putting in new plugs, I was wondering if these symptoms might indicate too high a fuel level in the float bowl. Thanks for suggestons.