Export thread

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.

Click to expand...

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

Click to expand...

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.

Click to expand...

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.

I don't guess a worn cam lobe could cause this, could it?
That would probably be more constant.

SlopeMan2 said:

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.

Click to expand...

Sounds more like a lean condition to me, I would try spraying WD40 or carb cleaner around manifold to block/ carb to manifold gasket areas to see if it makes a difference in rpm indicating a leak. Also makes me wonder about head gasket.

Walt Conner

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. ... .

Click to expand...

to set the record straight. Going to a race in Atlanta back in the 70's, in Texas, and pulling a small camping trailer into a very strong 50 mph head wind in a 1971 VW bus we could not get over 40 mph. We found a semi that could not get over 40 mph either and he had no problem with us sitting 4' off the back of his truck for about 2 hours since both of us were able to speed up to about 60 that way. He used his turn signals to tell us what was going on at a distance ahead. With a full set of gauges on the bus - oil pressure, oil temp, cyl head temp etc., I can assure you that we actually ran cooler because their was less effort pushing into the headwind. Air cooled engines need air volume to cool, and the best benefit comes at a spot where the fan is most efficient. A heavy load at low RPM is hardest on air cooled engines.

As to the OP - probably changing the air filter, and fuel filter would be where I would go with this. That said, I have seen this behavior in some of my air cooled engines around the landscape and garden. In each case it turned out to be a worn out fuel pump (diaphragm) or loose carb jet. Modern fuels attack many of the fuel diaphragms and this causes an inconsistent supply of fuel - which in turn causes the surging.

Scrubcadet10 said:

I don't guess a worn cam lobe could cause this, could it?
That would probably be more constant.

Click to expand...

Thanks Scrubcadet10. I hadn't thought about a worn cam lobe. Since it started this snorting all at once, I wouldn't think it was that. But, since it has over 1000 hours on it, I might check that. It only takes one little thing. Sure a good feeling when they run nice and smooth. Thanks

Tinkerer200 said:

Sounds more like a lean condition to me, I would try spraying WD40 or carb cleaner around manifold to block/ carb to manifold gasket areas to see if it makes a difference in rpm indicating a leak. Also makes me wonder about head gasket.

Walt Conner

Click to expand...

Ok Walt, will try spraying some WD around the intake. I remember having a friend that did that to my 250 Honda that was fussy. One cylinder did have a little oil coverage around the head, which had happen over time. It wasn't fresh. The other cylinder was dry. I forget which cylinder had the highest compression, but they showed 150 and 160. If it were a head gasket, wouldn't the compression be much lower? I mowed again with it tonight. When I am mowing at higher revs, you hardly notice it. And, at idle, it sounds fine. But at about 1/4th throttle, it starts to spit, and sort of has a lope to it. I really hate it, because it was so perfect before last week. I will check the compression again. I will try the WD and check the compression again.

sgkent said:

to set the record straight. Going to a race in Atlanta back in the 70's, in Texas, and pulling a small camping trailer into a very strong 50 mph head wind in a 1971 VW bus we could not get over 40 mph. We found a semi that could not get over 40 mph either and he had no problem with us sitting 4' off the back of his truck for about 2 hours since both of us were able to speed up to about 60 that way. He used his turn signals to tell us what was going on at a distance ahead. With a full set of gauges on the bus - oil pressure, oil temp, cyl head temp etc., I can assure you that we actually ran cooler because their was less effort pushing into the headwind. Air cooled engines need air volume to cool, and the best benefit comes at a spot where the fan is most efficient. A heavy load at low RPM is hardest on air cooled engines.

As to the OP - probably changing the air filter, and fuel filter would be where I would go with this. That said, I have seen this behavior in some of my air cooled engines around the landscape and garden. In each case it turned out to be a worn out fuel pump (diaphragm) or loose carb jet. Modern fuels attack many of the fuel diaphragms and this causes an inconsistent supply of fuel - which in turn causes the surging.

Click to expand...

Thanks sgkent. Enjoyed reading about drafting with the VW bus, ha. I remember tucking behind a travel trailer on a 90cc Honda in 1965. When we stopped at a station, I thanked him, and said I was getting over 100 mpg. He laughed, and I should have, because he was pulling me, ha.

I will check the fuel filter, since you said it sounded like a fuel supply problem. I does surge. And, yes, the fuel pump. That is original, so I will check that. I love this engine, so hope you are right. It doesn't knock, or have mechanical noises, and the compression tested 150 and 160. I did recheck the valve clearance. Thanks a lot. Will let you know what I find.

SlopeMan2 said:

Ok Walt, will try spraying some WD around the intake. I remember having a friend that did that to my 250 Honda that was fussy. One cylinder did have a little oil coverage around the head, which had happen over time. It wasn't fresh. The other cylinder was dry. I forget which cylinder had the highest compression, but they showed 150 and 160. If it were a head gasket, wouldn't the compression be much lower? I mowed again with it tonight. When I am mowing at higher revs, you hardly notice it. And, at idle, it sounds fine. But at about 1/4th throttle, it starts to spit, and sort of has a lope to it. I really hate it, because it was so perfect before last week. I will check the compression again. I will try the WD and check the compression again.

Click to expand...

Head gasket starting to leak will not show drastic compression loss. Just trying to think of something which might result from getting hot under load. Long shot, might run some SeaFoam thru it per instructions on container.

Walt Conner

Tinkerer200 said:

Head gasket starting to leak will not show drastic compression loss. Just trying to think of something which might result from getting hot under load. Long shot, might run some SeaFoam thru it per instructions on container.

Walt Conner

Click to expand...

I rigged a small temporary gas tank, and mixed gas and seafoam half and half. I mowed some with it last night. Will try this a few time before I dig further. I do remember doing this to a 250 Honda, and it did the trick then. I had forgotten about that, but never had any more problems with the Honda. Thanks, and will let you know.

SlopeMan2 said:

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.

Click to expand...

Well guys, finally got the 26 hp B&S to quit belching. A new carburetor solved the problem. Some of you suggested that it was a lean problem. Well, I guess it was. I checked everything else first - leaks, valves, new fuel pump, lines, filter, plugs etc. I understand the little o rings around the jets go bad. It seemed easier to find a new carb. on-line, than parts. So - got it all back together, prayed and turned the starter. Oh what a feeling. It started quick, and was it's old self. Sounds great. Mowed 2 hours with it today without a hic-up. What puzzles me, is why it started that all at once, while mowing on that hot day. The only thing I can figure, is that some extra heat affected the little o rings - or, that it was just time for one of them to get distorted.

Thanks again to all you guys. These little engines can make you pull your hair - kinda complicated like human bodies. Much better that the problem was just a motor, than a problem in one of us that the doctor has to figure out. Thanks, and hope you all get help with your problems.

Glad you got her going!

Scrubcadet10 said:

Glad you got her going!

Click to expand...

Thanks Scrub. Sure enjoy those sweet exhaust notes after all those weird sounds, ha. The tougher the chase the sweeter the catch. Good luck.