Lawn-Boy S21BSN, SURGING

[waynieboy]

Been using the above mower for two years after my complete rebuild, cannot stop the engine from surging. I've searched the net for instruction on adjusting my WALBRO 2728 carburetor however, not much luck. Anybody out there having success on adjusting this S Series LB??

 

[Rivets]

I’m going to make some assumptions, WHICH I KNOW I SHOULDN’T DO, as it can get me in trouble very quickly. Here they are. Your surging is the result of a lean running condition, not rich. Closing the choke slightly helps a little. Carb has been cleaned and rebuilt with new float needle and seat. New manifold gasket and you have tested for leakage. Internal filter screen is clean. Using fresh fuel. Three things that I’ve found when working on these engines. 1. Float level not set properly, I normally set them slightly rich. 2. Mixture screw not adjusted properly, again I normally se it slightly rich. 3. Governor not set to correct speed, I like my engines set at 3200 RPMs.

 

[sgkent]

my edger was doing that a couple years ago. I looked to adjust the mixture but there was no spot to adjust the mixture. The small mixture jet had unscrewed and fallen who knows where. Local dealer had one and putting it back in solved the issue.

 

[FuzzyDriver]

1. Float level not set properly, I normally set them slightly rich.

As long as the float is keeping the jet in fuel, how does changing float level change fuel:air ratio?

 

[Rivets]

It has to do with air pressure, air movement and the Bernoulli principle. if you look inside the air horn (tube) of the carb you will notice where the air horn gets smaller. This area is as the carbs Venturi and the main nozzle opening is located at this point. As the air is pulled from the outside of the engine into the cylinder, by the piston going down, it must pass through this restrictive area causing it to speed up. As the air speed increases the air pressure decreases in this area. We now have a high pressure outside the engine and a low pressure inside. Because the float bowl is vented to outside air, it will have the same air pressure. Air and fluids always flow from a high pressure to a low pressure. High pressure pushing on the fuel in the float bowl, forces it up the nozzle to the low pressure in the Venturi, where it mixes with the air. The higher the fuel is in the float bowl the less pressure difference needed to draw it up the fuel nozzle. Less air needed to move more fuel, high air/fuel ratio.

Hope this makes sense, or I’ll have to find the diagrams from my teaching days. This site will explain bern principle, fluid movement. https://en.wikipedia.org/wiki/Bernoulli's_principle

 

[bertsmobile1]

As long as the float is keeping the jet in fuel, how does changing float level change fuel:air ratio?

To lift fuel from the bowl to the top of the emulsion tube against the force of gravity takes energy
The energy to do this comes from the low pressure zone at the venturi in the carb throat
If the fuel level is lower more energy is needed to get it out of the emulsion tube .
It is the opposite effect or storing water high up to increase the pressure head .

 

[FuzzyDriver]

Rivets: Thank you for the complete reply. I'm sure it will help others to understand how a carburetor works. I was actually wondering how such a small change in water column (gas column here) could make a detectable change in fuel/air ratio. It just doesn't seem like it would make enough difference to matter, especially since there is a range of 'acceptable' float levels. Why doesn't the manual tell you that this makes a difference? I have to admit, I just make sure it's in range and never give it a second thought, but I'll mess with it in the future if I get surging! Thanks again, and I have to ping just a little: In your last sentence, less air gives a higher fuel/air ratio, not a higher air/fuel ratio. ;^) Have a good one!