RPM adjustment

Back in 1997 my next door neighbor said he liked my D series Lawn Boy so he picked one up at Lowes. I remember him saying that the new one didn't rev up as fast as my 1976 mower did. His was a V series with a steel deck. Twenty years later he gave his Lawn Boy to me because it caused him too much trouble. I got it running good and remembered what he said about the slow RPMs. I twisted the knurled wheel on the air vane a few clicks and its running like a top. I also
was given another v engine lawn boy that when I got it running it would rev way too fast and I adjusted it down by turning the little wheel the other way to get it running at a normal speed. I don't have a tach but Iv'e been mowing for about 50 years and I know about what the right speed sounds like. Any way my question is how does that control the engine speed? I'm guessing that turning the wheel a few clicks changes the tension on the spring but I still don't understand how that changes anything. I don't understand how that if the air vane is moved more forward, how does that increase the speed of the engine.
I don't know why it gets more gas by moving the air vane more. Could anyone explain how it works?
Thanks,
Jerry

Hi Jerry,

The air vane is connected to a butterfly valve. Opening the butterfly allows more air to the engine, which along with more air is more fuel.

Regards

Jeff

Only thing I will add is that as you load the motor the flywheel spins less decreasing wind and the vane will open applying more air/fuel to motor, less load the flywheel will spin faster creating more wind and closing butterfly slowing motor. The spring will wear out, have to pull the copper throttle plate out with a needle nose plyers to replace spring. Each click is 50 rpm adjustment. You can get a cheap tach on amazon for 15.00 or so, I have one for both of my lawn boys. My dura-force will fluctuate from 3100 to 2900 rpm until motor is at operating temp (about 5 minutes of mowing) after that is settles in to 3130 or so unless I climb my hill. My other LB is a M-series that has an internal governor that pulls on some linkage when motor needs more throttle or if it needs to back off throttle. This system hold Rpm much better than the vane style. However the vane is the simplest and easiest to fix if something needed to be repaired. The M series will hold 3150 to 3200 after engine is at operating temp.

Thanks Jeff for your reply.
Thanks Lewb, that was going to be my next question about how the engine speeds up when you get into heavier and thicker grass. I didn't know that the flywheel slowing down would open up the throttle more. It's starting to make sense. Thanks again fellows.
Jerry

Hi Jerry,

Here is a more scientific answer to your question. As the butterfly opens, more air and the velocity of air flows through the venturi design of the carburetor. This causes a higher vacuum pressure at the throat of the carb, which draws more fuel up through the main jet of the carb. I've had experience with both type of governors, air vane and centrifugal types. Lawn boy used the air vane and Suzuki and Tecumseh used the centrifugal type. I can't honestly state one worked better than the other.

Regards

Jeff

Thanks for the explanation Jeff. I'm starting to get a handle on it now.
Jerry