Hey everyone. I have a Scag STT-31BSG with a Briggs-Stratton 580447 engine that is roughly 10 years old. We recently have been having problems with it as it has been throttling in RPM up and down at high settings, and if you attempt to quickly change speeds from a lower setting, it will stall and die. At low speeds and idling, there is zero surging, and if you carefully raise the speed, you can avoid these two problems, but not at full speed.
Also, with most motors, you can tell a difference in RPM as you slowly increase the speed. Currently, it's throttle has to be moved very slowly to make sure it doesn't surge and die. During that time, I do not hear a real difference in RPM, even after moving the throttle from slow(idle) to medium (middle) speed. After a 'threshold,' it kicks in and increases in RPM quickly, causing a surge, and occasionally death.
I also recently had it miraculously work at full speed for a day without any surging after mowing for an hour, but after lowering the speeds to turn off the mower, the surging returned. My father figured it was the fuel pump as a faulty fuel pump can cause surging at high RPMs. Since it was an easy part to replace, we made the gamble, but it didn't pay off.
I've been staring at this thing for a while, and it almost looks like the governor lever's spring might be oscillating, causing the problem. However, the spring appears to be in good shape. Honestly, I don't know if that's the problem, as it's just a spring, but I have read in a few other manuals that you could adjust the spring tension in order to help the governor do it's job properly. The manual for the scag states not the mess with it and consult the Briggs manual, which has nothing about the governor. At this point, I wouldn't be surprised that this engine was simply so dirty, that there is friction on the throttle lever, causing the governor level to fluctuate heavily when it passes a certain point.
Lastly, I am not entirely convinced that it is the carburetor, as you can just hold down the carburetor throttle lever on the motor itself and it holds a constant RPM. I am under the inexperienced notion that when the carburetor gets dirty, it will surge at any speed.
I am not experienced with motors and I feel a little over my head. I have debugged a couple of systems (computers and electrical) in the past successfully, so hopefully my technical degree is worth it.
Thanks for the help!
Also, with most motors, you can tell a difference in RPM as you slowly increase the speed. Currently, it's throttle has to be moved very slowly to make sure it doesn't surge and die. During that time, I do not hear a real difference in RPM, even after moving the throttle from slow(idle) to medium (middle) speed. After a 'threshold,' it kicks in and increases in RPM quickly, causing a surge, and occasionally death.
I also recently had it miraculously work at full speed for a day without any surging after mowing for an hour, but after lowering the speeds to turn off the mower, the surging returned. My father figured it was the fuel pump as a faulty fuel pump can cause surging at high RPMs. Since it was an easy part to replace, we made the gamble, but it didn't pay off.
I've been staring at this thing for a while, and it almost looks like the governor lever's spring might be oscillating, causing the problem. However, the spring appears to be in good shape. Honestly, I don't know if that's the problem, as it's just a spring, but I have read in a few other manuals that you could adjust the spring tension in order to help the governor do it's job properly. The manual for the scag states not the mess with it and consult the Briggs manual, which has nothing about the governor. At this point, I wouldn't be surprised that this engine was simply so dirty, that there is friction on the throttle lever, causing the governor level to fluctuate heavily when it passes a certain point.
Lastly, I am not entirely convinced that it is the carburetor, as you can just hold down the carburetor throttle lever on the motor itself and it holds a constant RPM. I am under the inexperienced notion that when the carburetor gets dirty, it will surge at any speed.
I am not experienced with motors and I feel a little over my head. I have debugged a couple of systems (computers and electrical) in the past successfully, so hopefully my technical degree is worth it.
Thanks for the help!