725EXi throttle application ?

J

JKP63

Forum Newbie
I have a 725EXi replacement engine for my old John Deere walk behind. Model 104M020250F1 ...... This variant uses a governor with a preset mowing rpm. I would like to add a manual cable throttle to work with the governor. Does anyone know if this possible, or a B&S option for this motor ?
Thanks for any advice.
 
Photos would be helpful.

Does it have a control plane?

I believe I had this engine and it was fixed speed using a spring and a tab.
You could bend the tab to get 2800-3200 rpms
 
Not a very good picture, best i can do at the moment. This shows the governor spring, attached to an arm with linkage to the carb. If I connect a throttle cable to this, then I can manually set the rpm, but it would override the governor, as the linkage would not be able to respond to governor spring tension. What I'm wondering, is there any way to add manual control, while still allowing the governor to function as load dictates. Like maybe a B&G add on, or aftermarket device
 

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There is a very and I mean very high chance that the engine will not run smoothly outside of it designed RPM range because most of the fixed speed engines lack an idle circuit in the carb, which will result in poor running performance.
 
makes sense I guess ... what is an "idle circuit" ? ..... while running at the governed speed, I can reduce or advance the rpms manually, and it seems smooth, but I don't know how it would run under load while pushed outside of the range
 
Last edited:
JKP63 said:
makes sense I guess ... what is an "idle circuit" ? ..... while running at the governed speed, I can reduce or advance the rpms manually, and it seems smooth, but I don't know how it would run under load while pushed outsidie of the range
Click to expand...
Variable speed engines use carbs with what is called the idle fuel circuit and the high speed fuel circuit. The engine runs on the idle circuit until the engine is placed under load like engaging the pto on a rider, or pushing the mower into grass, which caused the throttle butterfly to open and exposes the high speed circuit or jet to vacuum which allows the engine to draw fuel from the circuit.

When an engine doesn't have the idle circuit and only the high speed circuit closing the throttle butterfly beyond a certain point cause disruption in the vacuum air stream over the high speed jet which cases a lean run condition and surging or misfiring due to the lack of fuel. So when the carb is designed to supply the proper amount of fuel at say 2800-3200 rpm going above that range can result in the demand overwelming what the jet size can actually flow resulting in a lean run and surging. Also when the engine is slowed down below what the engine is designed to run can result in the throttle butterfly closing to the point that it basically shuts off the vacuum to the high speed jet resulting in a lean run condition and surging.
 
Thanks! .... very good explanation. Do you have any idea where I could get technical info on this engine ?
 
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