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Discussion Starter #1
Have an older John Deere generator with a Honda GX390 engine on it. With the idle control off, it runs fine and powers whatever needed. With the idle control on, it runs fine until it idles down. When it the idle control trips to go back to high-idle or full throttle, it spits sputters pops and dies. The carburetor has been rebuilt, carburetor has been replaced with new, fuel lines and tank have been cleaned out, a new fuel shut off valve on the tank, the little choke control diaphragm has been replaced and the check valve between the manifold seems to be working. Any help with ideas of what I need to do to get it to stay running coming off-idle?
 

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I'd go back to the factory carb. Unless you got a factory replacement that matches exactly, you may be chasing your tail trying to make it handle the 'idle up' demand by the generator. The engine may have been equipped with a carburetor designed to handle the problem you encountered.
Going from idle rpm to demand rpm requires transition circuits that give a shot of liquid somehow to enrich the mix when the throttle opens. Air can get going quickly, and flow right in, after all it has 14 psi ambient air pressure pushing it, and it is less dense. Liquid fuel, in the float bowl, or in a passage, takes more time to get going. It is being pulled by venturi pressure, not air pressure, and is just harder to get going. Result is good airflow with no fuel to burn... brp mfll bap. silence. A 'transition' circuit will add extra fuel, most using simple ports right 'behind' or 'after' the butterfly. As the butterfly/throttle plate opens, these ports are exposed, and a suction forms to pull some liquid fuel into the airflow. It doesn't mix perfectly, but gets by for that short time until regular flow takes over. It 'transitions' the carb from its idle speed system(idle mix screw, idle speed) to its working higher rpm system(main jet, etc).
Take a look inside the venturi of the old carb, look for ports on the 'side wall', right after the throttle plate. Make sure you can blow carb cleaner backwards through them. Use a bread-bag wire tie, stripped of paper, to poke in if you think they are clogged. Make sure the fuel level in the float bowl is correct and that the needle moves freely. Low fuel level will make pulling fuel into the carb harder, and the 'transition' will be less successful than factory if the level is low.(you will be just a smidge leaner)
tom
 

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Discussion Starter #4
It is a factory Honda carburetor. The original was rebuilt due to this problem, problem still persisted so it was replaced with a new Honda carburetor. Supply is good from the overhead tank, it is gravity fed to the carburetor. Has plenty of fresh clean fuel getting to the carburetor
 

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Was the replacement carburetor an exact replacement? I have a different brand generator with the 'econ' setting that lets it idle down when demand is lower(inverter type). It has a stepper motor that operates the throttle in response to changes in demand. If the 'base idle' is too low, or too lean, it can stumble when the load increases. The base idle rpm can be set using a hard stop on the carb. The stepper seems to let it go to the hard stop idle rpms, whatever they are, and then opens the throttle as needed, varying by load, but it will go to WOT if demand added is high enough.
I think you might want to find out the suggested 'idle' rpm setting, and use a tachometer to see if you are below that number. An alternative would be to screw in the hard stop(where the throttle controls will move to in the fully closed position) to adjust the no-load idle a bit higher. Could be it has moved over time and use.
There may be a Honda specified idle rpm, and if there is, I'd set to that and see what happens.
You can get an app for Android or iphone that listens to the engine to tell you rpms. Look up member "Realtime" or see his posts for info. It works well enough to know if you are in the ballpark.
tom
 

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Discussion Starter #6
Well, figured I would come back here and update the fix. So the final fix that corrected the issue was a new spark plug. The plug had been cleaned and checked, but installing a new spark plug fully fixed the unit. Just a reminder that the old KISS method still works, even after you think you have all your bases covered.
 

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I have had one start and run, after a carb cleaning, needed from being stored with fuel...

Started, fiddled with the adjustments, shut down. Went to demo to SIL, silence, lots of Swedish Steam trying to get it to start. Nope. Not gonna happen.
Checked the plug. No more spark. New ignition orderd, and it worked. I think Honda engines are a bit 'different' in that they will unexpectedly do things you are not aware of. For no apparent reason.
Ya never know.
tom
 
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