Honda GX390 Surging Problem

Casey Anderson

New member
My machine has a Honda GX390 13-HP motor with a General 4040 direct drive pump.

The pump and unloader probably have less than 100 hours on them. I am guessing the motor has around 500 +/-. The motor has always ran great until the end of last year.

It seems to hunt or surge when I release the trigger after spraying. You can hear it and watch the governor arm move back and forth. The problem is not too bad when the machine is cold but gets worse the longer it runs. I have actually had it die in me in the past which is a huge PITA to stop what you doing and have to restart it. It seems to get better after the motor cools down a little.

Here is a video of the machine running and surging. Is is pretty apparent about :25 in.

Here is what I have done and tried so for.
-Disconnected the oil level sensor
-Cleaned out the gas tank, line, and fuel filter in the small bowl. (Everything was clean.)
-Tried fresh 93 octane gas.
-Tried running with no gas cap.
-Air filter and spark plug look like new.
-Removed the carb and cleaned the jets. Blew out all the passages in the Carb. Carb looked spotless. Carb to intake gasket did not have any tears or holes.
-Checked the governor arm where is goes into the block. Everything seems tight.

Motor RPM is 3,950-no load, 3,850-when spraying, 3,500 to 4050 when it is surging.

About the only thing I can think of is to order a new carb. Does anyone have any other suggestions? Thanks!


New member
Is there an idle mixture adjustment screw anywhere on the carb? Are there are other holes where the governor spring attaches, try moving the spring over.

Art Oliveri

Roundtable Host 2009
I started having the same problem Today. I just found this YouTube wonder if this is the fix and where do you buy the Spring

Russ Johnson

Equipment Expert
That engine should run at MAX 3600.

I have seen the insulator crack. Easy to test. Let it surge and spray carburetor cleaner in the space between the carb and the head. If it's cracked, it'll suck carb cleaner instead of air and smooth right out.

If that doesn't work, try closing the choke slowly and see if it runs smooth.

Casey Anderson

New member
Ok, had a chance to run the machine again tonight.

It appears closing the choke to about 1/3 closed makes is run right. No more surging when I let off the trigger and it also seems to run smoother.

I tried spraying carb cleaner all around where the carb meets the motor while it was surging. That did not seem to stop the surging or change anything.

Also got the RPM adjusted down to 3,600. Wish I would have known a few years ago how cheap and easy it is to put a tach on these things!

Any suggestions on what to do now? Take the carb apart again and try cleaning it better? Thanks!

Casey Anderson

New member
Pulled the main jet and stem that sits above it. Took some small wire to all the holes. Also hit both pieces with a few more rounds of carb cleaner and compressed air. The jet and stem look squeaky clean. Still runs the same. No improvement.

Never heard of ethanol poisoning. What is that about?


New member
Just a bit of sarcasm this stuff is rough on tiny jets on this smaller engines it could be the governer has went bad I had one of those along time ago that the governor went haywire on.

Casey Anderson

New member
I just ended up ordering a new carb on Ebay for $20. It comes with a new insulator and gaskets. I figured that was worth a try rather than investing more time in mine. I'll let you guys know how it works out next week.

Ralph Q

New member
I just ended up ordering a new carb on Ebay for $20. It comes with a new insulator and gaskets. I figured that was worth a try rather than investing more time in mine. I'll let you guys know how it works out next week.
Better check the estimated shipping for that. I ordered one from ebay and it turned out to be a chinese company who ships them out on shipping containers when the container is full. My estimated shipping time was 7 weeks and I didn't notice until 2 weeks later when I heard nothing.

Casey Anderson

New member
Finally had a chance to swap out my carb, gaskets, and insulator with new ones from ebay today. Absolutely no change! it still surges with the choke off and runs fine with it about 1/3 closed.

I am assuming this means it is running lean? Any suggestions on anything else to try?

Casey Anderson

New member
Just did a static adjustment on the governor. No change. I know I adjusted the governor right because adjusting the shaft all the way in one direction makes in run at a real high RPM. I had to shut it off immediately. Adjusting it back the other way makes it run like it has been. This thing is driving me nuts! There is not too much too these engines.