Honda gx390 surging

Honda gx390 surging DEFAULT

Calling all GX390 experts

Well today my gx390 let me down and left me embarrassed. Had to tuck my tail and tell the homeowners I would have to come back to finish their driveway. Basically my motor is revving high and “surging”. Hard to explain really. After I use a high pressure tip, surface cleaner, basically anything other than my jrod it starts waning and trying to recover. It flat out would not run my surface cleaner long enough for me to finish the job. So y’all that are familiar with these motors. Can you throw me out some possible problems/solutions? Or could It be my pump? So frigging frustrating. Here’s what I’ve done. I’ve checked the needle and bowl for debris. (None). Checked valves and I’m pretty sure they’re adjusted like they’re supposed to be. I will do a fluid change on motor and pump. And change the spark plug. The carb is new with only like 30 hours on it.

I’m about to say efff it and buy the 8gpm a lot sooner than I wanted. A lot of options. For mostly residential and light commercial would y’all recommend the 630 or the 690? I know I want a Honda motor. Can’t figure out which pump would be good. I’m wanting to stick with cold water but have the option to add a burner down the road. Sorry for the long post but it’s been one of those days from hell.


Several years ago I purchased a Black Max 5700 Watt Generator with a 414CC Honda GX390 Engine so I could power my house during power outages. I bought it at Sam’s Club. I love the generator – it was small enough to move around and just large enough to run my whole house while keeping gas consumption low. I wanted 30 amps of 240 volts so I could run everything.

But it had a surging problem. If you are in a hurry, here is a video of the before and after:

Here is the detail on what happened.

Bad News – Surging – Warranty Useless
As soon as I turned it on, it would not idle. Unless it was under load, it would surge because it could not idle. The governor would notice it was about to stall and then give it gas – then slow it down, and it would almost stall and over and over again every second.

Under load the governor opened up the butterfly a bit and it would maintain RPMs nicely for hours. I was ticked off that this brand new generator could not idle. So I contacted Black Max and told them I had just bought it days ago and it surged. They told me that their warranty covered everything *except* the engine. They gave me a number to call about the engine. When I called the engine warranty number – they effectively said – I had not bought the engine from them and so I would have to work through Black Max. I called back Black Max and hit a wall. Lesson learned – if crap is broken as purchased – take it back to where you bought it – right away.

The workaround
But since I liked the generator so much I found a “hack” where I would use the choke to keep the revs up while it was not under load and then it worked well enough to get me through two power outages.

But then we had a winter with no power outages and so I wanted to run the engine for a while to warm it up, clear out the carbs, and get rid of condensation in the oil. I run all my engines a minimum of 30 minutes per year every year in the Fall.

Since it was warm, my choke trick did not work. If I choked it – the engine would not run – and if I let it surge for more than 10 minutes, the surging would get worse and choke itself out so bad that it would be hard to start afterwards.

This is for a unit that was well maintained with fresh gas, fuel stabilizer, and non-Ethanol gas when it was stored. It had a total of 20 hours and looked brand new.

Ah Ha! Buy a new Carburetor

The engine was running so bad that I wondered if some gum had built up – or perhaps there was just a manufacturing defect in the carb – and I figured a new carb might just fix this.

So I searched and searched for a L09 120621 D11866 carb but to no avail. But I got close and figured out that the engine is a Honda GX390 and found replacement carburetors for that.

It looked like Amazon had a carb that was almost perfect. The new carb had a fuel shut off valve that was not present on my old carb, and it did have an idle adjustment – which I liked. For < $15 I ordered one to see if I could make it work. When it arrived, the size was perfect - all the things like the choke extension, and the throttle to governor connection were exactly the same. 02-both-carbs

The only weird bit was the incoming fuel like for the new carb was 3/16″ and the incoming fuel line for the original carb was 1/4″ – so I picked up an adapter from a local parts store.


I put it all back together, adapted the fuel lines and started it up. It was easy to adjust the idle and minimum speed adjustment and stop it from surging.

Check out the above video for the pre-and-post.

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Honda GX390 Surging Problem

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!


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