Calling all GX390 experts


Thanks for the information!


@Steve. Update. So I put a “brand new” pump on my Honda. Still bogged and died under high pressure. So my conclusion is the motor is bad. What could be the problem? I think I’m just going to take both the motors I have to the small motor shop and see what’s up. My grand dad gave me this “brand new” washer today. He says it’s probably 22 years old and he paid $1200 brand new. It’s only been run 12 hours and last ran 5 years ago. I think it’s worth having it fixed over the Honda.


Same unloader?


Different unloader. The Briggs motor above, i just removed the pump/unloader and bolted it to the Honda.


The motor is probably not bad but it’s hard to diagnose the problem long distance. If you have already investigated the air filter, spark plug, & fuel filter, then it could be the carb needs a good cleaning because one of the jets might be gummed up. Honda carburetors are designed to feed fuel using both high and low speed circuits. The problem might be the low speed jet and circuit. Also, check the crank case breather tube that goes from the valve cover to the intake elbow. If it is cracked it will cause the engine to run lean and might stumble under a load because it’s sucking in air.

My guess is a carb problem but a good small engine mechanic will be able to tell you or you could try this:


I will check all those tomorrow. Didn’t even think about that breather tube. Thanks for the insight. I think I’m going to drop the Briggs and Stratton off at the shop tomorrow and maybe the Honda on Friday after I check it out


I read a review a while back about my pressure washer (GX390, like yours), where someone was complaining that the engine would start surging like you are describing. Turned out the vent on the gas cap had become blocked, and it interfered with the fuel flow. You can test this by simply removing the gas cap while it is surging.

Here is the text of that review, from Home Depot’s website:

Unit started to SURGE, or hunt in it’s RPM. Thought it pointed to a governor problem . This problem
occurred after only the second tank of gas, very odd, to say the least, on a 3 hour old unit
One will be surprised what the problem was, ““The Gas Cap””
The gas cap is a two part pressed together unit that apparently has some type of filter and/or
sound damping inside it (weird) … Not sure what the purpose is to design a cap like this
In any event, the cap “prevented” air from entering the tank, meaning that the tank eventually
became enough negative pressure (vacuum) to hold gas back from gravity feeding the carb
Take the gas cap off, and the unit runs normally, Put the cap snuggly back on and the unit
will hunt / surge , and I’ll assume stop due to lack of, or no gas being fed to the carb
Fixed it by drilling two .200 dia holes on the bottom side , near the outside edge
without drilling through the top surface
I Now assess that what’s inside the cap is ground carbon, similar to a water filter
(activated carbon) (it’s in granular form) similar to sand, That prevents gas fumes from escaping
to apparently make the unit as safe as possible
Doing this vented the tank to atmosphere
Although, most of this granulated carbon has bounced around and vibrated itself down the center hole and into the tank itself. A very fine inside tank filter prevented this carbon sand from going into
the gas line post the tank
Will need to pull the tank off to rid the particles out , and clean the gas cap out of all remaining
particles,or buy a different type of cap that will fit
Conclusion : Bad gas cap design
Other wise this is a powerful, well built unit, and I don’t care if some say it’s heavy


thank you I will add that to the list of things to check. I’m also going to remove my downstream injector to test as well. I’m open to any and all suggestions. New carb has already been ordered.


I just wanted to say thank you to everyone that commented on This thread. I did my trouble shooting with the list y’all made and now it runs like it should. I was all set to upgrade to a 5.5, but I think I’ll hold off a little longer and squeeze every last dollar out of my machine. Note to self: always have a brand new spare carb in toolbox. :grin::+1:t3:


Haha, Yep!


Was there anything particular you did that fixed it? Or are you saying that the new carb fixed the situation?

Also, I think I’ve read that you can upgrade this engine to a gear-driven 5.5 fairly easily… basically just a bolt on if i remember correctly. I’ve thought about maybe doing that with it one day, if it’s that easy. Not sure what the cost would be though. Would be nice to bump up the GPM, and also be able to easily draw from a buffer tank.


Basically I replaced the fuel filter at the bottom of the gas tank, new carb, new spark plug and ran it for 15 mins. It powered my surface cleaner like new with no stutter. Then I changed the engine and pump oil. I’m thinking it was 98% carb related which is frustrating because the carb only had about 20 hours on it. So I’ll be keeping a spare on me for sure.

I’m thinking about the same gear driven pump upgrade as well, but I priced out a brand new gx390 and 5.5gpm@2500 and all the nozzles I would have to change out and it came out to $1,700 from KEC. I honestly don’t know what all would be involved with the swap. I would want to do a UDOR gear pump swap and be done with it. Then I could draw water from my tank.


Nick, the Udor gear driven 5.5 is a little under $1k, last I checked. And I’m sure it’d be much cheaper to ship than an entire machine.

Glad you got the Honda running again. Do you use a fuel treatment? How was it stored over the winter? Was gas drained 100% and run dry with choke? Ethanol is murder on these carbs. I’ve also been told by a couple different small engine experts recently to run premium (91 or higher) octane in any air cooled motor. If you can manage to find ethanol free gas, that’s obviously the ideal thing to use.


I have no idea about how it was stored previously. I always run 89 with ethanol shield in it. I think I’ll step it up to 91 from here on out. I need to find a station that has ethanol free gas. I will look into the UDOR gear pump. That seems to be the top of the line. I have no idea how my motor was treated before I got it, but when I checked the valve clearance everything looked super clean so I can hope it was treated well.


Hey do you have a link to the one you are talking about?


Octane rating has no effect on ethanol. Keep your tanks full as much as possible to avoid moisture. Thus is awesome stuff. I use it everyday in the machines and in my boats and small engines.


Hell, I thought octane had everything to do with it. :thinking: I know my ecoboost runs better with 89 but it is a boosted motor. I’ll try this stuff out. Like I said I’ve been using that ethanol shield, but obviously that’s not cutting it. @Infinity yea post a link I can’t figure out which UDOR you’re talking about.


An engine will only ignite fuel at the rate it is designed for. If it is designed to ignite 87 octane then that it all the air flow and gas mix will ignite. Ethanol actually is an octane booster. This explains it better than i can

Run 87 but add star tron or something with an enzyme to counter act the moisture build up. Why the gov thought water in fuel is a matter for another topic lol


My best bet would be on restricted water flow or the governor going bad


@alabama, I have the same unit and it started to have low pressure. Appears to run fine just low pressure which doesn’t allow me to use a surface cleaner. I think it’s the unloader.