Machine misfiring on low pressure

I have a 4/4 with the Honda GX390 and started to hear some misfires whenever downstreaming or low pressure rinsing.

It started off 1 every 5 or 10 seconds. But has grown to kinda rapidly… like 3 a second now.

Only when using low pressure…

Does anyone have any idea what could cause this?

Thank you

Check your unloader.

May need to adjust valve lash. Those 390’s are notorious for needing adjustment. Could also be bad gas, clogged fuel filter, dirty carb, or fouled plug.

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All of the above, but try a fresh spark plug also with valve adjustment. Plug could be cracked.

Alright cool, so basic maintenance stuff.
Appreciate the advice.

Yeah I probably should do routine maintenance on it, I haven’t looked over everything in a couple months… been busy late season!

I was thinking it may be a unique problem because it runs fine when under high pressure load, like surface cleaning. But when I put on my low pressure nozzle and it’s under maybe half load, its gets to missin.

Please let us know what the fix was so future readers can do the same with their machine.


I haven’t had much time to mess with anything yet, but I did find that moving my throttle back about 1/4 inch, quiets down the misfiring alot.

I’m thinking it may just need a throttle adjustment.

I dont have a working tach anymore, so I dont know exact rpms. But I think it may be over revving slightly…

Hey :slight_smile:

Are we talking misfire as in pop! Like a backfire/unspent fuel?

Or uneven idling/low speed?

I’ve had lots of issues with the GX390 over the years - if you could post a video of the sound, that would be awesome!

Have you checked the throttle return spring hasn’t snapped? (The really thin long one under the governor rod)

I have not checked that spring… can you describe what the governor looks like?

The misfires are mostly uneven idling sounding, not a POP like a backfire.


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I’ve had a few snap, and they cause low idle issues when they do. It’s the tiny spring in the pic, definitely worth a check…

Is thats the case, definitely reset valves clearance.

Ok the spring is still there… I checked valve clearance and they were only .01 off. Did my best to get them perfect… I adjusted the throttle stop screw, that sets the max throttle, to stop the throttle a little sooner. That got it sounding alot better, but not like it use to.
Checked oil, checked air filter, and sediment cup. All Is well…

Theres 1 more throttle adjustment screw that faces the exhaust side that I might mess with a little… i think it adjusts the actuation of the throttle under load If I’m not mistaken…

Belts are also In decent condition.

So far it seems to be a engine speed problem based on throttle.
My tach is broken sadly… but I know it likes to run around 3600rpm… any more or less and its not as smooth.

I will update more once I try adjusting the throttle screw and check the spark plug(its only 3 months old though, so probably fine)

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Nice work :+1:t2:

Often it can be a simple thing, other times a rebuild :tired_face:

(That throttle screw is factory set and won’t be the problem)

…another thing springs to mind (pardon the pun!) I had a problem with the fuel tank/small filter inside it.

My GX390 would run rough at idle then stall eventually. That was obviously a fuel restriction.

I basically took the tank off and turned it upside down and flushed it out. That went away immediately.

How old is the GX?

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Its almost a year old and ALOT of hours. I full time pressure wash and, and iv been running though jobs recently.

Ok I looked with a flash light into the tank and wow… it has sediment all over the bottom… possibly problem found? Getting late but first thing tomorrow I’m cleaning that tank out…

Any tips or tricks? And I had no idea there was a fuel filter other than the sediment cup… do I replace that or just clean it?

Remove the four nuts/bolts. Pull off the fuel hose at the carb end and plug it or just your finger.

Take the tank off, swish it around with the filler cap off and the turn it upside down while sloshing it :slight_smile:

(Obviously not with a full tank)

Pour some fresh fuel in, take your finger off the fuel line and let run a bit - turn it upside down and swish it out again.

It’s probably not the “way” to do it, but I was on a job at the time.

Air compressor might help, but it needs swishing somehow I think :sunglasses:

Don’t worry about the filter if the fuel is running out nicely.

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The carb bowl will probably have a ton of sediment as well.

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Alright, took off the tank… it is completely full and I’m draining Into a gas can…

The flow seems slow for a full tank. I have the gas cap off as well so theres no vacuum.

Is this slow?
Never done this before…


Is the carb bowl different from the sediment cup, and if so how, what does it look like?

Sorry I have no experience with small engines…

I rebuilt a fuel injection pushrod engine in my firebird so I know I can tackle this!

Just no idea about carbs…

It should be pouring out - that’s not right.

The carb bowl/sediment cup is the same thing and it’s after this problem, so I wouldn’t worry about it right now.

Pour some out, then blow backwards through that fuel line. See if you can feel/hear anything.

You can always take the filter off and check with a deep socket, but be careful as they are easy to strip or misalign and then they leak. I wouldn’t bother unless replacing.

It’s a hard nylon mesh that can’t get internally blocked (like foam etc) just externally.

Flushing/blowing should work.

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Alright I checked the sediment bowl and it was totally clean… so I assumed the filter was clogged… poured some lucas carb injector cleaner and gas into the tankn, screwed the lid on and shook(the mix of the 2 creates pressure) the tank hard… I had the tube plugged and it blew out SHOT OUT EVERYWHERE … lol… now covered in gas I feel like a real mechanic…

I tried blowing through it before that and It was tough but I did hear air and got some through…

After the cleaning I can now blow through a little better.

I’m gonna try inspecting the filter, carefully.

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It’s odd that it built pressure as it’s a vented cap to alleviate pressure build up and allow the fuel to gravity feed too.

Seems like there’s definitely one problem here, if not the main one - that flow should be much, much better.