Help me diagnose my pressure washer issue

Machine Repairs
1

Here is a link to YouTube of what’s happening. Pressure washer surging. Help me diagnose. - YouTube I bought this unit used, a Karcher 4000psi 3.6 gpm pressure washer with Honda GX390 motor and the motor ran great. I thought there was an issue with the pump or unloader valve. Once I got it home I realized it was running with full choke. It was missing the low speed jet and had a plug rigged in the low speed hole. I replaced the carb with an oem Honda and replaced the two gaskets and spacer plate. The motor hunts a little unless I apply a little choke. I sprayed starter spray the best I could around the head gasket and no change. I didn’t see any noticeable cracks on the intake manifold either. Also running brand new gas.

The pressure keeps cutting in and out when you turn the pressure up. I pulled the unloader valve out and didn’t see anything obvious. The o rings were good and the spring was still stiff. My supply is about 7.5gpm, 5 gallon bucket method. I planned on initially replacing the pump, but I don’t want to drop $350 on a pump if it’s not the problem.

Also if you turn the throttle down you can turn the pressure up higher but then it still cuts in and out. For initial testing I did not have the air filter on, but it runs the same regardless.

Thanks everybody!

2

Had a similar issue, it ended up destroying my $800 pump. The unloader could be bad, if that’s not the issue then there could be air getting into the system or clogged filters in the pump. Worst case scenario is bad pump. Check these issues first.

3

I’ve seen gx390 behave that way with a faulty oil level switch. Try running it with the little silver box by your on off switch unplugged.

4

Unfortunately that made no difference.

5

I may just tear down the pump to look for something obvious. The rebuild kit is over $200 which is stupid. And the unloader valve is over $45. I would rather spend a little extra and get a triplex pump.

6

Ouch sorry to hear that it was worth a shot.

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I have the same engine and Karcher pump. As you did, I concluded that repairing that pump makes no sense (and I think that the more you look at repairing that pump the less sense it will make). I replaced that pump with a triplex (Cat 66DX40G1) and am quite happy with the change. You can get a good 4gpm/4,000psi-rated pump for a lot less than that Cat and for not much more than the cost of the repair kits for the Karcher pump.

I recommend running the engine after removing the pump from it. That way you can determine whether you have any problems with the engine. Edit: That’s not quite right about running the engine with the pump removed. If the engine surging is from a problem with your governor, that might not show up when the engine is run without a load. I’m out of my depth on diagnosing many engine problems, so maybe someone with more talent in this department will chime in.

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8

Fuel problem. Have you flushed the tank out and checked the fuel line/filter?

9

Drained the tank, cleaned out all the dirt and debris. New gas.

I did not change the fuel filter, but gas pours out pretty quickly when I disconnect the line.

10

Fair enough. Is there still a spring under the throttle arm? Can’t see in the video…

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3 things, either the carb has gotten some crud in it,.or wrong gasket. Or your water supply is inadequate.

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12

Yeah, I was thinking about the water supply too as it’s direct drive and won’t suck.

However, the surging says fuel to me still…

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The carb is new, no crud. Gaskets are new and correct. Water supply is about twice the pump rating. Also this does the cutting in and out at lower throttle. It does not cut out without a tip. I checked and the tip is slightly undersized. It’s a 3.0 I believe I need a 3.5 It cuts out well before the pressure is even all the way up.

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There is a spring to the governor mechanism and a little spring in the throttle linkage that takes up the slop.

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Is carb absolute correct unit? I second the testing it without pump. Seems someone else had kinda similar issue with a bad pump valve being the culprit.

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Is the engine surging or is it the pump or water surging?

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I used the part lookup tool from jacks small engine and cross referenced Honda’s site ordered it from jacks. It looks identical to the previous carb.

It seems to run fine when not under load or with no tip on the gun. What would removing the pump do?

The pump transmission get super hot in no time by the way. Yes it has oil.

This is frustrating.

18

Well I think the engine is slightly surging a little choke helps it but under load it feels like the pump is going in and out of pressure.

19

Did you set the unloader?

You mentioned the nozzle your using is too small? How small is it? I would order the correct one to set your unloader. I just wonder if too small of nozzle could be causing it? If

20

Just got home with bigger tips. 4.0 which are slightly oversized. And boom it’s working now. I need a pressure gauge to set the unloader and I’m ready to rock and roll. Thanks guys for all the responses.

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