Daily Archives: October 1, 2017

Fixing Surging Generator – BlackMax 414CC / 5700 Watt / Honda GX 390

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.