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Hey, folks!
I'm working on a BlackMax 7000 Generator with a GX 390 Honda that was recently converted to propane.
It runs fine on LPG, but when switched to run on gasoline, it surges and backfires a little when under load. The load is approximately 4500 watts, so well within the capacity of the power plant.
Has anyone else run into a similar issue? I was a bit concerned that maybe the problem is that the donut that supplies the LPG is restricting the air when it is running on gas, as it is a perfectly round opening, and the carburetor has different shapes in the opening to allow for ports to get air.


Blackmax BM907015
Honda GX 390

Chances are it is just a dirty carb but to test it just remove the donut and see.

Surging usually means lean, and a restriction at the intake would richen the fuel mix not make it run lean. Maybe the jet that the gas goes through is partially clogged causing not enough gas.

I did a good carburetor cleaning, even though everything in there looked good. The jet was clear, all the tiny holes in the nozzle were good, and no dirt in the float bowl.
I did, however, find the youtube video on the installation of this particular conversion kit. The video was put out by Century, who made this exact kit, and by nice coincidence, they were using the exact generator I'm working on as an example. https://youtu.be/Ip7ckWAZ3xQ It looks like the folks who set up this one put a spacer in the wrong place.
Thank you for your input, and I'll see here in a day or so when the customer comes back for his generator and we run the load he uses, if I got to the bottom of this one.

Either way, I would have expected the donut to either be bigger, or shaped more like the mouth of the carb.

Ok, so the guy came, and we plugged everything in. There was some improvement, but still some hunting. I checked the voltage, it was set a bit high, like 134 volts, so I brought it down to 120, and that helped a little.

The funny thing is, it runs fine at no load, and runs smooth at full load. Only at partial load does the engine seem to be happy only for a few seconds, then the rpms drop just slightly, enough to trigger the governor, which kicks it back up a little violently, causing the rpms to bounce a bit then steady out, then the cycle repeats, gradually dropping ever so slightly until the governor gets pissed off again. So like, ever 8 - 10 seconds.

My verdict: The engine doesn't care much for the propane conversion. There should be a better adapter plate or donut that is more ergonomic for the carb. The whole thing just felt kinda awkward, like putting a round gasket on a square hole and calling it good.