Hi, thanks for the response.
Our state does not allow ethanol-free gas so yes, I am using 10% ethanol gas from Shell which gives me the best car mileage. I tried their 87 and 89 octanes which work fine in my two lawnmowers and snowblower.
The snowblower I have has the same carb and engine. (gx200) and the last time I mucked with that carb on that snowblower I had to chip the crud out of the bowl with a screwdriver but it never warbled like this.
This is a new carb, new fuel line, gas tank cleaned with acetone, new fuel filters, new gaskets, new insulator, new head, new valves, new valve sprins, seals, retainers, and a new spark plug and fresh gas.
I don't think this is a fuel quality issue. It appears to be more of a design flaw. On a regular mechanical governed engine, the governor continuously adjusts the throttle to maintain RPM. The electronic throttle seems to use the same principle but the way the software appears to be working is to throw the throttle on and back it off rapidly. Its not a smooth operation. There is a tiny damper spring that seems to want to compensate for this but it isn't enough and it seems to accentuate any issue. The only way to fix it would be to have a perfectly tuned engine which is almost impossible with a non adjustable carb.
Here is a video of a fairly new generator doing it. The guy is blaming cold weather but I have seen these up in Alaska with a foot of snow on them run better than this. When I shot my video it was 40 degrees.
And the guy did an earlier review of it in the summer and it was doing the same thing!
And here is a brand new one, with Eco off.. whirr.. Whirrr Whirrr.. very annoying if it can be fixed.