First, you are breaking the law. You must have an automatic or manual interlock that keeps the utility from being backfed. Just relying on a non-interlocked opening of the main breaker while using the generator does NOT fit the bill.

I suspect there's something very wrong with your generator. If you haven't overloaded it, there's something wrong in it's regulation circuitry.

Or a missing neutral connection.

Good call Brick....That would do it as well.

Thanks for the input. When your utility company tells you it will be 24 hours before service and its 10 degrees out-- legality seems a little less important. However, since it didn't work anyway its a moot point. Looks like a new generator, transfer switch and microwave are in my future.

Consider your self lucky, you only killed a microwave, not a utility worker.

If you haven't already done this work... An interlock is cheaper than a transfer switch if you just want to do what you had already done before, except the interlock will make sure the main is off before the generator circuit is turned on. The tricky part might be finding an interlock kit for your model of panel. Be sure to shop around, mine for a Square D panel was priced between $65 and $185 for the same brand.

BTW: there are some Chinese made generators I'm seeing now that do not have a neutral connected on the 240v outlet. That is, they are either 120v or 240v but not 120/240v.

Be careful with what you get.