Odd issue between Generator and Inverter/Charger

So this past week, Canadian Tire had a special on Xantrex HF1000 Inverter/Charger units -- selling them at half-price for $149, so we picked one up. These units are a smart 3 stage charger that have a 20Amp charging capacity along with a built in 1000W inverter. We need a good standby battery charger once in a while, so figured this unit would do the trick and can act as a redundant inverter should the need arise.

The way this unit works once it's hooked up is as follows: when shore power is connected (via the grid or generator), the unit has a "safety" where it will not switch to shore power until it has been connected until the 20 second mark is reached. It will then pass through AC power and activate the battery charger.

Ran into a weird problem with our generator, and I am sure someone here knows the answer to what's going on. We have an older Honda generator -- an EG1400. This unit belongs to my in-laws and is well in excess of 25 years old -- however, it starts like a pro. It has a 1400W AC output and an 8Amp DC output. When I connect it to the HF1000 however, it doesn't sense the AC input when it's connected to our battery bank. I checked everything -- all wiring was correct. Connected various power tools to the AC outlet on the generator and they worked. The inverter on the HF1000 worked, but it would not detect shore power and pass it through or activate the charger. On a whim, I connected a variable-speed drill to one of the AC outlets on the generator, while at the same time I watched the panel on the HF1000 -- ran the drill at about 1/2 speed and voila -- it detected shore power and after 20 seconds, began charging.

But I can't quite make sense of this. If I connect anything to the generator, power tools, lights etc.... they all switch on immediately -- so it doesn't appear that the generator has some sort of load sensing switch. Also mentioned, that with the drill I used, I had to keep it at the "perfect" speed -- initially started full speed, but had to back it off to just below half way, otherwise, although the HF1000 sensed power it would "hiccup" in that the 20 second counter would continually reset itself if I didn't hold just the right amount of power on the drill. Anything else I connected to the generator -- various types of lights or other tools - none would allow the HF1000 to see shore power (the variable speed drill I used just happened to be the first thing I tried -- lucky).

Can anyone explain or surmise what's happening here?

Spec for the HF1000 is here: http://www.xantrex.com/documents/Inverter-Chargers/Freedom-HF/DS20080521_Freedom_HF1000-1800.pdf