Sounds like the idle stop screw needs to be turned in a bit.
Unless you messed something up or have air leaks or misadjusted carb jet screws.
A better description of what it does would be helpful too. Does it run fine at high speed?

Okay Bill- I reread my post; not so clear...sorry.
My question really is related to how low the idle rpm should be. The hi speed is good; smooth, etc. The low speed... well, I can't seem to make it as low as I have on other tecumseh horizontal engines. When I back out the screw for the idle speed where I have on other machines (by ear) I find the engine wants to quit. I'd really like to know what microfiche 30 says about the HM80 series. I was curious if my observation about the engine needing to run faster was related to the fact that it is constantly connected to the chipper disk/impeller. In other words; it's never really a "no-load."
It runs well at around 3500 rpm; and it seems to run at a low of what must be 1500 to 2000 rpm.(My tach is AWOL this week.) I would have told you that this is about double what I have set oh, say my 6hp tiller engine (granted it's not the hm-80.)

So; does that help?
-br

I don't know what the spec is, but a little on the "fast" side could make sense.
Since you have so much weight to accelerate, a slow idle speed would probably just kill the engine if the throttle is suddenly opened wide. Too much loss of vacuum might not be the correct description, but I think you may see where I'm coming from.
Also, with a faster idle speed, you don't get the rod "hammering" as bad, when accelerating.

I used to have an 8HP Tecumseh on an MTD chipper.
I'd try to start it cold at a slow idle because of the hammering effect. It seemed so abusive to start it faster.