In misc.rural Todd wrote:

When I engage the deck the engine slows down a lot. If it were not at full
throttle it would stall. When the deck is disengaged mower is strong as a
mule, and runs very well. It is a 13hp 2-cylinder water-cooled engine.

30 year old John Deere 140, 14 horse 1-cyl Kohler engine, 48" deck.
Slows down momentarily when I engage the electric clutch for the mower
deck, but goes back up immediately.

Does your engine have a governor on it? Is it working properly? I
would think that it should adjust the throttle position at the carb to
compensate for the added load (the deck). Look for something
between what you control (your throttle control) and the carb itself;
if there's something between and it's just directly linked from your
control to the throttle itself, that should be the governor.

Does the throttle plate linkage move (open) when you kick in the deck?

Dave Hinz