I've got a buddy that just aquired a 9 hp GD longtail. He's having problems when running it wide open. If he's right at the surface it runs great. If he tries to push it down deeper it dies right out. Is this normal or is there something wrong with it?

Governor on small motors sometimes needs tweaking - as you
load up it opens the carb buterfly more and more - verify that
it is working properly.... If it is - then carb might have some
trash in it or float level issues - when butterfly is open fully
it has to deliver more fuel and often out different venturi ports...
A bit of "dip" will open up all those tiny fuel passages - along
with a basic carb kit and gaskets - not a very hard job....

Not sure what brand of engine is running the GD but if it has a low oil cutoff switch built in that could be his problem.

Fill the oil to the high side and try it again. Sometimes that "dip" changes the oil level enough to trip the low oil cutoff.

We see this quite a bit with some of our smaller hp longtails, it is usually an overloading issue. Kinda like when you push your lawnmower over too much grass and it kills it. A simple fix for this is bend the entire cavitation plate down so that you have about a 0.5" clearance between it and the prop (assuming it has a new prop). Then adjust the trim tab accordingly to get back to the sweet spot. I would however rule out some of the easier solutions, like stated above... but if I had to guess I would say overloading on the engine.

Agree with the others, I would start by going through the entire fuel system. New gas in a clean tank. No air leaks, new inline filter, flush fuel lines, dissamble and clean carb, ..................Duckdon

If it is the 9hp Honda listen to usfgeology. As an alternative call Godevil they will probably tell him the same thing.

as stated before check the oil level to make sure it's not low,if the level is at its normal full level unplug the sensor and see if that helps.if is does replace the
sencor.had to replace my last yr.