I have a spray rig that runs a small 5hp honda with a comet mp30 diaphragm pump. This afternoon while spraying we got into some bad gas and it started running rough and spitting black sooty smoke. Eventually the engine started to lose rpms and power. When the pump was taken out of gear the rpm would come back up but would fall back when back in gear. At times it sounded like a hit and miss honda! Finally she just quit running. I am sure the plug is trash and some fresh gas would help but what kinda of damage am I looking at?

 

Shouldn't be any mechanical damage.
Drain the tank and the bowl of the carburetor, fill with fresh gasoline and put in a new plug and you should be good to go.
If it doesn't run well, I'd suspect a pre-existing problem.
Your exhaust valve may be "carboned-up", preventing it from closing properly and thus causing compression loss. Carbon buildup is fairly common on constant speed engines as the expansion/contraction of the heating up/cooling down cycle that occurs on engines run at varying speeds helps to break carbon deposits loose. The additional deposits from poorly burning fuel may have been the straw that broke the camel's back here and the exhaust valve and seat may need to be cleaned up.

 

Did just that this afternoon. New gas and plug and it fired right up. Do you think I should run a little seafoam in just for good measure?

 

Sorry, can't comment on SeaFoam. I have never used it. I have read good things by people that have, but have no personal experience with the stuff. I am not aware of its abilities to loosen/burn off carbon deposits.
Don't see how it could hurt though.......

 

Bad gas does not cause problems besides it won,t run well or not at all.Any other problems you have now where their before this happened.