Re: Johnson
There's a world of difference between how an engine runs on the hose and how it runs in the water (as you've found out). When running on the hose there's zero backpressure on the exhaust, when in the water the engine has to overcome a fair amount of water pressure to push the exhaust out, which robs the engine of power.
First off check the basics. Take compression readings on all cylinders (if you don't have a compression gauge they're available at most any auto parts stores for under $20).
Check the spark on each cylinder with a variable gap spark tester set to about 7/16 inch (again, spark testers are available at most any auto parts store for around $15). The spark should be able to jump that gap with a good healthy spark when cranking the engine with the starter. Pull the plug leads off all the spark plugs when your doing this.
Look at the spark plugs, do they look the same? You can find many many examples online of what a spark plug should look like from a 2 stroke outboard.
Also, it's very very handy to have a factory service manual when doing anything on these engines (not the aftermarket manuals but the actual Johnson/Evinrude shop manual).
As for the black oil coming from the exhaust, that's not uncommon when an outboard is idled on the hose, that's unburnt 2 stroke oil.
