What Ferm says is a good way to do a general over all cleaning but the kind of decarb you need to do is very easy.
First find out if your VRO has the injector tube for doing it. Eveinrudes and Johnsons after a certain year have a tube with a little screw on piece on the end that screws down onto a can of de-carb you buy in the store. Find out if you have this feature on the side of your carbs, if you do follow the can directions for set up
Second if you have the tube use it and set it up, if not you use the spray on kind of can instead. First run your engine on the hose for a couple of minutes to warm it up. Then tilt your engine up as far as you can go, pull off your caps for the plugs and remove them. spray a little decarb in each cylinder to saturate it. Put the plugs back in but do not put the caps on yet. Turn the engine over a few times just to get everything in there coated.
Walk away for ten to 30 minutes.
When you come back lower the engine, put the caps back on the plugs, hook up your water hose.
Pull off your air filter box, usually held on with rubber type stretchies or maybe a few wing nuts whatever used your engine.
You should be able to look right into the carbs.
Start your engine, it will idle very rough and may give you trouble starting at first. Let that sucker choke out or idle poor but get it started giving it a little choke if needed and let it run and you will look like a WhiteSnake video minus Tawney Kitaen for about 4-5 minutes.
After it clears rev the engine a little, remember you are on a hose so no long high rpm blasts.
When it is running clear you take your spray can, either hooked up to the outlet discussed earlier or if not you will be useing the little squirt straw that comes with it.
Now if you are muy taliented you can use the throttle lever alongside the engine (throttle linkage) or have someone help you at the binnacle control and what you will do is build up rpms to about 2G's and you will see the carbs open a little. Spray that decarb directly into the carbs, a good shot into each one, it will try to stall, as it sputters and loses rpms back off with the spray and let it catch up with itself. Keep an eye for Tawney Kitaen at this point

You keep doing it to you finish off that can, sometimes you won't be able to do all the carbs at the same time before it starts stalling, in that case do one at a time or whatever you need to do.
After you finish killing every bug in the county, and the engine is running with clear exhaust and normal idle shut her down, let it cool down, pull the plugs and install new ones if you got them or give the ones you got a good cleaning and make sure they are gapped right if they are not stud type.
If you do this most of the crap that is in that engine will be gone. Worst case scenario is you go thru the process twice. But if that does not correct your compression ratios enough then it is time for an overhaul.
On a side note, I would check my own compression before I dumped that engine. Mechanics, even good honest ones make mistakes, and a large percentage of them want to sell you something, build you something or charge you big buck repairs and will exagerate the situation.