Ok motor guru's...

My 90 Merc (96 model) was running great, then dropped a thousand rpms, and really went to junk. Thought it was a gonner, just blew it up.

Symptoms: it's puking fuel out of the exhaust vents, no leaks detected under the cowling. No fouled plugs, it actually started and ran fine after a rest, then acted up again shortly (5 minutes) after starting.

Where do I start?

Pat...I would be eyeballin either the igntion or fuel system. If it is a CDM ignition they can do some crazy things and the fuel I would suspect trash in one of the carbs not letting seat to shut fuel off....OPTI

First, I would run a compression test mainly because the results are like a 'quickie' on telling you about the general health of your motor. If you don't have a spark tester to test spark, I would do a drop cylinder test. With the engine running and with a pair ofinsulated pliers, pull one ignition wire at a time. When you pull one that doesn't effect the rpms, you've found the offending cylinder. Install a new spark plug in the offending cylinder. After that swap a coil lead from a good cylinder to the bad one and the coil lead from the bad to good. It will be out of time, but all you want to find out is if the problem moves with the wire. If the problem follows the wire, you know the coil is okay. If the problem doesn't follow the wire, you have a bad coil (or perhaps a bad ignition wire).

If the problem follows the wire, you will then need to check the stator, trigger with a multi-meter. If you intend to go to this level of troubleshooting, I suggest you purchase the appropriate shop manual for your motor for the proper way to make the tests and the resistance values. If those test out okay, then by matter of elimination, the problem more than likely is the switch box.

I just went through this with my 150 Merc. The tests aren't complicated, just to much for me to type.

BTW, when you do the spark test, after finding the first one, continue with the rest to make sure you don't have more than one problem cylinder.

If it is also a fuel problem, like Opti says, I haven't had to troubleshoot yet so I can't help there.

It's most certainly in fuel delivery. Opti is probably correct, it's likely a stuck float, because the fuel puking is after it's shut down.