Mercury didn't make a 75hp in 1972. What's the serial number of your outboard? Is it a four-cylinder?
Have you checked the prop, it may have been damaged, causing vibration. Check propeller blades for damage and turn the prop in Neutral with the ign OFF to check for a bent propshaft or distorted prop hub.
If you don't find any issues in the drivetrain, the next thing to check for is spark in all cylinders. Easiest way to do that is with an inductive timing light which you can just clip over a spark plug wire, or with a "firing indicator" commonly found at most auto parts stores.
http://www.walmart.com/ip/Calterm-E66331-Tru-spark-Ignition-Firing-Indicator/41281772
If you don't have spark, you'll have to troubleshoot that problem.
Here's some info on that:
http://issuu.com/cdielectronics/docs/troubleshootingguide?mode=embed&layout=http: %2F%2Fskin.issuu.com%2Fv%2Fcolor%2Flayout.xml& backgroundColor=000000&showFlipBtn=true
To properly troubleshoot, you'll need an analog or digital VOM (on sale most days at Sears for less than $20) and a DVA adapter, which allows you to measure the high-voltages generated within the ignition system.
http://www.usatoolwarehouse.com/usatoolwarehouse/ESI-640.html
If all plugs are firing, pull the spark plugs for inspection. They should all look pretty much the same, perhaps with a few deposits or normal coloration; any plug that is sparkling clean probably is seeing water in the cylinder. Water drops on the plugs are a sure sign of an issue.
One more test you can do is a "drop test" in which you pull a spark plug wire off the plug and see if the cylinder in question causes a drop in rpm with the wire pulled. If not, there's something going on there, even if you found that you had spark. In that case, suspect fuel or other mechanical issues.
NOTE: you're dealing with very high voltages when you do this; you can get badly shocked unless you're using the proper equipment. I'd recommend using a set of insulated spark plug boot pliers such as this inexpensive set on eBay:
http://www.ebay.com/itm/190717757918
Use a thick leather glove as well as the pliers, for added protection when pulling wires.
Another check you can do with the engine running is to pour a small amount of 50:1 pre-mix fuel into a spray bottle. Spray into the carb throat and see if the engine speeds up; if so, the fuel system isn't giving the engine enough fuel, for whatever reason. If the engine slows, that carb is likely OK. If it makes no difference, the cylinder(s) being fed by that carb are not firing and bear investigation.
All of the above fall within normal troubleshooting techniques when you have a mytserious running problem. That being said, many times a bad stator on an ADI-fired engine is cause for failure-to-rev. But many other things can cause that problem, so you just have to do a systematic check to narrow down the possibilities.
HTH & G'luck..............ed
