Hello, and welcome to Justanswer. My name is XXXXX XXXXX I will be helping you today. If you need clarification on something please post back here.
There are about 15 diffent things that can cause lack of full throttle RPMS, instead of giving you all 15, we need to do some basic checks to get us pointed the right direction.
That being said, we need to do some preliminary troubleshooting to figure out which system is causing the condition. When testing you should do it in the order I am writing it here, jumping around can have misleading conclusions.
What we need to is engine compression and spark, and you will need these tools to accomplish this.
1. Compression. Your results should be 100psi or better, all cylinders should be within 10-15% of eachother. No cylinder can be below 100psi
2. Spark. These are the testers we use in the industry.
But you can also get cheap spark checkers at most good autoparts stores.
You need to hook the spark tool up and set the gap to 7/16ths of an inch. The spark should easily jump that gap, be hot and blue in color, and be consistant with engine rpms. Weak looking yellow spark will not cut it. Items such as cap, rotor, points and plugs are once a year items, so if you have weak spark, you should start with replacing those items, spark plug wires are good for 3 to 5 years.
Also, we should check the quality of the fuel itself. Either pull a sample of it and put it in a clear jar overnight. In the morning, look for any kind of dirt/debris/water in the fuel, or any kind of seperation line.
Or, Get a small outboard tank (like a 3 gallon) and fill it up with known new fresh gas and try it.
Lastly, if compression and spark check out, we need to look at the fuel system. First we look at the boat side fuel system and test it. Here is how;
Boat side fuel system test - click here
If the problem does not appear to be in the boat side after testing, then the problem is going to be in the carbs. Carb jobs are very common on marine engines. They make up 80+% of all my work. It takes a piece of dirt the size of a piece of sand in there to gum up the works.
You will also want to have a quick look at the prop to make sure it is not all dinged up, as that will effect rpms.
After you perform these tests we will know what direction we need to go in.
The last bit of advice I have is that if you wish to work on this motor yourself, you really should get a copy of the factory service manual. That is the most important tool you will need.
Post back here with questions or test results.
Good luck my friend.