All engines need 3 things to run properly. Compression, spark, and fuel. The problem with a lack of power situation is you have to do some troubleshooting to figure out which of the 3 the engine is missing, giving you that lack of power, as all 3 can do it. Just 1 or any combination of the 3. There is no rule that says there is only 1 thing wrong with the engine that you have to find.
I can also tell you from experience, about 75% of the time it is fuel related somehow. Wether it is bad gas, water in the gas, weak fuel pump, a clogged carb ect... 3/4's of the time it has something to do with fuel. However, you can't jump to that conclusion right off the bat, because if you do, things get missed.
If you brought this to me here is how I would go about troubleshooting it.
I would do some preliminary troubleshooting to figure out which system is causing the condition. And where I need to start with is engine compression and spark tests, and you will need these tools to accomplish this.
1. Compression test - Your results need to be 100 psi or better as a bare minimum. A like new engine would be 160+ psi across the board. Used but healthy engines are typically in the 120 to 140 psi range. And they all need to be reasonably close. If any cylinder comes in below that 100 psi magic number, or you have a huge difference between 2 cylinders. Then the engine is worn out, or has mechanical problems, or there is a bad head gasket. And you can stop your troubleshooting here, as there is nothing you can do to an engine with low compression to make it run better. And the compression issue has to be addressed.
2. Spark test. You have to check for 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. Weak yellow/orange looking spark can mean the cap and rotor are burnt out or the plug wires are bad. But it can also mean a weak ignition coil. What I do next is pop the distributor cap off and see if it looks pitted, corroded or burnt in there. And if it does, the engine gets a full tune up.
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. As contaminated or old fuel simply does not burn as it is supposed to. And people get bad gas all of the time.
Or, Get a small outboard tank (like a 3 to 5 gallon) complete with fuel line and primer bulb, fill it up with known new fresh gas and try it. This is the easy way to do it as it requires minimal tools. If the engine immediately runs better on the portable tank. You have bad gas.
If nlo change, pump the primer bulb over and over, if the engine picks up, the fuel pump is weak.
If you have gotten this far, and there is no fix yet. Then its safe at this point to pull the carb and open it up and see what it looks like in there. And then decide if you want to try the rebuild route, or just replace it. Not all carbs are rebuildable. They are made out of cheap cast pot metal, and over time will leakdown internally. What we say as a rule of thumb is any carb over 10 years old has a 50/50 chance of being rebuilt successfully.
That is the quick, easy and most efficient way to troubleshoot this. I don't know how much of this you can do on your own. And obviously I am not there to do this work for you, but I work for tips so I do want to make sure you are happy with me and the answers I give you here before doing the rating. If you have a further specific question on this, by all means feel free to fire away. If not, please let me know if the ratings box gives you any troubles.
Thanks in advance, and good luck!