"It could be a fuel starvation
"It could be a fuel starvation issue. Causes can be a faulty quick disconnect fuel line fitting or a faulty bulb pump causing air to be sucked into the fuel line ahead of the engine's fuel pump. It could be a piece of junk stuck in the inlet of the float valve in the carb, blocking normal flow. Here's a story of mine I had happen this last weekend. The procedures below are a little risky, so read on and heed the cautions I present below. You are fooling with gasoline so always have a fire extinguisher ready to discharge.
This sounds almost like an intermittent lean condition resulting from an intermittenly low carburetor bowl level, possibly as a result of the fuel going into the bowl not being able to keep up with the demand of the engine. I had this happen the other day with a 75 hp 3 cylinder to one of the three carbs. Looks like the 125 force is a single carb engine. When the engine starves for fuel during an upward throttle transient, it will emit a louder than normal noise air gasping noise from the carb throat when you open the throttle butterfly and the engine hesitates to accelerate. The engine will either eventually jump up to higher power level and rpm, or it will fall on it's tail (decelerate) with that same open throttle air gasping noise as it winds down.
When you back off the throttle, the fuel level in the carburetor bowl is able to recover to a more normal level, and the carb is able to supply fuel through the jets. Too low a fuel bowl level will mean less fuel into the throat, and too high a fuel level will flood with an overly rich mixture of fuel flooding into the throat.
In my case the other day, the second carb had a piece of junk lodged between the fuel inlet fitting and the float shutoff valve so that it couldn't fill the carburetor bowl fast enough at high demand. Since I had 3 carbs on the engine, figuring out which one was interesting. I used a can of ether (starting fluid) and used a very short burst of less than 1/4 second when the engine starved at high power. I could tell it was running on two cylinders. When I gave the very short ether burst to the 2nd carb, the engine recovered momentarily. Starting fluid can ruin an engine, so you need to be very cautious with it and keep your face away from the carburetor throat. Use it with respect and use it very sparingly for troubleshooting. Once I figured out which carb was the problem carb, I ran the bulb pump into the engine with two carburetor bowls removed with the floats sagging, and a good amount of fuel came from the top carb float valve and almost none came out of the 2nd carb float valve. I blew air through the float valve backwards and a piece of something came out the inlet port. Then the bulb test flowed a normal amount of gas out the valve and I put it all back together and all was fine after that. This procedure should be done with the battery disconnected and removed from the boat! Also, a fire extinguisher needs to be ready to dishcarge in the event of the unthinkable.
In your case, you might not be able to determine a problem with either the starting fluid method mentioned above or by pumping fuel to the carb with the bowl removed because you have only one carb and no other carburetor to compare it to. If you are mechanically inclined and can disassemble a carburetor, you might try removing the float and the float valve and blowing compressed air through the valve backwards toward the fuel inlet.
Other causes can be the quick disconnect fuel tank fuel line fitting, or a faulty bulb pump, in either case cause air to be sucked into the fuel line. You might try pumping the bulb while the engine is exhibiting the unwanted behavior and see if you can affect the symptom.
If you suspect or want to monitor the ignition system, you can actually listen to the spark signature by tuning an AM radio to an unused part of the dial with no stations present, and the ignition pulsing can be heard pretty well. A nice steady pulse beat that follows engine rpm is what you want to hear. If the beat disappears for a fraction of a second when the engine cuts out, maybe something is going on in the ignition system. Get familiar with what it sounds like during normal operation, and maybe use some headphones. Just sometimes, if the spark misbehaves, you can actually hear it happen if you become familiar with how the ignition noise on the radio is supposed to sound like under normal conditions. It won't work on the FM radio band. AM band only.
Good luck, and don't squirt any more than 1/4 of a second of ether into that engine at any time. Your cylinder walls and piston rings won't like it at all. And make sure you have that fire extinguisher when playing with gasoline.
Bob"
