I have a 1992 Yamaha 25hp 2 stroke I bought recently.
It did have carb issues when I purchased it as it has been sitting for 8 months and needed starting fluid to get it running.
I have the motor running well in neutral after thorough cleaning of the carbs.
I have replaced the connectors and bulb on the fuel line.
My problem I discovered the first time I ran the motor was when I put to motor in forward or reverse it bogs down and stalls out and the motor has a hollow sound to it as it is dying, the only way I can get it to run is to put it in gear and pull the magneto arm forward and open the motor to full throttle. It runs good if your about 1/3 throttle but lower than that it starts to bog down
The confusing thing is out of the water with muffs on the prop will go into gear and rev up with no problem. And in neutral there is no hesitation to the motor when you throttle up. The problem shows up when there is a load on the prop in the water.
When I got home and started working on the motor I discovered that in neutral the prop wound not spin freely which prompted me to change out the gear case oil which was low and I also checked the shift shaft as it was not freely moving.
Also I have adjusted the timing in accordance with the shop manual I have for the motor as I found that someone before me messed with the timing arm and cut it an inch shorter.
With the timing fixed and the carbs completely disassembled and cleaned the motor sounds and runs awesome in neutral.
After reassembling the lower unit and changing the oil it is now spinning freely and turning properly when in gear and I will be testing in the water this evening.
Some of the solutions I have been offered were bad head gasket, bad anti siphon valve, or an air leak in the fuel line. What would you test next?
When i got home and started working on the motor i discovered that in nutral the prop wound not spin freely which prompted me to change out the gearcase oil which was low and i also checked the shift shaft as it was not freely moving.
After reasmembling the lower unit and changing the oil its is now spinning freely and turning properly and i will be testing in the water this evening.
Sounds like you are running too lean to carry a load,
You can check fuel delivery system with a pressure/vacuum gauge to see if there is a restriction or a bad pump.
But i believe you need to go through the carbs again and make sure you clean all passages and jets not just some of them and then do the sync and link procedure in the manual after reinstalling if it has multiple carbs. or set carb to manual spec if it is a single