2000 Yamaha F100TLRY Carburetor/Accelerator Pump circuit

[philly304]

This is my first time posting so thanks for any help anyone can provide. I’ve had this motor for about 5 years. I’ve had suspected carb issues for the entire time I’ve owned it. At idle, motor runs great and at full throttle the motor runs like a top. When you punch the throttle and quickly accelerate, the motor is being starved of fuel and will stumble or stall. If you baby it up to full throttle it’s fine. About 3 years ago, I rebuilt the carburetors with no success in correcting the problem. After this, admitting defeat, I took it to a reputable shop who went through the carburetors again with no success. The shop went through a series of troubleshooting steps replacing the TPS and eventually the CDI. Eventually the shop gave up, charging me only for parts. I continued living with the issue until recently I decided to try and fix it again. I pulled the carbs and went over them in more detail… I found the check valve on the hose downstream of the external accelerator pump (that serves all 4 carbs) clogged. I also began investigating this flow path of the accelerator pump and found 2 connection points in the bottom carburetor but only 1 in the other 3. Upon further investigation I was able to remove the pressed fitting where this line ties into the bottom carb… I found a small ball and spring, almost like a ball check valve assembly. This is not on the parts diagram and is internal to the bottom carburetor assembly best I can tell. Cleaned everything and put back together…. Boat ran like it just came off the factory floor. 2 days later…. Back on the water with the old issue!! I hypothesized that this accelerator pump line with in line check valve and ball check in the bottom carb was getting clogged easily or getting gas back flow into when it shouldn’t? I blew the line out with a little compressed air without removing carberators and it ran great again. This series of events has repeated several time over the last few weeks. My question is does anyone out there have more info on how this accelerator pump, check valve(s) and tubing are supposed to function? Should gas be getting in the tubing? Carbs have been throughly cleaned, synced with vacuum sync tool, gas source ruled out. Please don’t waste your time telling me to check my fuel filter! I know this a very specific issue on a motor that Yamaha only made a for 3-4 years. Anyone out there that has any knowledge of this specific setup?