1987 50hp Pro LH Dead Overnight?

[crbeasley01]

I recently acquired a pretty low hour 87 Thundercraft Trihull with a 50hp Yamaha Pro LH on the back. I've had the boat out about five times and then took it to Lake Powell. The first day we ran for about 5 hours, no problems at all, as normal. We refilled once at the marina and continued burning the new fuel no problems either. After parking on the beach overnight, the next morning the engine was a pain to start which was odd but eventually got it going. After about 15 minutes of running and motoring out we stopped to launch a towable. When I idled down and put it in neutral it coughed and quit. It started back up again and we towed for about another 15 minutes. It was difficult to tell but seemed a little rough and down on power. We then switched riders and ran into major issues. The engine would only start back up when I gave it a lot of throttle. Idling it back down to shift into gear caused the engine to cough and cut out. A couple of tries I was able to get it into gear but it wouldn't run for more than about 30 seconds before quitting. Most attempts ended with a rough start with lots of gas and it coughing and cutting out as soon as I brought the throttle back. I tried switching tanks and manually pumping the primer bulb but this had no significant difference from what I could tell. We ended up getting a tow back to the beach. It is odd to me how one day it ran perfect and the next we ended up with issues. What changed overnight?

[TownsendsFJR1300]

Very possibly the issue may be your fuel pump diaphram leaking into the cylinder and causing flooding: https://www.boats.net/catalog/yamaha...lh/fuel-system (part #14) That part may be 32 years old

Also, check the under cowl fuel filter for water / debris

As your engine is carbureted: https://www.boats.net/catalog/yamaha...0lh/carburetor I would remove the drain bolts (Part #17) and catch what fuel comes out of the bowl just to rule bad fuel out...

[99yam40]

sounds like the cough could be a lean sneeze,
if that happens on #3 it could rupture the fuel pump diaphragm.

may have problems with the low speed jets(pilot/idle)