Hello. I just bought a 2009 15hp Mercury 4 stroke and it ran rough on the test drive. (11.5' dinghy) It wouldn't idle with the choke in, but started on the 3rd pull. I took it out, about 5 minutes no wake speed, then WOT for another 5+ minutes. It planed fine and went pretty fast. I could push the choke in if I was over 1/2 throttle. I got the price down another $300 and told him I'd deal with it. He seemed to think that it was caused from storing the engine for 3 days on the side that it says not to store it on. Some oil leaked out. I changed the oil and took a look a the spark plugs. they were dark tipped and black on the end of the threaded part. I bought new ones, but they were the wrong type so I cleaned the old ones. It had the same symptoms, on the way out (choke out). I nailed the gas and it started to bog bad, so I put the choke in and it took off with no issues, going faster than on the test drive. I thought it was cured, but then it got much worse. It wouldn't run at all with the choke in, at any throttle, and with the choke out, it would no longer plane. The bulb on the fuel line was still hard, and pumping it did nothing, but when I would goose it to hit the accelerator pump, it would rev up. It would also rev up and run way better if I pumped the prime bulb continuously. I pulled the fuel filter and I could blow through it easily. The engine probably has 20 hours tops.
I'm wondering if there is any type of foam air filter in that plastic piece on the carb. I reached around and I could feel a plastic piece with holes in it, but it looks like I pain to remove so if there is no foam material in there it would be a waste. I'm not sure that a clogged air filter would ever cause it to run lean, only rich. I'm thinking it might be the fuel pump, or the carb, but some input from you guys might help me save time troubleshooting this. Thanks in advance!