I'm having a similar problem to whitesnooky's 25hp merc. The engine starts fine, idles decent sometimes but was sputtering quite a bit last time I tried it and it runs good up to about 1/2 throttle and then bogs down. It doesn't completely stall just goes to idle speed and then I am able to throttle back up to 1/2. I had this problem a few months ago after I bought it used so I had the carb cleaned and it was running fine till now. I took it to the shop again and they said the carb was clean and that they wanted to test run it to see if they could figure it out but I picked it up before they could because they were taking forever. So after reading the repair manual and looking around online, I'm thinking there might be problem with the float in the carb. When it bogs down, it seems like the engine is flooding instead of being starved of fuel. I think the fuel pump can be ruled out because when I pump the primer bulb when running, the engine seems like it bogs down (floods) even quicker. Could the timing be an issue? Any ideas?