installed 15hp carb on my 2005 merc 9.9, no improvement..???

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ohiobass

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I have a 2005 Mercury 9.9hp 2 stroke, and after talking with a couple mechanics, I was told that the only difference between the 9.9 And 15 horsepower is the carburetor. The exhaust ports are the same, they both have the larger exhaust ports of the 15hp. so I purchased a new in box 15Hp carburetor online. The part number is 1392-9726A 4. It is an older model ( manufacture date of 1995), but supercides to the new part number, and was told it is the same part.
after installing it, I get no performance boost at all. Speed and rpms are about the same as what the 9.9 horsepower did ( top speed with me only is 8.2 mph) . I have done this conversion on Johnson outboards and seen a world of difference ( usually went from 8-9 mph to 17-18 mph, because the wxtra hp would actually get the hull " on plane" and that was the trick, and gained around 500-600 rpms). I have it on a 17 foot flat-bottomed Xpress aluminum bass boat ( Xpress H51), and the ones that I put the Johnson outboards on with 15 carburetors we're on the same type of boat, and about the same weight give or take 30 pounds.

What can be the issue???
 
I would check fuel flow.. Make sure your pickup hose in tank is in good shape and replace your tank to motor hose ( they can collapse inside)
Also, do you possibly have too much prop for a 17 ft boat? If the motor can't rev out then you can't get up to the recommend operating RPM
 
thanks,
Already installed a brand new hose and bulp, from tank to motor. Also, running a new 9x9 3 blade prop, which should be the correct prop.
 

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