[bigwillie]

Mercury 20hp, only about 3 years old, just got it tuned up....now, when I put it in the water and go to full throttle, the motor will go for a few minutes and then it seems to suck air. Then the the motor wont push past wake speed..It will run at idle speed and starts on the first pull....but this dosn't happen all the time..Sometimes I go out and it will work with no problems...I could understand it if the motor did this all the time..but its only a select number of times....Brought it back to the boat shop and they ran it in their tank with no problems..I took it back out and it happed 2 out of 4 times I was in the water...Its seems that when the boat begins to plan off, the motor will start slowing down like its running out of gas....Any clue to what might be the problem..

[petryshyn]

Sounds like you're losing one cylinder. Before getting too involved, try putting the old plugs back in. I've seen new ones defective which develope intermitent missfire. I assume new ones were put in at the time of tune up....

[12Footer]

Does it act like a slipping clutch on a car? If it does, the hub has spun out of the prop.If the engine is not stumbling, but the boat just doesn't want to go, this is the problem.

[bigwillie]

it feels more like a fuel problem than a motor problem.....the plugs are fine...the engine isn't miss firig..its when you go to throttle up it will go but then the power will come down to about idle speed...

[kevlar3000]

How is your tank and your fuel line? A tiny air leak can cause lots of problems. Water in the tank could also contribute.There has been more than one bag spark plug sold, the idea with using the old ones is to see if the problem persists. Or even try a different new one if the old ones were tossed. I too would think if it worked ok at the shop it has to do with your fuel delivery...Keep us posted....