Top speed problem

[James matthews]

Guys I have what I believe is a 1976. Its a 20hp mercury tiller handle. Motor runs flawlessly but will not go faster that 8mph. Its mounted on a 14ft fiberglass boat that's open hull and has a dry weight of about 400lbs

 

[racerone]

They run very well on one cylinder ----Some inspection needs to be done.----Check spark.

 

[DblEagle]

What style of hull is it? Deep v, flat bottom or catamaran like a Livingston?

Does the hull have a max HP capacity plate?
What is the max rating?

Add yourself, gas and the weight of the motor and your close to 700lbs total weight.

 

[James matthews]

I will check spark in next few days. I do have a spark tester. Its a like a semi tri hull but flat mostly

 

[racerone]

Is the " boost venturi " in place on top of main nozzle ?

 

[James matthews]

? Elaborate please. I lack knowledge on outboard engines

 

[Chinewalker]

Make sure the throttle linkage is giving you full advance on the carburetor. There are some plastic bits in there that can break...