lacking power

[2dogsnight]

First of all: I sold my boat , getting old and to many toys to take care off. The truck sold yesterday.
New owner is much younger but likes old stuff, so I am happy for my old boat and old Atomic 4.
Last summer motor started showing a lack of power. Noting happened to justyfied that. Starts, runs, but boat will not go at max speed of 17 mph. The fastest was around 9 mph. Doesn't overheat,
Any ideas what should be look at??

[zellerj]

A number of things could be wrong.

An engine is an air pump. Any difference in RPM under load or in neutral? If it revs higher in neutral it could mean a blocked exhaust. Dirty flame arrestor?

An engine needs sufficient gas supply. A weak fuel pump? Fuel blockage?

An engine needs a strong spark. Leaky plug wires? Points corroded? Fouled plugs? A spark test should give a nice blue spark. A yellow spark indicates weakness.

And an engine needs all to come together at the right time. Is the timing off? Do you get higher revs if you rotate the distributer a touch? The timing needs to advance with RPMs- do the weights under the points plate in the distributer move freely, does the rotor under the cap snap back if rotated by hand? Are the plug wires in the correct firing order?
.
Do you have strong compression numbers on all cylinders? Should be near 130 to 150 psi or so and not differ very much between the four numbers.

And finally the engine needs to turn the prop. Any binding of the prop shaft that would restrict its rotation? Cutlass bearing?

[2dogsnight]

thank you Jim !

[tenders]

It’s late August. Most every year around this time we get questions about power losses. The first thing to check is for a generally clean bottom and unbarnacled prop. If they haven’t been scraped so far this season, that’s almost certainly the issue.

Though complaints that the boat will go “only” 9mph are fairly rare. That’s the max my boat has EVER gone…full throttle down the East River in NYC with the tide in our favor.