Assuming your boat speed is less than reasonable at that RPM, it sounds like carb tune or ignition timing, or an incorrect prop. The carb and timing are easiest to deal with first. I printed the directions below, and made the adjustments myself successfully.
How do I adjust the ignition timing on an Atomic Four engine?
The ignition timing on an Atomic Four engine can be adjusted in several ways, but no matter which method is used, before the timing is adjusted the distributor should be serviced to make sure the distributor shaft bushings are lubricated with light oil (a few drops of oil on the little piece of felt under the rotor) and not worn beyond specification, the centrifugal ignition advance is properly lubricated and working smoothly, and the contact breaker (points) gap is adjusted to specification using a feeler gauge, or a dwell meter if one is available. Once this is done the engine can be timed.
Static timing can be carried out by turning the crankshaft so the piston for #1 cylinder is at the very top of its stroke (TDC) with both valves closed (firing #1 cylinder), the distributor rotor is pointing at the distributor terminal for #1 spark plug, and the distributor points are just beginning to separate. You can connect a battery powered light or buzzer across the points to determine exactly when they open. When #1 cylinder is at the top of its stroke the split pin in the front of the crankshaft will be pointing straight up and down.
Dynamic timing can be carried out by either tying the boat firmly to the dock or taking it out an area where it can be run at top speed for several minutes, connecting a tachometer to the engine, slightly loosening the distributor clamp screw, opening the throttle to get maximum engine speed, gradually rotating the distributor back and forth until the engine is running at the highest rpm possible, and then retightening the distributor clamp screw. At full speed the engine should reach at least 1,800 rpm (if it can''t, see "What size propeller
should I use?" above).
How do I adjust the carburetor on an Atomic Four engine?
Early model Atomic Four engines were fitted with a cast iron Zenith 61 carburetor with 2 adjustment screws, one for the high speed main jet, and another one for the low speed idle jet. Atomic Four engines after 1968 were fitted with an aluminum alloy Zenith 68 carburetor with a fixed high speed main jet (some Zenith 68 carburetors have had adjustable main jets installed), but the same adjustable low speed idle jet as the Zenith 61. Assuming the flame arrester is not partially blocked by dirt and oil, and the carburetor is clean and in good condition with the float level set properly, the proper procedure is as follows:
Carburetors with fixed high speed main jets should have the main jet removed and checked to see what jet number it is. The jet number is stamped in very tiny numbers on the face of the jet. If it is not a #21 main jet, it should be replaced with one.
To adjust the idle mixture screw, initially adjust the idle mixture screw (the upper adjustable jet with the slot for a screwdriver) clockwise (in) until it bottoms lightly, and then out 1 turn. Connect a tach to the engine, start the engine, run the engine in gear until it is fully warmed up, then close the throttle, leave the engine in forward gear, and adjust the carburetor idle speed screw (the screw on the carburetor throttle arm that controls the throttle stop) until the engine is idling at 700 rpm, turn the idle mixture screw slowly out until the rpm drops noticeably, and then in again until the fastest idle speed is obtained. Readjust the idle stop screw to again set the idle speed at 700 rpm. Repeat the idle mixture adjustment again.
Carburetors with adjustable high speed main jets should be initially adjusted by turning the main jet adjusting screw (the lower adjustable jet with the little cross on the adjustment screw) clockwise (in) until it bottoms lightly, and counter-clockwise (out) 3 turns, and the idle mixture screw (the upper adjustable jet with the slot for a screwdriver) clockwise (in) until it bottoms lightly, and then out 1 turn. Connect a tach to the engine, start the engine, and with the boat running at top speed in forward gear (or tied to the dock) slowly turn the high speed mixture screw clockwise (in) until maximum rpm is obtained on the tachometer and then counter-clockwise (out) about 1/8 of a turn until the rpm just starts to drop.
Close the throttle, leave the engine in forward gear, and adjust the carburetor idle speed screw (the screw on the carburetor throttle arm that controls the throttle stop) until the engine is idling at 700 rpm, turn the idle mixture screw slowly out until the rpm drops noticeably, and then in again until the fastest idle speed is obtained. Readjust the idle stop screw to again set the idle speed at 700 rpm. Because the idle mixture setting has an effect on the main jet setting, repeat the main jet adjustment, and then the idle mixture adjustment 2 more times.
The rationale for setting the idle speed at 700 rpm in forward gear is to ensure the engine is idling fast enough to prevent stalling or erratic running when docking. An engine speed of 700 rpm in gear will also automatically increase to about 800 rpm when the transmission is shifted to neutral, which is high enough to allow the alternator to continue charging the battery.