"Carac....If the compression a
"Carac....If the compression and ignition on all four (4) cylinders are as they should be (compression = approx 100lbs & even... and a spark that will jump a 7/16" gap with the plugs out), I would suspect that possibly the carburetors are out of synchronization. If a carb jet was clogged, the rpms wouldn't be able to jump around like that.... but if the linkages between the top & bottom carb was incorrect, the rpms would go up, down, back and forth, etc etc. You may have other problems but do this first.
The idle set screw on the vertical throttle arm, (the arm that leads from the throttle cable to the timer base under the flywheel)Back the idle set screw off (usually about half way up that arm) so that the metal cam is nowhere near the bottom carb roller.
Loosen the roller on the starboard side of the bottom carb and move it away for the metal cam that guides it. Now, loosen the screws that retain the rod that connects the top carb to the bottom carb, also on the starboard side. Now, on that rod, tighten the bottom screw... then carefully tighten the top screw of that rod. Make sure that you didn't move the throttle butterflys of the carbs when you tightened those screws. This can be verified easily by slightly moving the top butterfy shaft. If there's a ever so slight bit of play there before that rod starts to move the bottom butterfly (a few thousands), that's good.
Now, move the throttle control arm at the control box, or at the engine itself if you wish, and align the scribe mark on that metal cam so that when you move the carb roller against it, the center of the roller is aligned perfectly with that scribe mark when the bottom carb butterfly starts to open. Tighten the roller screw to keep that adjustment.
On a flushette (hose) with the water turned on full blast, start the engine. After it warms up, set the idle with the throttle cable trunion, then screw in the idle stop screw on the vertical throttle arm to where it just hits the stop, which prevents the idle from dropping any farther.
The above proceedure will sychronize one carb to the other, and both in relation to the timing. I hope that cures your problem.
Note, on a flushette... with a 20" shaft engine, set the idle at 1000rpm. With a 25" shaft, set the idle at 1200rpm. When actually in the water, the rpms will drop to a normal idle rpm.
Joe
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