"Matt.... Your engine is a 196
"Matt.... Your engine is a 1964 40hp Evinrude. Although there could be many reasons why a engine would not perform properly at full throttle, the most likely reason is a clogged carburetor which needs cleaning and rebuilding. However, first (with the s/plugs out), check the spark with the throttle set at various settings which include full throttle. If you have a strong blue flame that will jump a 1/4" gap, the ignition should be okay. Also check the compression... it should be 100+ psi and even on both cylinders.
It's always possible that at some point through the years, a different (eariler) carburetor has been installed on the engine, hence the instructions are listed below for the old type carb that has the high speed (adjustable) valve.
If you have the original carburetor, the H/Speed valve/jet is a brass fixed jet that is located near the center of the carb behind the 7/16" hex bolt that you'll see at the front bottom of the carb float chamber. If that brass jet is ever so slightly clogged/restricted, it will most definitely cause a problem at top end (full throttle). Make sure that it is perfectly clean. If you can remove it without damaging anything, fine. If not, clean it with a single strand wire (steel) wire (paper clip etc), then pump fuel to the carb that will cause fuel to flow into the carb and out the entrance hole, thustly cleaning out the debris.
(Your Carb)
The top needle valve on the carburetor is the slow speed valve.
(Old Type Carb)
The bottom valve is the high speed valve. If the compression and spark test proves okay, do this before rebuilding the carb.... Back out the high speed valve another 1/4 turn. Run the engine (in the water of course, on the boat, under way), in gear at full throttle. Slowly (in segments of 1/8 turns), turn the H/S valve (bottom) in until the engine starts to die out, then back it up approximately 1/4 turn.
(Your Carb)
Now... After you've cleaned or adjusted the high speed valve/jet, at idle, in neutral, slow the engine down as low as it you can get it without it stalling. Slowly (in segments of 1/8 turn), slowly start turning in the S/S valve (top) until the engine either starts to die out or it spits back through the carburetor (sounds like a small mild backfire). At this point,back that valve out approximately 1/4 turn. NOTE... If at the start, the engine already has that spitting back effect, back out that S/S (top) valve 1/4 turn or so (just to get rid of that spitting effect) before starting this adjustment.
Assuming that the compression and spark are normal, If the carburetor is okay, the above adjustments should correct your problem.
Joe
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