Re: "Newer" 25/35hp OMC compatibilities

[Don]

Will try to address each of your questions, let me know if I mess up:
OK, you were running with a test wheel. I am not sure what the RPM is that you are supposed to see when running with the correct test wheel, so we don’t know if perhaps 4800RPM is the correct RPM or not. Nonetheless, you mention that the engine was pulsating anywhere above 2/3 throttle. Let’s keep in mind that sometimes our test tanks are our worst enemies, it may just be a mixture of breathing exhaust and test tank water ventilation causing the pulsating RPMs you are seeing. It is always best to put the engine on a boat with a prop and try it before "getting in too deep".
In any event, I don’t think swapping the mag plates is going to cause the engine RPM to pulsate, even if WOT timing is off a few degrees. I realize you cleaned the cleaned the carb, but again, there may have been stray debris in the line or somewhere that might have lodged in the high speed jet after you cleaned it. It only takes a few minutes to remove/inspect/clean the jet, without having to remove the carb (if you have the correct tool), so this might be worth doing. Keep in mind that a loose high speed jet (or not completely seated) can cause a lean/pulsating condition as well, use a pipe cleaner soaked in engine tuner/carb cleaner to clean any residue from inside the jet, it sounds like you have already readjusted the float. It sounds like you changed the fuel pump, but it may not be the correct one. What was wrong with the original pump? How about the screen/filter in the fuel pump? Could be an air leak in the inlet fuel lines somewhere also. Keep in mind that running the engine in a test tank raises it high above the fuel tank, which may exceed a normal operating fuel pump’s ability to lift the fuel so high. In other words, the fuel pump is OK, but it is unable to "lift" the fuel that high. Simply place the fuel tank on a table near the test tank if this issue is in question, make sure the fuel line does not sink to the ground after leaving the high fuel tank though. And there is always the simple method of pumping the fuel primer bulb while the engine is pulsating to confirm pump/air leak issues, it is best to have an assistant pump the primer bulb while you run the engine. You could always feather the choke partially closed while the engine is pulsating too, a positive result may indicate carb restrictions/problems.
OK, back to the mud dobbers. Yes, this could surely have created and exhaust blockage down in the gearcase. Pull the prop and hub off, have a look inside the prop shaft bearing carrier (exhaust area) for signs of mud/crud. You could always use a coat hanger to probe up inside the gearcase exhaust area for signs of mud/crud without having to pull the gearcase off. Have you had this gearcase off? I’m guessing no, or else you would have seen mud/crud. I would recommend a water pump job, but I don’t want to open another "can of worms" for you, let’s just say these early 35hps had cooling issues.
And finally, yes, these engines have those two water relief holes just above the gearcase hidden behind the lower mount clam shells, seeing some water come out of there is normal. The driveshaft tube is sealed (or at least supposed to be), so I don’t think mud dobbers could possibly get in there. In other words, seeing water come out of those relief holes has nothing to do with mud dobbers.
PS- I just looked in the 1976 35hp service manual, but could find no reference to a test wheel, or test wheel RPM, probably an early edition of the manual. Again, please be sure the pulsating isn’t just a test tank/exhaust issue before getting in too deep. And yes, you have found another one of OMC’s gems, those early 35hps were not great engines, even though they did become bullet proof in 1980. At least the one you are working on has CD2 ignition instead of the beloved "low tension" system.