Re: 1975 4.5hp dies when throttled up.
Maybe there's still some trash left. Did you take apart the main jet, remove the main jet emulsion tube as well, and the idle emulsion tube?
Any crud in these and the carb won't fuel correctly.
The fuel/air passages in the carb are teeny-weeny so it'd be easy to miss something.
Take a look at this parts breakdown:
http://www.mercruiserparts.com/Show...png&inbr=469&bnbr=5&bdesc=CARBURETOR+ASSEMBLY
The parts in question are the main jet (31), the main jet nozzle (emulsion tube) (29), and the idle (emulsion) tube (22). Also note the itty bitty copper washer (23) under the idle tube. It may stick in the carb when you pull the tube, so be careful not to lose it when blowing the carb out with compressed air or carb cleaner.
There may or may not still be a washer (32) under the main jet (31), don't fret if it's not there, the jet will seat fine without.
All the passages in the main jet, jet tube, idle tube, and carb must be super-clean and not blocked by any debris in order to work properly.
Use a small 'Gordon'-type wire 'toothbrush' to mechanically clean the parts. Some passages are so small that you may have to strip out a strand of copper electrical wire to use as a poking device.
Also pull the idle mixture adjustment needle (35) and blow thru that passage. Take a look at the manifold side of the carburetor, where you'll see the throttle butterfly and some very small passages drilled into the bore of the carb. A series of small holes will be the idle passages, and you should be able to blow air/cleaner from the idle mix needle side and have it come out the holes in the throat of the carb.
I bet you find something plugged that you didn't notice the first time.
I recall having fits making an old Honda XL250 single run right, can't recall how many times I took the carb apart when at last I pulled the main emulsion tube, only to find it full of old varnish and the holes in the side mostly plugged up. She ran like a dream once that was fixed.
There could of course be internal engine problems, but since it was running ok just a few weeks prior it's not likely unless there was some sort of catastrophic failure.
I'd look to the carb first, it's a lot easier to deal with than tearing apart the powerhead!
HTH...........ed
