2001 50HP 4S high speed problem

[mmscitrus]

As title states, its a 2001 Merc 50hp 4 stroke engine, carbs. My problem is this: Engine idles ok (after cleaning carbs) but will not make power. Under load, advance throttle to full, revs smoothly and peaks, then begins to lose power. The engine will not shut off, it just runs poorly under throttle. For those of you who have experienced this it acts exactly as if it were starving of fuel, too much air not enough gas. I am reasonably sure it is not rich, the spark plugs look fine. The needles are not grooved, and float height looks ok. As I write this I cant remember if I checked for a rich condition at WOT. I am sure I would have but I just cant remember. When the weather clears I will check that, in the mean time this is what I have done:

Cleaned carbs again(they're perfect)
Synced carbs(I have gauges, not Vacuumate, but they're synced properly)
Checked to make sure the right jets were installed(they're correct)
Checked compression(190-195)
Checked timing
Replaced spark plugs
Balanced checked engine at WOT under load(all cylinders balance equally)
Run from aux fuel tank
Completely bypassed the engine fuel system by running fuel from aux tank directly to carbs(pumping primer bulb to ensure max fuel, no change at all. Not a drop in rpm, nor a rise.............)

I dont get it. Compression is good. Spark and timing are right on. That leaves fuel/intake and I think I've covered all my bases. I'm in need of a fresh perspective.


Im pretty sure thats the complete list. Any thought and comments are appreciated.

 

[JohnSC]

Re: 2001 50HP 4S high speed problem

Just a suggestion. My brother had a 50 HP 4 stroke Honda with a WOT problem which had been looked at several times. The problem was in a carb. Take each carb off one at a time and hook a hand vacuum pump up to the gas inlet. Pull a vacuum of 20 and see if it hold for 30 seconds. If it does turn the carb upside down to release the vacuum and retest. If the vacuum holds several times go to the next carb. If all three test the same the seat test has passed. One of his failed which was letting gas bypass the seat under max gas pressure fludding a cylinder. I replaced his seat and problem cured. Good luck.