I have a 1976 Mercury 500 which is experiencing power loss at WOT.. its a long story, to get to where i am at, but many thanks to this forum, and the great responses, i am making headwayLast year the engine would idle, and run smooth at WOT. It would fail (die out) during transition from Idle WOT, and use of the electric choke was required to bridge this gap. the following has been done to the fuel system (in order)to fix this issue. -replaced tank with new merc model- replace all fuel lines/bulbreplaced fuel diaphram and installed new check valves in the fuel pump.-installed new plugs - retimed unitAll this effort do not fix the problem. This year:- installed new carb kits, adjusted floats etc and had the motor out this past weekend. Idle thru to about 60% rpm's worked perfect (trolled for hours).. however, when i go WOT the engine stalls out or runs very slow, and sounds like it is running on two cylinders (vs 4). During failure bulb is hard and sweezing does not increase speed. - this year i used fresh 92 octane fuel,and did a total flush of all lines before starting as well,any suggestions to next steps would be most welcome. Are there high speed jet adjustments for this Merc?. Carbs are back drag types, KA model
Hi, BEL.Do you have a Manual?I'm no Merc-spert, but it sounds to me like you have exhausted suspects in the fueling system and have a heat-related failure somewhere in the ignition system that is taking out a couple of cylinders.I have successfully located heat related failures by heating parts with a hair dryer while idling on the muffs.Might be worth a try. Good luck.
Thank You. Yes, I have a full factory manual which i use for reference, however aybe this is what is getting me a bit to deep. As the vacation season is approaching, i will be reviewing all the work performed last summer. I have had the motor running on the muffs, and have the same issue. (thought the load of the boat may be the problem. Looks like i will start on the electrical side to isolate any issues. I inspected the CDI, and it looks to have cracks around the terminals... would this cause any High tension leakage?
I have the same problem with my Mercury 500. I have done the same things that you have. New diaphram, Check valves, Fuel lines, Pump ball, Plugs, Wires, Different tank. Except when I apply firm constant pressure to the pump ball the engine will pick up. It is like the Float bowls are emptying and can't keep up with the demand for fuel. Doesn't make sense. If you find an answer let me know.
I have the same problem with my Mercury 500. I have done the same things that you have. New diaphram, Check valves, Fuel lines, Pump ball, Plugs, Wires, Different tank. Except when I apply firm constant pressure to the pump ball the engine will pick up. It is like the Float bowls are emptying and can't keep up with the demand for fuel. Doesn't make sense. If you find an answer let me know.
Bel, Can you run the engine at night? If so you may be able to see any arcing. Cracks are not good. If its not the CDI, its very possible on that old engine that your exhaust passage from the power head to the lower unit is clogged up. I learned this from someone that went thru the same thing you are going thru. He was able to clear it by dropping the lower unit and digging all the crud out.
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Fair Winds and Following Seas
Bill
PTC USN ret
Thanks for all the feedback. Spent the last part of my vacation last summer..."working" on my problem. I re-did all carb adj, install new plugs, fuel etc. Re-ran the unit, and managed to get up to WOT, only once. I pulled the lower plug, and it appeared to be "cleaner" than the top 3. Think I am getting water in the lower cylindera. The day before my vacation ended, I pulled the lower leg and investigated the bottom of the power head. It was covered with a gray paste. I removed the power head (try this in a camp ground some day!) and the bottom gray paste appeared to be an OIL/Water mixture from the exhaust ports. The gasket in the bottom of the head appeared to have blown thru (water intake to exhaust channel). I believe I need to change out the gaskets. I have purchased a complete gasket set for the unit, and will begin installing/testing this spring. It has been a long haul, but I am one stubborn dude. Cannot let a mechanical item get the best of me... Been making me think, and if I am successful... will provide an update when the ice clears from the lakes!!! Wish me luck..
just cannot leave this alone. During the non boating season I meticuosly replaced all the gaskets i could, save for the crankcase mating seam gasket. (did not have one.) I added in new fuel, and went to the Lake last weekend to enjoy my successes The engine immediatley started, purred like a kitten, and i could travel anywhere at low speed. After engine was warmed up a bit, moved to full throttle, and was very disapointed that my symptoms did not change. Still getting power loss ( machine Bogs) as i go to full throttle.... The only things i have not changed to date are the Coil, and distributer ( they do check out according to the manual). Any additonal suggestions would be appreciated. If i could place the unit in a test tank, it could help me diagnose sooner. I would like to obtain a test wheel. Does anyone know how/where I could obtain one? Can i modify an old prop to perform this function?
Just a thought-the fuel pumps appear to be driven by crankcase compression via a small hole in the carb/block mating face gasket. Is this hole blocked on either carb? I SUSPECT (correct me if I'm wrong) that if the lower pump isn't working at full capacity because of a blocked tube then the fuel being pumped to the top carb will also fill the bottom carb-probably right up to the point where it can't fuel both and will then feed to the top carb only. Any thoughts anyone?
Do you know for sure that you are getting fuel through the high speed nozel on both carbs? I had a similar situation occur after my carbs were rebuilt. One of the bowls loosened up on the top carb. I took the carb off, fixed that and then re-installed it on the engine. It idled fine. Took the boat out and couldn't get past about 1800 rpms. After checking everything under the sun, I discovered that the needle valve or the float on the upper carb was stuck and it wasn't getting enough fuel. When it was running on the muffs, I would advance the throttle with my hand (be careful!) and look into the front of each carb. Once your past idle, you should start to see fuel rising out of the high speed nozzle in the middle of the opening. Also, how do you know the powerpack isn't bad? You mentioned earlier in the thread that you saw cracks in it. That was my drama last season. It would run fine for a while and then loose power. This was the result of my powerpack failing which resulted in one or more cylinders not firing sporadically.
Mercury 50hp-500 power loss at WOT
however, when i go WOT the engine stalls out or runs very slow, and sounds like it is running on two cylinders (vs 4). During failure bulb is hard and sweezing does not increase speed. 
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