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!977 Merc 500 thunderbolt

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Fuddyduddy

Regular Contributor
My 50HP Mercury Outboard runs at about 65% of full power for the first 15 minutes of operation and then it will kick up to 100% after a warm up period stated. I have two fuel pumps to install in the carbs as these are the only things not done to the carbs during the motor's renovation. Does anyone knowledgeable think the fuel pump or pumps could cause this power problem? Thanks for any help.
-Mike
 
Could be. It's either running lean (bad for pistons) or too rich thanks to a pinhole in the diaphragm. Hope it's the latter.

Jeff
 
Just took the starter out to try to get to the carbs better. I know I can get it all apart but I also know I will need my retired marine mechanic friend to help me put it all back together properly. Glad you think the fuel pumps could be causing it. We have just about tried everything else except a new stator. Thanks Jeff.-Mike
 
This is what I have found with the project so-far. One of the main attaching nuts was loose on the bottom carb. Port side.The fuel lines at the dividing "yoke" were very hard and brittle to the point of not being serviceable. They could be spun on their connectors so there was no seal. I have a feeling that this is the problem. It will be two weeks before I can get some help putting things back so it runs OK. I was not able to loosen the starboard side nut on the lower carb. I can't get a wrench on it. Silly to have something like this on a potential maintenance part but I didn't get to sit in on the design meeting. I thought I followed the fuel hose/ bulb and air rules conditional on a motor running well. The motor runs like my old Evinrude with a damaged fuel tank "O" ring seal. I had to wiggle the hose couple to get it to run out.-Mike
 
That's the result of bad design--those carb nuts are hell to get to. Merc smarted up MUCH later on with the new design that has long bolts, accessible from the front, that run right through the carb. My favorite! I can pull all three carb off my triple with nothing more than a 1/2 inch socket, and in 30 seconds.

Jeff
 
Finally got the bottom carb off. Had to slim down the box end of a 12 inch wrench with a grinder. Set to rebuild carbs and also bought some fuel line. Update when I can get it to my "test track". (small lake that is way too high now due to 6" rain recently.-Mike
 
Got the full power problem taken care of with the last bit of TLC. I was real proud of myself when I got it all back together with no help and didn't have any parts or screws and such left over. However........ It will not idle now and my friend the marine mechanic says I messed up putting it back together and was focusing on the little wishbone like "thingy" on top of the upper carb. I have the appropriate parts removed all ready for the correct rebuilding to take place in the morning.Hopefully with help. Mike
 
Help arrived and we sorted out my small errors and started the motor. Started easily and ran smoothly and idled down nicely. Spotted a piece of plastic in the splash well and discovered a venturi from a carb. Both were missing the venturi so I think since it ran well without them , they will just not have a venturi. Hope this is a proper move but time will tell?
 

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Thanks, Jeff- I can give a quick run down of parts thrown at this project some needed badly. First off: set of plugs a new rectifier and tank hose and bulb and two cans of carb cleaner. I didn't need the plugs. A new wiring harness and removed the kill switch. Very bad wiring as insulation was lost and crumbling on nearly every wire. 2 new fuel pump sets and 2 gasket sets and two needle valves and 1 foot of fuel line. All badly needed as I found out only the top carb was cleaned thoroughly during an initial run out. I also bought a set of four coil and wire sets on flea bay that were not needed but exceptionally hard to find. I will keep a spare and resell the others. After my test track run I will be back for a few more words. Thanks so much for the parts and the time.-Mike
 
I am cursed (and cursing)Finally took it out for a test and I am back to square 1. Same thing with it running about 40 % and will not go fast. Not even after a warm up. It runs slow smoothly and starts very easily but no go when the throttle is pushed forward. I am kind of down right now but anyone with an idea would be good. Seems to run great at home with muffs but not at the lake. Thanks, Fuddyduddy
 
Here's what you have to do: Run it WOT for a few minutes, then shut it RIGHT down--no idling allowed. Next, pull plugs (keeping track of where they were) and see what's happening.

Jeff
 
I've got the mechanic going out to the lake tomorrow and we will be looking at a potential electronic problem. We will run it on the trailer with the cowl and cover off and we will take the plugs out to see what's going on with those. I can't understand why it runs and starts so well and so smoothly but only goes about 7 mph. The motor did kick up and then down again while running today but never more than 7 mph tops. Last fall it would do that but would go full out after it warmed up. Not today. I will weigh in after it gets another test.-Thanks, Mike
 
Took it out today with the mechanic. Ran like it should then dropped off a bit the up again then down. Perplexing but he has diagnostic gear left from his old marine business so we pulled the flywheel and then I removed the stator. There were a couple wires that go to the rectifier from the stator that were missing old rotten spots of insulation. I used some high heat tape and rewrapped them . My motor is in the 4707239 serial # for 1977. The pics are of the stator and the trigger and I need to order a service (shop) manual for it. Which is best to get? I will more that likely need to order a stator and /or trigger also but the mechanic said to wait till he gets some equipment together. His shop manuals went away when he sold his business.
Thanks,Mike
 
Those yellow wires are solely for charging the battery. The ignition system is self-contained and does not need a battery at all.

I suspect your stator is shot. Sorry.

Jeff
 
Jeff, Should I get a new trigger when I order the new stator. I will probably go with Sierra or Merc Parts . Will I also get some form of decent instructions for installation. Thanks, Mike PS, Is there any chance those two wires could be arcing and causing a power fluctuation?
 
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Ordered a new stator from our host at the forum. Great customer service and a very livable return policy. I'll update with the next test run that may take place in the next 10 days. The turnaround on shipping is excellent.-Fuddyduddy
 
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