1975 Merc 500 Has No Power. NEED HELP PLEASE

[MADMERCOWNER]

Hello everyone I have had enough. I really hope someone can give me some ideas because I'm all out. I have a 1975 Merc 50 HP Serial#7060655 and my problem is this. Out of the water she runs great and idles fine but when there is a load on it I have no power. I have compression from 130 to 140 accross the 4 cylinders. I have replaced the fuel line the fuel pump the plugs and plug wires. I bought some carbs on ebay and I'm about to try that but if the motor is running fine out of the water why would it be a fuel problem! But thats why I'm hear to see if some can help me. Oh yah I've changed the fuel and the gas tank is vented. If you have any ideas that would be awsome and I will owe you big time. Also I was wondering if it could be a bad coil or ignition box or distributor? I'm running out of nerves and money. Please help me!!

 

[dmessy]

Re: 1975 Merc 500 Has No Power. NEED HELP PLEASE

Sounds like you may have spun a prop hub if it revs up Ok but you'er not moving. What does the motor do on the water? Does it idle? Does it accellerate cleanly? Whats your tach telling you?

 

[Hooty]

Re: 1975 Merc 500 Has No Power. NEED HELP PLEASE

Tell ya what MM. I'd bet you've got a dead hole . They can run on three and sound fine with out a load but lay down when loaded. While you've got it running on the muffs, start shorting out sparkplugs. The one ya short out that doesn't change the sound is the one not hitting. Another thing to look at is the plugs themselves. Pull them and note where they came from. The one that looks different (wet, dry, dark, light, etc.) is the one not hitting. Best to start with new factory spec plugs. No substitutes. Let us know.<br /><br />g'luck & c/6<br /><br />Hooty

 

[JB]

Re: 1975 Merc 500 Has No Power. NEED HELP PLEASE

Howdy, MMO. <br /><br />I agree with Hooty. <br /><br />Your symptoms are classic and repeated here several times a day.<br /><br />Good luck.

 

[sprintflyer]

Re: 1975 Merc 500 Has No Power. NEED HELP PLEASE

mmo,<br /> Must be a lot of those old 500,s around.I just finished answering another post about one.<br />Just by reading your post I look to see if you were anywhere near V.A. because I could swear you had my old boat! Mine had the same symptoms and I never did get it to run right. I just gave up and traded. I didn't check for a dead cylinder but I did try a new fuel line and pump ball, playing with the timing and changing the prop. (2 blade and 3 blade) <br />It sound like your getting good advice.<br />When this motor was right it ran great. I wish I had kept mine.