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Old Merc 500 50 horse doesn't rev up in water


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I've got an old ('61, '62, '63?) Merc 500 Thunderbolt 50 horse 4 cyl outboard on my boat and I'm trying to troubleshoot a problem with it. It will troll ALL DAY LONG but won't rev up to go if I want to get anywhere faster than 3.6 SOG. Once it hits that point it sputters and dies. Put it in neutral and it fires right up, no cranking needed and will continue to troll with no prob. On the trailer it winds right up and revs right out. In the water under pressure, dies. I've checked and tightened the fuel line fittings and clamps, that helped keep it running, must have been sucking air. Now I'm trying to fix this problem. Is there any other simple things that I should check before I get into rebuilding both carbs? On the other hand, I've trolled about 25 to 30 hrs on less than six gallons of gas. I just want to get up and move when I want to.

Thanks, Andy

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I would check your plugs. The bottom plug on 3 cyl johnsons will load up after a few hrs of trolling and will not fire. We could still troll fine but it would not have any power to run. We used to carry extra plugs and change the bottom plug before we could run. There are many other things that it could be but, the plugs are the easiest starting point.

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