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

[Broadhorizons]

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

[dieselhp]

Sounds like the problem is when your have a load on it. Most times that is fuel or air.

You need to check them first

hope this helps

[LongLine]

BH - Are you sure it's firing on all four cylinders? Most 2-strokers will idle all day on two or 3 cylinders but won't take off unless all are firing.

Tom B.

(LongLine)

[Broadhorizons]

I can check that, I haven't checked the plugs. I just don't want to have the down time for the carbs until winter. I'd rather fish if I can and deal with the problem. Any other suggestions?

[dieselhp]

Make sure the gas tank vent is clear

[ray koziatek]

sounds like head gasket check the plugs after running in the water .

[Ray]

The following is a great place for DIY boaters....

http://forums.iboats.com/

Ray

[KOPPERKLAD]

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.

[Broadhorizons]

Thanks guys, I looked at the site that Ray suggested and I found a similar scenario with a Mercury motor. It looks like it probably is an ignition problem. Thanks for the help.