Cant plain off

[fogie27]

1974 Mercury 850, 85hp. Two years ago I was having problems with the engine staying running even at full thottle. You had to keep priming it or it would die. I took it in last year and spent over a $1000.00 for the carbs to be rebuilt, a new fuel filter and fuel pump, I also had reverse that was out so I swapped a lower unit with a '76 Merc 115hp that was the same shaft lenth and bolted right up. I was told that gears in a 115hp lower unit are so close to the same that it wouldn't make a differance. So last year out in the water it was falling on its face when I first got it out there and I called the shop that just fixed it and they told me to turn out the idle mixture screws until it doesn't do that. Well I got it running fairly well until I had two people or more with me then I noticed that it would not plain off. The RPMs wouldn't go much over 2800 and we would only go 5-10 mph. I kept thinking that it was fuel related so I kept playing with the carbs and found no differance turning the IMS in or out. My prop pitch it 15 and I've never had a problem like this before. I used to pull 2 skiers out no problem. The plugs had some carbon on them and I have some new ones to put in but the old ones didn't look that bad. I do have a cracked bowl cover plate on top carb but I have been told that it wouldn't effect due to being above the float level. I always run 50:1 mix with fresh high octane fuel. Never have had a problem like this and I am stumped. Please any thoughts

 

[fogie27]

Re: Cant plain off

My boat is a 16 foot fiberglass tri-haul that has a cap rating of 1100lbs. Max Engine rating is a 90hp.

 

[WillyBWright]

Re: Cant plain off

That cracked cap is interesting. I think a good question would be how it got that way. May there have been water in it that froze and cracked it? If so, then internal damage to the carb is fairly likely. It may be in a passage that you can't see into. You might try locating a used carb.

 

[fogie27]

Re: Cant plain off

A couple years back I was replacing the needle and floats in the carbs and cleaning them all out. The bowl cap is made out really soft aluminun and there is a little bolt in each corner of the square to hold it down. When I tightened one of the bolts it cracked that corner. I don't think this is the problem. Sure it leaks a little fuel but there shouldn't be much because it is above the float. I could try putting some gasket material that can withstand gasoline over the crack just rule this out.

 

[Chris1956]

Re: Cant plain off

Dennis, First some news. The gear ratio in your 115HP lower unit is likely 2::1. Your old one was likely 2.3::1 or something like that. However, I used my 85HP inline four with a 2::1 lower(from my inline 6), on a 16 ' runabout. I used a 17 " pitch prop, and that boat would hit 40MPH+. I therefore suspect you have "lost" a cylinder for some reason. I would check compression, and it that is adequate, check to assure there is spark to all plugs, plugs are good etc. Also, those spark plug wires like to arc to ground when they get old, especially the #4 wire.<br />If that doesn't get it, maybe you have water leaking into your cylinders.