Merc 115 No power

[Prairie Lake]

We have a 1977 merc 1150 thunderbolt outboard on a 16 ft Glastron Carlson ski boat. We can't seem to be able to get more than 4500 rpms out of it. We had it in the shop. He said it dyno'd great and changed the plugs, rebuilt carbs. It had an electric fuel pump on it which he didn't like and took off. Said the boat should be good to go. Got the boat home and it ran poorly. Put the electric fuel pump back on. Tried a smaller prop went from a 19 to 17 to 15. Still not enough power. Starts good, idles fine. Has new distributor cap. Have a manual.

Please help. Summer is almost over.

 

[Yepblaze]

Re: Merc 115 No power

What ratio is the lower unit?

Put a bit of masking tape on one blade of the prop and another bit on the edge of the flywheel.

Count how many times the flywheel goes around to make one revolution of the prop.

 

[Chris1956]

Re: Merc 115 No power

Prarie, Check the spark plug wires for arcing to ground. Especially the lower ones. Check the plugs for color. If any look brand new, you may have water infiltration.

 

[Prairie Lake]

Re: Merc 115 No power

Hi,
Boat runs great in the shop with the muffs on, all plugs are firing. 2 to 1 ratio. (One motor rev to 1/2 a prop rev). Put her in the water and she's sick. Think its the top carb. If you 3/4 choke the top carb it picks up 1000 rpm under load in water. Can get up to 5000. Would it be the float in the carb? Screen in the fuel pump is good. The other two carbs if choked foul out.
Carbs were supposedly rebuilt in the shop.

 

[Yepblaze]

Re: Merc 115 No power

Double check the float level with the top section inverted I believe it is supposed to be level.

And or you may need to find something to probe the main jet to make sure it's not plugged with a chunk of hardened stuff.

Also consider that the top carb needs the most oooph from the pump to get there, so a marginal pump might make the top one go lean first.

After fighting my OEM fuel pump for years I run an electric on mine and am very happy I do each time out.

 

[Chris1956]

Re: Merc 115 No power

Prarie, If you suspect the top carb(or any of 'em) is dirty, clean them up. After that you need to set the idle mixture correctly. It needs to be rich enough to accelerate and lean enough to idle well. First set all to 1-1/2 turns out from lightly seated. Start the motor in the water and warm it up. Now adjust the idle needles until you achieve the fastest, smoothest running. It will likely be too lean to accelerate, and stumble. Open the top carb 1/8 turn and try again. If it still stumbles, open the mid carb 1/8 turn and retry. Continue with the bottom and then the top carb. On those motors the top carb runs leanest, the mid carb is a bit richer, and the bottom carb is the richest. You need to take this into account when adjusting the mixture.