Mercury 85hp runs fine on Muffs but Boggs down in Sea

[thelostkiwi]

Hi All!
I have a Mercury 85 HP Thunderbolt 1977(I think)serial 7101664
It runs fine on muffs connected to a hose but when I took her out in the sea it starts well but runs rough and will go into gear and can increase speed slightly but once I open her up abit more she bogs down and will stop if I dont pull the throttle back fast enough.

Once I got her home and back on the muffs she ran perfectly and would get up to full throttle in gear with no problems at all.

The only thing I noticed is that the tell tale is a little weak on muffs and in the water, and that water is coming out round the prop right from when I start it before its had a chance to warm up.
This is my first Mercury so Im not sure how strong the telltale should be.

The previous owner had some work done a year ago so its got new plugs and new ignition panel thing. He had it running well on a freshwater lake last year with no problems and said its never seen sea water.

So as I use it in seawater could this be the problem or something else?
Any Help would be much appreciated

Thanks,
David

 

[Chris1956]

It is not the seawater. It is not the cooling system. it is likely the idle mixture is too lean. The PO had some work done and the mechanic (or PO) didn't know how the set the carbs. The part about running well on the lake was BS.

Take her out on a calm day. Warm her up and try to accelerate. if she bogs, open the top carb idle mixture screw 1/8 turn. Try to accelerate and if she still bogs open the bottom carb screw 1/8 turn. Continue to open top carb mixture screw, test, open bottom carb mixture screw, sequence until the motor accelerates smoothly.

 

[racerone]

It only sounds " perfect " to the untrained ear on a hose.----Agree you need to adjust low speed mixture screws till bogging stops !

 

[thelostkiwi]

It is not the seawater. It is not the cooling system. it is likely the idle mixture is too lean. The PO had some work done and the mechanic (or PO) didn't know how the set the carbs. The part about running well on the lake was BS.

Take her out on a calm day. Warm her up and try to accelerate. if she bogs, open the top carb idle mixture screw 1/8 turn. Try to accelerate and if she still bogs open the bottom carb screw 1/8 turn. Continue to open top carb mixture screw, test, open bottom carb mixture screw, sequence until the motor accelerates smoothly.

Chris, thanks for your comment! It’s interesting that the first thing you mention, or that anyone has mentioned is to adjust the mixture screw. I say that because when I was in the water I took the carb cover off to expose the mixture screws and at the bottom of the casing was a socket from when someone else was in there, probably doing exactly what you described but possibly not on the water. I will get it back out on the water the next nice day I can and give this a go.
Thanks mate!
Also do you by chance know what would be a good starting point for the mixture screws? ie. 1 1/2 turns from bottom?

 

[Chris1956]

Yes, 1-1/2 turns open is the initial setting. They will need 1/2 - 3/4 turn more, but must be adjusted individually, under load, fresh 50::1 fuel mixture, in the water.
You might check the timing as well. Set the throttle such that the carbs are ready to open, but still closed. Crank the motor, plugs 2-4 out, prop removed. Timing should be 0-9* ATDC. Set the throttle full and the timing should be 21*BTDC.

If they are not those settings, you should get a service manual to do a full link and synch.

 

[thelostkiwi]

Yes, 1-1/2 turns open is the initial setting. They will need 1/2 - 3/4 turn more, but must be adjusted individually, under load, fresh 50::1 fuel mixture, in the water.
You might check the timing as well. Set the throttle such that the carbs are ready to open, but still closed. Crank the motor, plugs 2-4 out, prop removed. Timing should be 0-9* ATDC. Set the throttle full and the timing should be 21*BTDC.

If they are not those settings, you should get a service manual to do a full link and synch.

Thanks Chris, Ive never done timing before but youve explained it really well.
That being said, and the fact I dont have a timing light, I will try adjusting the mixture screws and hopefully that will do the trick.
When I initially got the boat it had a high idle problem which was caused by a straw stuck in the back of the bottom carb from when it was fogged.
The previous owner didnt find this problem, I did, so I wouldnt be surprised if they mucked about with the mixture screws trying to remedy the high idle, or at least thats what Im hoping and also hoping they didnt touch anything else lol.

 

[thelostkiwi]

Yes, 1-1/2 turns open is the initial setting. They will need 1/2 - 3/4 turn more, but must be adjusted individually, under load, fresh 50::1 fuel mixture, in the water.
You might check the timing as well. Set the throttle such that the carbs are ready to open, but still closed. Crank the motor, plugs 2-4 out, prop removed. Timing should be 0-9* ATDC. Set the throttle full and the timing should be 21*BTDC.

If they are not those settings, you should get a service manual to do a full link and synch.

Chris, I’m taking the boat on the water tomorrow to adjust the carbs. After adjusting the mixture screws do I need to replace the carb cover each time or can I leave the carb throats exposed when trying to accelerate?

 

[Chris1956]

You should be able to leave the carbs open. Can you not get to the mixture screws with the airbox installed? just curious.

 

[thelostkiwi]

You should be able to leave the carbs open. Can you not get to the mixture screws with the airbox installed? just curious.

Hi Chris,
I have to remove the carb cover to get to the mixture screws as I have the square bowl carburetor. I ended up buying a manual and found the base setting was just 1 turn so I used that when I took the boat out.
And she was much better!
I started making the adjustments you recommended and she just got better and better. I think I added about another 1/4 turn to both screws before she went really well.
Spent the next 2 hours towing the kids around on a kneeboard.
Thanks so much for the help Chris!
Cheers!

 

[Chris1956]

Glad it worked out. Unfortunately, current marine mechanics (and owners) are not well versed in adjusting the older carbs.

 

[merc850]

Please don't rev it up to full throttle in the driveway, more that 2000 rpm's with no load can be destructive.