1977 Mercury 850 thunderbolt

[Bayliner101]

I bought a 850 thunderbolt that had some issues with it, after many hours and throwing some money at it to get it running right, I have come down to one last problem that is giving me a headache and i am hoping that somebody on here has had the same problem or knows exactly what it could be, I have put in all new spark plugs, new wires, new packs, the right oil/gas ratio, had the carbs proffesionaly rebuilt and set to the right air/gas mix, new gas tank, new gas line, basically everything has been replaced and set, and the compression on each cyl is at 115-125, now with that all being said here is the problem, i can get it to start up just fine on dry land and throttles great but as soon as it goes in the water it will start up but as soon as i put it in forward gear and try to give it full throttle it will bog down and die, any insite will be greatly appreciated

 

[cell]

Re: 1977 Mercury 850 thunderbolt

http://forums.iboats.com/showthread.php?t=168855

 

[Bayliner101]

Re: 1977 Mercury 850 thunderbolt

well we checked the timing and that is all correct, any other ideas

 

[cell]

Re: 1977 Mercury 850 thunderbolt

Go through the carbs......

 

[Inline6]

Re: 1977 Mercury 850 thunderbolt

Adjust the carb mixture approx 1/4 "out" / richer then see if she quits bogging. If that doesn't take car of the problem give them another 1/4 turn.

when the engine is in the water the exhaust has "back-pressure" and that makes the engine fuel mixture requirements a bit different.

1/4 to 1/2 turn should take care of your problem.

Regards,
Inline6

 

[Chris1956]

Re: 1977 Mercury 850 thunderbolt

I agree with Inline, however, do it in 1/8 turn increments starting with the top carb, followed by the bottom carb. Try to accelerate after each adjustment.

 

[Bayliner101]

Re: 1977 Mercury 850 thunderbolt

well we are redoing the carbs as i speak, hopefully this will take care of the problem, i will let you all know what the results are, hopefully this weekend we can try to get it back in the water to see, thanks everybody for the help

 

[Bayliner101]

Re: 1977 Mercury 850 thunderbolt

welp after all this time and money we have finally got the boat in the water, after redoing the carbs and a little adjustments here and there we were succesful, but i think we are going to have to change the floats, i think either one or both have lost there bouyancy and is flooding one of the carbs but for being 32 years old and they are the originals what do you expect, as soon as i get the money for them and get em installed i will let you all know if that has changed anything.

 

[cell]

Re: 1977 Mercury 850 thunderbolt

When I took out my floats I put em in some fuel for a few hours to see how good they were.They are original,and floated so i stuck em back in.Check to see if they are binding in the carb.They should move up,and down with no rubbing.Make sure to use the manual for a reference on the spring hight,and needle adjustment.Glad to hear the needle adjustment got you good results,but keep in mind if you take the floats out,and readjust em you might have to go through the adjustment again for best results.

 

[Moody Blue]

Re: 1977 Mercury 850 thunderbolt

Bayliner that is good news. When going to the trouble of "redoing the carbs" why didn't you check the floats? That is part of the overhaul process is it not?

What did you do to the carbs? Are you confident this solved the stalling issue? I may have to go through the same thing soon myself.

 

[Bayliner101]

Re: 1977 Mercury 850 thunderbolt

my dad rebuilt them for me, so i wasn't able to check the floats at the time, i did go back and check the floats and after having them submerged in gas overnight i found one of them had a small pin hole in it and was holding gas, he completely tore them apart and soaked them in paint stripper, he said he didn't like the fact that all they had was bio friendly carb cleaner that they sell in the one gallons, i guess he figured that stuff was not going to be as good as the old stuff they use to sell. after having them soaked for a day he use compressed air and blew out all the holes and everything else, put everything back together and turned then out to 1 1/2 turns, i put in two new floats today and tried it out and that thing shot out of the hole like a bat out of hell, just got a few more adjustments to make to the idle, thanks everybody for all the help

 

[cell]

Re: 1977 Mercury 850 thunderbolt

tried it out and that thing shot out of the hole like a bat out of hell

lol.Good news.I like it.That was pretty much my description when I dialed in my carbs too.

 

[Moody Blue]

Re: 1977 Mercury 850 thunderbolt

Thanks for the feedback. Looks like I'll be doing rebuilds on my carbs just to make sure the previous owner did it right.

 

[Chuck0101]

Re: 1977 Mercury 850 thunderbolt

Hello,
Congrad on fixing your mtr, I have the same 1977 85hp mercury motor for 9 years now and mine runs good it does need to have new wire motor harness plug, and control wire plug from the motor to the shifter etc; would be all my motor needs but still runs good .

My motor had a problem this summer it started skipping and loss of power after going through several checks electrical , carbs, my problem was a ignition coil I replaced it all better now the kids can take it out again.
I did go through my carbs just to make sure there was no build up and they looked new inside out I just wanted to note here according to my Mercury manual for this motor (never use carb cleaner on the floats in the carb) something to do with the materal they are made of will destroy them good luck .
Chuck....