1984 75 hp Mercury wont go past half throttle...

[ebassett74]

I have rebuild the carbs twice and still have the same issue. Once I go above half throttle it starts to bog down. I have noticed at an idle I can see the gas and oil residue on the surface of the water. Today I covered the top carb air intake and then she went full throttle and now cant duplicate it. I have run the motor underway spraying cleaner and I am at wits end. At idle I can go to full RPM while in neutral. Would rebuilding the fuel pump help? What about finding a company to send the carbs to so they can be sonic scrubbed. Thank you for your help.

 

[big boost]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

stator bro test that first

 

[lwbensend]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

i have a similar issue with my 1980 mercury 80 hp thunderbolt what is the best way to test the stator if there is a way

 

[donzi gt230]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

The manual has the test procedure for the stator; involves an ohm meter and the specs. I think the thunderbolt has a distributor and therefore no stator, BUT I don't know that for sure. Covering the carb is dangerous BTW so be careful, engine vacuum (negative pressure) can cause a very serious injury.

 

[aussieflash]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

My mate went through similar prob last year.Cleaned carbs,fiddled with jets,complete replacement of carbs.Still bogged at 1800 to 2000 RPM.Turned out it was the switch box.Took a year to figure out.Check your whole ignition system.Stator,Trigger,Switchbox,coils etc.

 

[big boost]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

lol donzi man dont post anymore, distributor r u kidding thunderbolt, coil packs . stator trigger switch box , go post on a four stroke v8 site thats all u seem to be referencing nothing wrong with covering the carb a couple time on a little two stroke, and for those of u who disagree if u cant do something like cover a carb alittle bit while cruising, y would u own it id scrap it cause it would be a kids toy to me if its that weak

 

[donzi gt230]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

lol donzi man dont post anymore, distributor r u kidding thunderbolt, coil packs . stator trigger switch box , go post on a four stroke v8 site thats all u seem to be referencing nothing wrong with covering the carb a couple time on a little two stroke, and for those of u who disagree if u cant do something like cover a carb alittle bit while cruising, y would u own it id scrap it cause it would be a kids toy to me if its that weak

Is there really a need to get nasty about it? As for the distributor, I had an 800 with a distributor and I'm pretty sure it was called a Thunderbolt. As for his motor, I don't know and that's why I clearly stated that I don't know. As for covering the carb at speed, the vacuum created could cause an injury and would have nothing to do with it being a weak kids toy. In fact if it were a weak toy then it would have less possibility of injuring one's hand. There is some possibility of doing engine damage as well. Even if I'm completely wrong in everything I state, at least my grammar and use of punctuation makes it easier to figure out that I'm wrong.

 

[ebassett74]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

I have done some of the suggestions and I am trying to get to all of them. Stator has tested fine on the ohm test. I will look into the switchbox tonight. The timing was a bit off and I think that the person before me thought they may have been adjusting the idle when in reality they were adjusting the spark advance. I am no mechanic in any way but I am doing my best. I could not get the required 23 for the max timing so to the screw being bottomed out, no more room to go. It is floating around 27 BTDC. This was just realized today that the person that rebuilt the top carb never took the core plugs out. I did clean a spider out of the needle and am wondering if there is a way a part of it got logged up in their somehow? The second issue is that I would adjust the idle mixture screw on the carb and it would not respond the way the service manual said. Two three turns and nothing. I would lightly bottom out and it would start to bog down and I tried many times slow, fast, half way in-between and it was not responsive. Any information would be greatly appreciated again.

 

[moosehead66]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

I just got done dealing with a 1979 merc 850 inline 4 that I picked up for close to nothing. I was told it needed a switchbox. Compression 135 across all 4 cyl. Has newer stator, trigger and harness. Rebuilt carbs over the winter, set mixture screws at 1 3/4 from lightly seated. Put in new fuel pump and water pump impeller and all new fuel lines. Bought new switchbox and rectifier (rectifier was shorted). Ran great on the hose, would not accelerate on the water unless I kept bumping the key to enrich it. Anyway adjusted the mixture screws out 1/4 turn at a time and at about 2 3/4 turns out it came to life, turned them out another 1/4 turn. This motor runs great now, had to bump the idle stop up slightly because it runs a bit rich at idle and wanted to stall when put in gear. There are alot of posts on here about adjusting the idle mixture on the water, you can't do it on the hose with no load. I have this on a 17 ft alum deep v and a 21" prop. I am seeing 34 mph GPS speed at 5600 rpm perfectly trimmed. Also installed a CMC pwr tilt/trim over the winter and it works perfectly, no more breaking my back lifting this heavy motor from inside the boat. These old mercs are great outboards, but you have to be familiar with them.

 

[moosehead66]

Re: 1986 70 hp Mercury wont go past half throttle...

Re: 1986 70 hp Mercury wont go past half throttle...

I just got done dealing with a 1979 merc 850 inline 4 that I picked up for close to nothing. I was told it needed a switchbox. Compression 135 across all 4 cyl. Has newer stator, trigger and harness. Rebuilt carbs over the winter, set mixture screws at 1 3/4 from lightly seated. Put in new fuel pump and water pump impeller and all new fuel lines. Bought new switchbox and rectifier (rectifier was shorted). Ran great on the hose, would not accelerate on the water unless I kept bumping the key to enrich it. Anyway adjusted the mixture screws out 1/4 turn at a time and at about 2 3/4 turns out it came to life, turned them out another 1/4 turn. This motor runs great now, had to bump the idle stop up slightly because it runs a bit rich at idle and wanted to stall when put in gear. There are alot of posts on here about adjusting the idle mixture on the water, you can't do it on the hose with no load. I have this on a 17 ft alum deep v and a 21" prop. I am seeing 34 mph GPS speed at 5600 rpm perfectly trimmed. Also installed a CMC pwr tilt/trim over the winter and it works perfectly, no more breaking my back lifting this heavy motor from inside the boat. These old mercs are great outboards, but you have to be familiar with them.