merc '79 115 carb rebuild

[Georgesalmon]

I have a merc 115hp inline 6. It starts and idles OK but higher than that it stumbles and sometimes runs good and stumbles and runs good,,,,,, When I pump the fuel bulb it never gets "hard" just firm. If I keep pumping it I can get fuel to flow out of the carbs (at least two of them). The fuel comes right out of where the air goes in (throat?) Does this mean I need a carb re-build? Thanks in advance

 

[Chris1956]

Re: merc '79 115 carb rebuild

George, It sure sounds like you do need a carb rebuild. Purchase new gasket sets and inlet needles, seats and floats for each carb. Should be about $60 for the parts, and get a Merc service manual to help you thru it.

You might rebuild the fuel pump as well. The kit is $10-12.

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

Thank you. I have ordered all of the above. Just waiting for the parts. I would like to soak them in carb cleaner but I can't get the main jet out (I think its the main jet, it sticks straight up in the mddle about halfway up). I even got out my gun screwdriver set to fit the jet perfectly and no luck. Problem is that there is a plastic part kinda around the jet where it sticks up and cleaner says "no plastic parts". How important is that plastic part? If I broke it out to soak would I really screw things up? Another question if you don't mind. There is a small hose the goes from the top of the bowl across the carb. I read somewhere I should put new hoses on and put a BB in to plug the hose. Should I do that?

 

[Chris1956]

Re: merc '79 115 carb rebuild

That is the main nozzle. The black plastic part is the venturi. I would leave them both alone, and soak it as an assembly. The carb cleaner should not harm it. The main jet is behind the brass hex bolt in the front base of the carb. Make sure you blow out the small holes in the carb throat, as they are for the idle jet.

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

Thank you, I'll do it. I'm waiting for parts and the carbs. fuel pump sitting on the bench all cleaned up. I have replaced all the hoses with new. Including the two verticle ones behind the carbs(all three fittings were loose so I tighened them. Don't know what those are for or how importand they are but the top one of them was cracked and when I pulled them off they broke into pieces. Would that have any effect on engine performance? Can any one tell me what else I can do while I'm waiting for parts and have it partially torn apart?

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

One more silly question. On all three carbs the main jet had 074 stamped on top. I can slide a #47 (.078) drill bit in them easily but a #46(.081) won't go. Is that a problem? Do I need new main jets?

 

[Chris1956]

Re: merc '79 115 carb rebuild

Normally jets do not need replacing. I am not familar with the drill sizes you cite, however.

Those hoses behind the carbs are balance hoses that connect sets of two cylinder crankcases to each other. An IL6 is really three 2 cylinder motors, sharing a crank. The crankcases are sealed from one another.

 

[gwukena]

Re: merc '79 115 carb rebuild

One more silly question. On all three carbs the main jet had 074 stamped on top. I can slide a #47 (.078) drill bit in them easily but a #46(.081) won't go. Is that a problem? Do I need new main jets?

Dude your overthinking the motor way too much. Are by any chance your an engineer? There's a tolerance in everything, odds are the jets aren't .074 exactly and the drill bit can pretty well guarantee it ain't .078 bang on

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

lol, actually I was an engineer. Retired awhile now.

 

[gwukena]

Re: merc '79 115 carb rebuild

lol, actually I was an engineer. Retired awhile now.

At least you can laugh about it, I have two of the 115 towers, a 79 1/2 and an 83. On both of those engines the balance hose fittings you referred to previously, were loose, never had a performance issue.

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

Thank you. OK, I have spent the last two days (waiting for carb parts) reading old posts. The thing I'm trying to decide is; should I block those little tubes on the top of the carbs or not? (back draft tubes?) Got new hoses for them ready to go. The carbs and fuel pump and all fuel hoses will be rebuilt and spotless. Do I put in the "BB" or not? I know this is old news. I'm not looking for an increase or decrease in fuel used or power, etc. I just want to do what is safest for my engine. My son bought me a brand new 2012 merc 9.9 pro-kicker with remote controls and that servo operated speed control thingie. All I need the big motor for is getting out to troll for salmon and walleye and getting back (lake Michigan). Not looking for speed just reliability. He has mentioned that he will buy me a new 4-stroke "big" motor in a "couple" of years. I just really need my current motor to last awhile longer. Should I or shouldn't I? The "BB" thing I mean.

 

[Chris1956]

Re: merc '79 115 carb rebuild

George, "To BB or not to BB" is a personal choice. I recommend praying on it, or else flip a coin.

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

beautiful

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

Well, Got all my parts carb's, fuel pump rebuilt, all new fuel hose outside and inside the engine, all the way from the tank including vent and fill. Ran the engine today (muffs) Question, did all the adjusting and the idle needles on #1 and #3 carbs make sense. Started at 1 1/2 turns, adjusted, yada, yada. Motor was running rough when I tried #2 I ran it all the way in to the stop with no change in motor sound. Backed it out until the motor started running much better, than in to stumble, then a half turn out. There is a big difference from the other carbs. #2 needs to be out almost 5 full turns to make things sound right while #1 nd #3 are only out about 1 1/4 turns. Is it "normal" to have that much difference? If there is a "normal" Oh, yeah, I can pump the bulb up hard now, not mushy with gas running out of the carbs and didn't put in the BB. "HE" said no BB when I prayed on it.

The new floats weighed 25% to 40% less than the old ones, Guess replaciing them was a good idea.

 

[Chris1956]

Re: merc '79 115 carb rebuild

That middle carb idle mixture setting doesn't make sense. You might check/clean the idle passages again.

When you water test that machine, be prepared to richen all the carbs a bit. When you set them on the muffs, they tend to be set for optimum idle quality, which is too lean for acceleration.

Normally the bottom carb runs richer (due to gravity) than the mid carb, which runs richer than the top carb. If the motor dies when you try to accelerate, the carbs are too lean. Richen them 1/8 turn at a time from the top carb (then mid, then bottom), testing after each adjustment.

 

[Georgesalmon]

Re: merc '79 115 carb rebuild

I did pull #2 apart and couldn't find anything wrong. Blew carb clearner and air through everything. Just got back from the water. Finished up the carb adjust. Got it to idle at 600 in gear real nice. No hole shot at first but with 1/4 turn more on all carbs no more hesitation. Motor runs and sounds really good (to me). Getting 35mph (GPS) at 4800 WOT trimmed up for max rpm. That was with a 19p SS prop. I have a AL 17p that came with boat and had to get a different thrust bearing and the outer one that is built in the SS one. The guy at the prop place told me its a really old prop with no cup on the blades at all. Thought it might even have come with the motor in 1979. I'm gonna run it this aft again to see where WOT and speed is with that prop should tell me something. Very pleased with performance so far but still don't understand the needle setting on #2. Better rich than lean I guess.