1983 Johnson 115 v4

I recently purchased a 1987 Larson 17 foot runabout with a 1983 Johnson v4 115.  This is my first boat. I’m in love with the classic boat but my issue is it will not go over 8 mph. I have found a bad coil which I replaced. New prop. Compression reads 103,105,110,114. I ran a whole can of sea foam in 1 gallon of gas which helped the idle a lot. Idles great and runs like a raped ape on muffs. Once placed in the water it won’t get out of its own way. Will rev high with the idle control but once it’s in gear it will not idle up much at all. The tach does not work but I would take a guess as to it will not rev past 1500 or 2000 rpm. I get 7-8 mph wether alone or with the family. Nothing more. I’m at a loss as to where to look next. Any help would be much appreciated.  

I did a 1 gallon gas can which was new and mixed the gas, oil and 1 while 16 ounce can of sea foam and ran it through. It idles much better now but still will not idle up at wot while in the water. In reverse it will go but still not idle up. Will rev in either forward or reverse just slightly over idle. Tach doesn’t work but the sound on muffs is my guess around 4,000 rpms and in the water less then 2,000 rpm. Going to pull the bowls and see what I can clean that way and may go ahead and order the carb rebuild kits and just start there. I did replace 1 coil and checked spark in all after and I have a strong blue spark. All new plugs as well. Had a co-worker tell me to check my lower unit. Would that cause the engine to not rev under load but run perfect out of water?  I have no idea what bad lower unit symptoms are besides lots of noise and no go at all. 

.I used to own a 1996 johnson 130hp v4 2cycle.  That motor is very similar to your 115 v4.  Across the years I needed to replace 2 of the coils.  Each cylinder should have it's own coil.  Just one coil not working well caused significant reduction in performance.  For carb cleaning, which could also be your problem, are you able to spray seafoam directly into to carbs.  Get a separate(portable) fuel supply that you can run with fresh gas/oil/seafoam mix.   They have formula for describing decarbing and carb cleaning, check that out.

This may not apply but I also used to have an older 70 hp mercury on 17ft boat.  The motor would not accelerate to full speed if I shifted into gear with rpm's too high. It had separate controls for gear and throttle.   I needed to have rpm very low in neutral then when I shifted into gear the motor accelerated to full power with no difficulty.  Some kind of linkage safety deal.  

Are you able to accelerate in reverse??  If you can it's not electrical or carb.

Good luck

With everyone’s help I am feeling much more confident that it is all carburetor related and I will start looking more into them. Thank you for everyone’s help so far. 

I have a 1986 120 Johnson was having the same problem.  I found a broken pick up line in one of the carburetors.  Rebuilt the other 3 carbs ond got a different one on ebay for 30 buck 

Welderguy, thank you. I will try that. Brent, when you mention coil pack, is that one coil for 4 cylinders?  Mine has an individual coil for each cylinder. I found one that was not giving any output and replaced it and am getting spark at each plug now. Is there something before the coils that would send the signal to each one that I am missing?  I am mechanically inclined when it comes to cars and motorcycles but boat engines and 2 strokes are new to me. I’m not sure what exactly to look for and what would be different. The muffs and under load is out of my wheel house. I got it running great until in the water lol. Cars either run good or they don’t. Lol. I appreciate everyone’s input so far. 

Had a very similar problem with my 1984 Evinrude 70 triple , turned out to be one of the 3 carbs were gummed up and needed to have kits put in . Once I rebuildt the carbs the motor ran like a top . Even though you ran sea foam through it , it sounds to me like a you  may have a carb problem. The other thing you can do is get a couple of spark checkers and run the motor to see which one is not firing good to eliminate that . I would get carbs done either way because of the age of the motor  , I did mine myself , with the help of a manual I found at the local library.  

I should add, that I used to have an Evinrude intruder 150 hp, and the power pack was intermittently failing. 

Sometimes I was running on four cylinders, and sometimes two...

I am guessing that is the case here, when you’re running your motor in the driveway, there’s absolutely no load on the motor...

Rule of thumb is that all cylinders should be within 8-10% of each other. Although you are not within that threshold, I don’t believe that is your problem, as all of your cylinders are above 100 psi. 

I believe you are likely running on two cylinders. It’s tough to diagnose on the Internet LOL, but I wouldn’t be surprised if you have a bad power pack. 

My guess is that you have a couple of cylinders that are not firing, and just going along for the ride...

If u can take the bowl off u should then be able to unscrew the jet. Maybe soak it in some carb cleaner then run a small wire through it then spray it some more with carb cleaner. Gas gums up from sitting a long time especially 2-stroke gas. 

It is running on all cylinders after I replaced the coil. Do I have to disassemble the carb to clean that jet?  I was hoping the sea foam would have cleaned it without takin them apart. 

Running in the driveway with muffs is much different than when it’s in the water with a load on it. I would start by checking and cleaning the high speed/main carb jet. 

Run it on muffs.  

One by one, pull a plug wire and make sure you ground it.  Start up the motor and listen for a difference.  Once you find no difference....you know that cylinder(s) are not firing.  I am guessing you are only running on 2 cylinders right now.