Johnson 120 won't accelerate past 10mph

[Jake Blood]

Hi all.. first time poster here and first time boat owner..

I just recently purchased a 1977 venture tri-hill with a Johnson 120hp on it. It also says V4 VRO on it.. if that might mean something to you..

When I got the boat, it ran great.. would reach about 40mph.. I was happy.
The guy I bought it from told me that for the 8 gallon tank I have, he adds one 16 ounce bottle of marine oil to it, and that is the correct mix. I ASSUME that is correct since it did run fine.

I have had this boat out about 5 times.

The secod time I brought it out.. It was with a buddy. We stopped and put fuel.. 3.9 gallons.. which cost 8 dollars. In his mind it registered as 8 gallons, and he put a full thing of oil. ( 16 ounces ).. the engine would not get over 10mph for that trip...

The next time out.. I put just gas.. about 4 gallons.. to compensate for the excessive oil he put in... The boat ran sluggish at first, but then ran great for the remainder of the third and 4th trip...

The 5th and last trip.. the engine is back to running sluggish.. It will sound smooth and run great up till about 9 mph.. as soon as I push it further it bogs down.. then after several attempts, it got up on plane.. for about 100 yards, then bogged down and would not go over 10mph...

When I got home, I removed the plugs and they were drenched in oil.. black.
I wiped them off, put them back in, checked the fuel filter.. clean as a whistle.. Not sure what to do next..

Someone told me there is a 'high speed injector' that might be pluggged, and that I need to take the carbs apart and clean them and put them back together.. etc...

One, I do not know how to do this... Two, what might that cost.. Three, is there some other lower cost alternative I can try first?

Thank you so very much for any feedback I might get.. as a first time boater I am starting to wonder if this is for me..

 

[ezeke]

Re: Johnson 120 won't accelerate past 10mph

edit

 

[ezeke]

Re: Johnson 120 won't accelerate past 10mph

The correct fuel mix is 50:1 87 or 89 US Octane to TC-W3 oil. That is approximately 1 pint to 6 US gallons. (128/50 = 2.56 ounces X 6 gallons = 15.36 ounces of oil)

The extra oil is probably not the cause of your immediate problem. Why don't you beg borrow or steal another tank, get some fresh fuel mix and run the motor. Be sure to use Carbon Guard or similar carbon treatment in your gas all of the time. Also add StaBil or other fuel stabilizer to your mix when you add gasoline unless you can use it all rather quickly.

If the VRO on your motor is still functioning, the oil should be coming from an oil tank and mixing automatically.

 

[Jake Blood]

Re: Johnson 120 won't accelerate past 10mph

hmmm.. How could I tell if the VRO is still functioning? Should I just assume it is not since I haven't added oil to it, so it undoubtedly has no reserve to pull from to mix it?

I can certainly do all the other steps and I thank you so much for responding.. This stuff seems like rocket science to me.. hate it.

 

[ezeke]

Re: Johnson 120 won't accelerate past 10mph

A lot of VRO systems were converted to premix pumps over the years and there would therefore be no separate oil tank if that were the case on your motor.

Keep adding the oil to your gas until you know for sure.

This will help and it is worth a few minutes of your time to read it: http://continuouswave.com/whaler/reference/VRO.html

 

[Brew2]

Re: Johnson 120 won't accelerate past 10mph

VRO didn't happen until the mid 80s, so your motor is newer than your boat.

Although I would highly recommend purchasing a service manual and following the instructions to remove and thoroughly clean each of the carb assemblies, if you want to try just checking/cleaning the high speed jets you could do the following...

Remove air box from the front of the motor (has about 10 bolts holding it on). This will expose the four carb assemblies. Each of the four has a large plug at the bottom of the assembly (the bowl) that can be removed with a large flat head screw driver. When you remove the plug, you should get what ever gas is in the carb flowing out (ie. have a cup or something ready to catch what you can and properly dispose of). Inside the opening that you removed the plug from is the high speed oriface (jet). This can be carefully removed with a flat head screwdriver, however, be very gentle and do not "force" anything as the jet is brass and can be easily damaged. The jet is essentially a threaded brass plug with a hole in the middle of it. The hole can become blocked or partially blocked preventing gas from getting to your cylinders. Clean out the jet with some aerosol carb cleaner and reassemble everything being very careful to not damage the jet when you screw it back into the carb.

As noted earlier, the proper thing to do is disassemble each of the carbs and give them thorough cleaning. This was the first "repair" I ever did on my motor and it was surprising easy as long as you had the service manual to follow. Having said that, I quite enjoy tinkering with these motors and figuring out how they work. If your comment of "hate it" is really the truth, I'd spend the couple hundred bucks to have someone clean the carbs for me.

The other simple thing you can look for is either a loose connection in the fuel system (follow the fuel hose and check each connection) or a kink in one or more of the fuel lines (typically the smaller fuel lines that feed each of the carbs).

Good luck!

 

[Jake Blood]

Re: Johnson 120 won't accelerate past 10mph

Thank you both very much.. I am going to go outside and try this now..

I will say 'Hate it" was a strong reaction.. I lov e it when I am cruising on top of the water and all is good.. but of course.. grrrr.. when I am not..

I have disassembled many 4 wheeler carbs, so I know the terms.. but I have never messed with a boat engine... I think this sounds simple enough though..

I will let you both know.

 

[Jake Blood]

Re: Johnson 120 won't accelerate past 10mph

Oh, and yes, the motor is a 1988... That would help right.. lol..

I was also told to try something called Sea Foam.. mif 3/4 can with gallon of gas and run it? The can says 1 ounce per gallon, but maybe that is just ongoing and not for unclogging ... thoughts?

 

[Jake Blood]

Re: Johnson 120 won't accelerate past 10mph

OK, first thing I noticed is fuel and oil mixed in the ai cover when I took it off... about two teaspoons.. normal? I will check the rubber hoses, but maybe if the jets are clogged, the fule that couldn't get through overflowed a bit ?

Be right back..

 

[Brew2]

Re: Johnson 120 won't accelerate past 10mph

Seafoam can certainly help, but it is no replacement for disassembling and cleaning the carbs.

If you've worked on carbs before, the ones on your motor will seem pretty straight forward. Basically, just a float with a needle valve and three jets. High speed jet at the bottom of the bowl meters fuel....low and intermediate jets meter air. The low and intermediate jets are on the front of each carb body and easily visable.

Good luck!

 

[Brew2]

Re: Johnson 120 won't accelerate past 10mph

Couple other thoughts....

If you have it idling, pull a plug wire with a pair of well insulated pliers....you should hear the motor rpm drop. If it doesn't, you got a problem with that cylinder (eg. no spark, no fuel, etc.). If rpm does drop, replace the wire and move on to the next cylinder.

While underway, just prior to accelerating and getting the bog, bump the primer (push in key) a couple times. If this overcomes the bog, then you definitely have a fuel delivery problem. You have to do it all fairly quickly though (ie. push key, push key, accelerate immediately).

Cheers.

 

[ezeke]

Re: Johnson 120 won't accelerate past 10mph

Don't pull plug wires on a running V4 looper, you will blow your powerpack. Use an inductive timing light instead.

 

[Jake Blood]

Re: Johnson 120 won't accelerate past 10mph

Pulled the carb plugs.. pulled the jets.. there might of been a little bit of junk, but I think it was just oil.. sprayed them out, all clean.. guess the next thing to do is try her out.

 

[scrampbell]

Re: Johnson 120 won't accelerate past 10mph

How did it go? All fixed up?