yamaha 115 totally stumped

[kory77]

i have a 1987 yamaha 115 2 stroke outboard. Main problem was that when it was at full throttle it would go for about 30 seconds then bog out like its not getting enough gas. i thought i could figure it out myself and have worked all summer on it. Here is what i have done so far...compression test good(130 on all) spark test good except on two it was questionable so i replaced two coils, replaced fuel pump,replaced spark plugs, timed motor, replaced fuel tank along with fill hose vent hose and supply line,replaced primer bulb, disassembled and cleaned both carbs, cleaned fuel filter.dont have equipment to check for vacuum leaks.. Now after all that it still does the same thing!!!! i feel very discouraged and a little stupid for putting all the money into it when i should of just taken it to a service shop to begin with but one thing led to another and i thought i was narrowing down the problem. i know now that its not related to the gas so its gotta be something in the motor.. the motor starts great and idles great. its almost like the bowls fill with gas then once its burned i have to throttle down to let them fill up again. wondering if this is carb related..everything looked ok when carbs were apart...please help...thank you..

 

[yamamarinetech40]

Re: yamaha 115 totally stumped

Well, guessing just won't work....can't trust what you've done without knowing these things....you say replaced tank-is this a portable tank or boat's tank? Have you pulled and tested/checked fuel pumps...these are very old. What do the plugs show? Did you buy engine with problem or did it just start happening with you? Alarms in working order? Have you tested the oil tank alarms and can see/hear warnings? How did you clean carbs EXACTLY....did you try to spray them out...be honest, it's your motor...have to be soaked and know what you are doing...ain't a Holley.

 

[kory77]

Re: yamaha 115 totally stumped

yes, i replaced the permanent under deck tank. i bought the motor and boat from a dealer thinking i was safe. i bought it in the winter and could not water test the boat and it had no warranty. the main problem i discovered was a deteriorated primer bulb and rubber bits in everything. carbs were removed and completely disassembled ,hosed down with carb cleaner and blown out with compressed air. the fuel pump i replaced and is new. alarms i am not sure about, i have an oil resevoir indicater light on my dash that shows oil level is good.

 

[Woodnaut]

Re: yamaha 115 totally stumped

Although I do not have an anti-siphon valve on my boat I've heard that they can be problematic. They can have an outward appearance just like a regular hose barb fitting, right? While replacing a tank and hoses, perhaps this fitting could be reused and be causing some of the problem. Am I making any sense here?

 

[Bob Gale]

Re: yamaha 115 totally stumped

Hi, had a simular problem with a Merc. It had a fiberglass fuel tank. I tore the carbs apart and they looked clean, I could see through the jets. After a couple teardowns i finally took one of the carbs to a mech, he looked at the jet and said run a drill bit through it. Turns out it had a build up from the delaminating fiberglass tank that was reducing the jet size. Runs great now.

 

[douglloyd]

Re: yamaha 115 totally stumped

No drills in Yamaha jets.

 

[SoFloStone]

Re: yamaha 115 totally stumped

Does your carbs have removable Needle Seats?

I just had a similar problem with a Force 120 that does not have removable Needle Seats, and over time at high rpms, the needle would rattle inside the carb and wallowed out the needle seat and over 4000rpm's the carb would flood out.

The Mechanic that worked on it mentioned that the Yamaha Carbs were similar, and did not have removable needle seats.

The wierd thigs was that the carb would hold 8lbs of pressure when it was re-built and off the engine.

I would take the boat out for a sea-trial withthe engine cover and flame arrestors off, and look down the carbs and see what is happening once you hit that magic RPM. If it is flooding out, you may have a similar problem and have to replace the carb.

Rick

 

[Jon D]

Re: yamaha 115 totally stumped

I have a similiar problem. I have 95 yamaha 115 2 stroke and she runs fine until you top out around 5500 rpms. Question what rpm does she putt out at also my problem was the temp sending unit or temp switch and for me it would trigger an alarm and the motor would slow down and i had to slow down and then i could speed up but could never full throttle for more than 30-40 seconds. Not sure if you have an alarm that might not work and its is triggering a over heat that might not be happening. that was my problem. also is the water hot that comes out of the water pump. just some ideas hope you find problem soon.

 

[kory77]

Re: yamaha 115 totally stumped

thanks everyone for the input. i dont know if i have alarms or removable needle seats?? the mechanic who has been working on it is at a loss so i think maybe my best bet would be to take it to a marine service shop. hopefully they can figure it out..it'll prolly cost me a leg though...if anyone has any easy diagnostic methods for the above suggestions, i would appreciate it. also if i do have to replace carbs,any suggestions on a good place to buy or how much it'll cost?? thanks

 

[SoFloStone]

Re: yamaha 115 totally stumped

Take the boat out, and look down the carb while it 'bogs out' and determine whether you are starving for fuel, or flooding the carb out.

 

[parrfive]

Re: yamaha 115 totally stumped

There are a few things you could try to eliminate some problem areas with some of the older Yamaha 4 cylinders. First thing to try is to eliminate the two fuel connections at the motor pan. Run your fuel line directly to the fuel filer inside the cowling. We have had lots of problems with those connections. If that doesn?t help try hitting the choke while at full throttle to see if the r.p.m picks up. If it picks up r.p.m while choking then its starving for fuel. The most likely reason is that the carbs weren?t clean enough. Resoak the carbs in carb cleaner and try again.

 

[kory77]

Re: yamaha 115 totally stumped

well i havent had it out to test yet ,but i do know that it is definitely starving for fuel as apposed to flooding out because when it starts bogging out you can pump the primer bulb and it goes like a bat out of hell. now to top it all off i have tilt/trim problems. damn thing will just stop then you have to tap on the motor and it goes again. now it barely goes down and wont go up at all.gonna try changing the fluid.

 

[tightlines]

Re: yamaha 115 totally stumped

sounds like your fuel line could be sucking air.replace any fuel lines older than 10 years with approved hoses. ethenol wreaks havoc on them.

 

[Woodnaut]

Re: yamaha 115 totally stumped

If you can work the primer bulb and get power back up then it is definitely starving for fuel.

- As already suggested, replace ALL fuel lines (100% with no exceptions).

- Fuel pump discharge and suction pressures would be very beneficial in determining the source of the problem.

- I am not familiar with that particular engine so I have to ask, what exactly is driving the fuel pump? A piece of hose connected to the crankcase, a mechanical linkage arm to the fuel pump, or is it directly mounted on a passage to the crankcase? If driven by pressure/vacuum pulses from the crankcase, is there something that is reducing (or thieving off) these pressure pulses? A cracked or leaking gasket between the fuel pump and the block, or possibly a restriction in the passage? Check the compression on this particular cylinder. There is the possibility that the fuel pump is not being driven properly/adequately.

- Double check any in-line fuel filters/strainers on the engine by back-flowing fuel through them. I had a terrible restriction in one on my Yam F90. It was causing a large pressure drop that only permitted a trickle of fuel through. I back flushed through it a got a LOT of trash out of the system.

- Do you have a fuel tank selection valve or a fuel shutoff valve? Are you sure it does not have any debris stuck in it? Could it be opening only partially instead of opening fully?

- What about the fuel tank pickup? Can you remove it, examine it and make sure it is clear?

- Finally, go looking for an anti-siphon valve at or near the tank. If you find one, then consider temporatily taking it out of the system to determine if this is contributing to the problem.

 

[Bob Gale]

Re: yamaha 115 totally stumped

Just a general note, for anyone needing a pressure/vacuum gauge. They are available for a free loan from the automotive chain "Advance Auto" and possibly many other chains. They loan out specialty tools at no charge with just a deposit.

 

[yamamarinetech40]

Re: yamaha 115 totally stumped

ok, we've gone all over the place here....let's get back to original point....I knew before you told me the carbs had been 'hosed down' or sprayed, but you have to SOAK them, you HAVE to, you can't not SOAK them...if a mechanic sprays them or 'hoses' them, RUN to another shop, and don't put anything but PURE copper wire smaller than whatever you are cleaning through any holes, jets,etc. Make very soft copper(can't enlarge things). Again, the carbs have to be SOAKED, and not for just an hour, at least two in good strong, environmentally unfriendly stuff, or overnight in the pansey stuff. Your trim is a bad ground probably, if you tapped on it to get it to work.

 

[douglloyd]

Re: yamaha 115 totally stumped

My guess on the trim is a carbon coated switch. I have to take mine apart and clean the contacts every couple of seasons. Keep your things simple it is a fairly simple and proven machine.