Mystery Problem on 115HP ETLD

I have a 1990 Yamaha 115HP ETLD motor that starts great, idles great, but when it hits 45-4600rpm, it starts to cough and stutter and falls back to 38-4000 rpm. It has run WOT at 5600 rpm.

Details as follows:

Boat = 19.5' Bayliner Capri

Motor = Rebuilt (3 yrs ago) 1990 ETLD 115HP

Work done = Plugs and wires have been changed
= Carburaters have been cleaned and checked three times
= Fuel pump replaced twice
= "O" ring on fuel filter replaced
= Primer ball and hose replaced
= Spark present on all cylinders
= Compression check shows 140psi on all cylinders
= Vacuum check on fuel pump showed 4lbs pressure until
boat hit 4600 rpm and then it falls to 2lbs pressure.
= Motor checked for air leaks with acetyline, but nothing
found
= Motor was disconnected from boat fuel system and run
off a jug, same problem at 4600rpm

The only time it runs to 5600 rpm is if someone squeezes primer ball while motor is running.

Yamaha tech suggest changing the crank seals, but I cannot afford to put any more money into this motor with no guarantees it will fix it.

Help?????????????????????????????????????????????

[neverenoughx2]

I had a problem like that and was convince it was fuel went through everything then I invested in the manuel the yamaha manuel $80 not after market and after reading it I started to believe it was the cdi. I fortunatly had a second motor and I swapped the cdi however while I was swapping them i found a ground wire loose that connected to the head and sure enough that was the problem. Take a look and see the manuel tells you how to test it with a multimeter and you will use that manuel all the time it is fantastic you can litterly rebuild your own motor with it (well worth it). Sorry thats all I got wish I had more keep plugging away.

[jb123]

air leak...?

[boats.net]

air leak...?

Yes if pumping primer bulb keeps it running there may be an air leak in fuel hose, fittings. Check oring in engine fuel filter cup.

"O" ring was missing on fuel filter, and mechanic was excited when he found it, but to no avail.

Out of frustration, I had them wire a Mercury electric fuel pump in place of the normal one. This pump put outs 5.5 lbs pressure constantly.

PRoblem still there and I'm sick of it.

[boatn]

clogged carburator jet?

[boats.net]

Does it still run good when pumping primer bulb?

Carburaters have been taken apart adn sprayed three times by the mechanic and he says they are really clean.

Once the electric pump was put on, I didn't bother pumping the primer ball. I will have to try that.

[GlennD]

I had a similar problem a few years ago and it turned out to be my gas lines running from the tank to the primer ball had small air leaks. Once I replaced the lines problem went away

[jb123]

Motor was disconnected from boat fuel system and run
off a jug, same problem at 4600rpm


this should indicate the issue is motor side...IF it was done correctly....did you use an NEW hose and ball hooked to motor filter ???

Also you COULD have issues with the hoses AFter lp pumps.

18 yr old hoses and clamps start to go bad.

I also had an issue with an '89 I had whereas the coil leads, not plug wires , were bleeding out...causing a highend miss,i just shrink wrapped em.

At this point, I am going to change the fuel line right from the tank all the way into the motor. I will also change the primer ball and fuel/water separator and fuel pump again.

It has been suggested to bypass the fuel filter on the motor itself as the fuel/water separator will do the same thing.

I will also try shrink wrapping the coil leads, though I have to wonder how you could even diagnose something like that.

[jb123]

still sounds like its not getting fuel at higher demands.