2003 yamaha f 115tlrb

[George J]

Hey Guys, thanks in advance for any help on this matter, 2003 F115 tlrb. Motor idles fine and runs up to 3k rpm. Anything above that it just bogs out. I have replaced the high pressure pump, the filter, cleaned the vst tank. The pressure at the rail during idle is about 35psi. it will jump up to 40-41 when reved up a bit. Just got back from running it on the water with the gauge hooked up. guage reads anywhere from 35-40 up to 3k rpm at 3k it drops down to 20. all filters are new all lines have been cleaned/blown out. Any ideas? Thanks

[fairdeal]

Gauge reads anywhere from 35-40 up to 3k rpm at 3k it drops down to 20.

Since you've already replaced the screen inside the VST, and the pump,
seems the problem is not getting fuel into the VST fast enough to keep up.

You didn't happen to try squeezing the primer bulb while running,
to see what difference that made?

in any case,
seems the performance of the lift pump is handicapped for some reason.
Could be within the pump itself - diaphragm, check valves

At some point the F115 got an in-line screen between the lift pump and the VST -
somewhat disguised as a thick piece of tubing -
do you have that? check that?

you might plumb in a gauge between the lift pump an VST -
see what you get while running
at idle, I think you should see around 7 psi once the VST is full.

Air leak is a possibility -
a piece of clear tubing patched in, just before the lift pump inlet,
can let you look for bubbles.

A vacuum gauge in the same place
will tell you how hard the lift pump has to pull while running -
should be very low, almost nil.

[George J]

Thanks for the quick response Fairdel. So the lift pump is brand new. The motor is new to me this year. The high pressure pump let go in the middle of the ocean with my children on board with me. That turned into a 4 hour tow. So once i got back i replaced a few things to try a prevent that again. the lift was one of them. Ok, so i found the inline filter underneath the rubber boot. i removed the filter and patched in a union with a schrader valve in it. I'm getting 10 psi between the lift and vst and still 35psi at idle at the top of the rail. I think i know the answer to this but theres really no other way to test this unless it's in the water and under a load?

[fairdeal]

Ok, so i found the inline filter underneath the rubber boot. i removed the filter and patched in a union with a schrader valve in it. I'm getting 10 psi between the lift and vst and still 35psi at idle at the top of the rail.

35 psi on the rail at idle is OK -
by design, the regulator lowers the pressure slightly due to manifold vacuum
as at idle, with the throttle valves closed.

10 psi between the lift pump and VST sounds good.

If you had not previously checked/changed that in-line filter -
that could be the issue.

But with a mechanical lift pump as on that engine,
I don't know of a way to check its full output, without actually running the engine under load.

[99yam40]

Yamaha makes test wheels for motors to test in tanks.
loads up the motor without having to be running across the water
might see if a shop near you has one for your motor

[George J]

Thanks for everyone’s responses. Looks like I’ll be doing an in the water test. One more question. Should they’re always be air coming out of the Schrader valve on the top of the vst tank?

[99yam40]

what do you mean by always air coming out?
the valve should seal when not depressed.

the float/needle and seat should keep liquid from getting high enough to come out

[George J]

Sorry. When pressed should you always have air that comes out?

[George J]

Sorry. When pressed, should there always be air that comes out?

[fairdeal]

One more question. Should there always be air coming out of the Schrader valve on the top of the vst tank?

hmmm.
I would say - "no".

If there is discernible pressure in the 'head space' of the VST
I wonder if the VST venting is clogged -
and - perhaps -
preventing the lift pump from being able to keep the VST filled.

There are actually two vent hoses coming off the VST cover,
that get Tee'd together -
and then there is a spring-loaded valve - see # 36 below.

The point of the valve is to prevent 'excessive' venting of the VST -
but if that valve is 'clogged' or 'stuck shut'
perhaps you are getting "no" venting

I would disconnect the vent hose at that valve
and then with a double-ended hose barb - or similar simple piece of thin tube -
extend the venting hose out of the cowling.
down through a drain hole - or out the alternate trim switch cover.

Then see if that makes a difference to how the engine runs.

[George J]

Thanks for all of the help guys. This is a great forum. I’ll give that a shot tomorrow and report back.

[99yam40]

hear is a question for those that know this stuff, is that shrader valve just for letting in air when draining the VST?
in other words, not there for checking any pressure?

[fairdeal]

hear is a question for those that know this stuff, is that shrader valve just for letting in air when draining the VST?
in other words, not there for checking any pressure?

Correct - on a four stroke.

but apparently on the Yamaha FI 2 strokes,
a valve in a similar location had pressure on it.

Never cared to understand the detail of that.

[99yam40]

thanks, never really thought much about it before.
on the 2 strokes it must be plumbed up to the inlet side of the needle & seat to read pressure to the VST or is it to the high/injectors