2000 Yamaha 115 four stroke low RPM

[FUNCAT-YAMAHA]

Help!
I have a 2000 115 four stroke that only revs up to 3300-3600 rpm.
I have changed fuel filters and the plugs (very black)
I have also noticed just recently fuel dripping out of the air intake box when the engine is running.
I have 38PSI fuel pressure at the fitting on the top of the fuel rail. I believe this is too low.
From all I have read should I be looking at the VST? Am I on the right track.
Thanks for any help.

 

[loyoki]

Re: 2000 Yamaha 115 four stroke low RPM

100% yes, perhaps pump filter is clogged but it seems you have a problem with float in the VST, perhaps it is wedged.

 

[FUNCAT-YAMAHA]

Re: 2000 Yamaha 115 four stroke low RPM

Thanks for the response.
I am going to take it apart this week end.
I will let you know what I find.

 

[FUNCAT-YAMAHA]

Re: 2000 Yamaha 115 four stroke low RPM

I replaced the filter at the bottom of the fuel pump in the VST and now it is worse.
The boat will barely idle and forget about reving up in the water.
The fuel dripping out of the air bax has stopped, so that appears to be fixed.
The fuel pressure at the top of the rail is now down to 24 psi from 38 psi, I believe both of these to be too low.
What could I have messed up now?

 

[rodbolt]

Re: 2000 Yamaha 115 four stroke low RPM

not much.
when you turn the key off how fast does the rail pressure drop and how low?
I just did one that the ECU was bad, held the injector on #3 on all the time.
the yellow red wires on all 4 injectors have 12V anytime the key is on.
with the key OFF check the resistance of all the other colored wires, with that injector unplugged, to ground. should see better than 18K ohms.
at 6 kohms the injector will stay ON.
you can also remove the rail. wire tie the injectors to the rail, turn the key on and see if any leak.
have a fire extinguiser handy just in case.
gasoline is very flamible and at 45 PSI even more so.

 

[FUNCAT-YAMAHA]

Re: 2000 Yamaha 115 four stroke low RPM

I believe the fuel pressure went up when I shut the engine off.
Do you think I should be looking at the injectors or is there still possibly something wrong, or I screwed something up, in the VST?

 

[FUNCAT-YAMAHA]

Re: 2000 Yamaha 115 four stroke low RPM

I am still having trouble.

I do not know where to go from here.
From the beginning. I bought a 2000 yamaha 115 four stroke used and
it ran smooth and quiet as I would have expected from a four stroke However, from the start the most I could reach was 4200 rpm. As the season went on it grew worse until it barely ran 1500 rpms and I barely made it back to the dock.
I have hooked up a remote fuel tank to rule out bad fuel.
I have cleaned out the VST tank 2 times and replaced the filter at the bottom of the pump.
Symptoms at this time are fuel dripping out of the air box.
Fuel pressure at the fuel rail is 38-40 at idle.

I realize the dripping fule could be a bad float and/or needle seat.
Do the pumps go bad? Would the pump work at a lower pressure or would quit working completely if it was bad. Besides filters what could cause the low fuel pressure?

 

[rodbolt]

Re: 2000 Yamaha 115 four stroke low RPM

you MUST test rail pressure at the problem RPM or the test is useless.
have you done a leak down test on the cylinders?
rule out a bad intake valve blowing fuel out the air box and if fuel is still dripping its gonna be a VST needle valve leaking.