Z250 Yamaha loses power above 3500rpm

[Bigtasty]

It starts, idles and runs up to 3500 perfectly. If I push the throttles further rpms drop to 2500. If I back off it will pick up and run fine again at 3500. I've changed the racors, the filters in the water separators and cleaned those in the vapor separators. I've noticed that there is air in the plastic water separator bowl (about 1 inch) while running at idle or speed. I changed to another fuel supply and it does the same thing so it's not pulling air in from a leak in the fuel line. I don't recall seeing air there before. Before I changed the filters it was doing this on the port engine, now with the new filters it's doing the exact same thing on the starboard. (both engines have air in the water separator now) Thoughts?

 

[cousinabe]

Re: Z250 Yamaha loses power above 3500rpm

Sounds like it could be the low pressure fuel pumps.
If I understand your post, you've cleaned the VST and filter inside.
Might not be filling the vst fast enough under a load
What is the full model number? When were the fuel pumps last changed?

 

[zzzzz]

Re: Z250 Yamaha loses power above 3500rpm

being a fuel injected motor,it has a vapor tank with a high pressure pump in it...the pump has a fine mesh screen/filter on the bottom- if you have any kind of residue in the bottom of the VST giving it throttle can suck the residue up on the screen and restrict the fuel flow

 

[Bigtasty]

Re: Z250 Yamaha loses power above 3500rpm

They are 2003 Z250TXRBs. VST screens were cleaned. I've never done the low pressure pumps. I have 500 hrs. on them.

I am curious as to whether the airspace in the water separator is normal. It seems to me that the low pressure pumps should pull that air right on through.

 

[cousinabe]

Re: Z250 Yamaha loses power above 3500rpm

It's normal to have some air in the engine mounted filters.

I am guessing LP fuel pumps. I change mine every three years as a preventative maint item.

Here are two good posts on fuel restrictions with HPDI motors that may be helpful.

http://forums.iboats.com/showthread.php?t=144600

http://forums.iboats.com/showthread.php?t=257011

Also, there is a filter between the VST and high pressure pump. Part# 18.
It's rare, but sometimes the fuel pressure regulator screen needs to be cleaned as well. Part# 13.
http://www.boats.net/parts/search/Yamaha/Outboard/2003/Z250TXRB/VAPOR SEPARATOR 1/parts.html

 

[rodbolt]

Re: Z250 Yamaha loses power above 3500rpm

on the HPDI the lift pumps simply keep enough fuel in the VST to feed the medium pressure pump which feeds the high pressure pump.
there is a filter before the lift pumps and yes its normal to see an inch or so of air in it.
there is a screen in the VST and a filter between the VST and the high pressure pump.
the water seperator in the hull should be serviced every fifty hours dont wait till it passes trash to change it then its to late.
the engine mounted filter should be checked every ten hours for sediments or water.
the VST and medium pressure filters need to be serviced at 200 hours OR yearly regaurdless.

once the debris is in the system the only way to clear it is physically removing it and replacing the non servicable elements.
there is a screen in the high pressure pump but its non servicable.
you need to either run it with the laptop, the winky blinky or a DVM and check high pressure fuel as well as medium pressure fuel and fuel system vacum at the lower lift pump intakes.
if it is a lean running issue and you continue playing with it you will post about detonated pistons no question about it.
the Z motors are run at the ragged edge of lean to start with and wont tolerate much of anything going wrong and they will detonate a piston.

 

[Bigtasty]

Re: Z250 Yamaha loses power above 3500rpm

Finally fixed! The screens in the pressure regulators on the side of the vst were clogged with crud and were putting out too much pressure rather than no enough. Now if the weather would cooperate...

 

[rodbolt]

Re: Z250 Yamaha loses power above 3500rpm

to much pressure?
kinda hard to belive.
the high pressure side of that fuel system runs about 1100 PSI. way more than the medium pressure side is capable of.
I still think erither the VST filter or the medium pressure inline filter is clogged.
medium pressure side only runs about 50 psi.