yamaha 90 hp fuel problem

[chompin]

when i start the engine it run's ok on idle and also on high rpm after the fuel tank reach half or
less of the fuel quantity the engine starts to go unbalanced until it's shutdown and it not hold idle also .
i try to clean the carburetors and change the fuel tank ,i verify that the fuel pump flow is ok (on idle ) but it's stil the same.
the fault comes and go whan fuel tank reach half or less of the fuel quantity
what could it be ?

[Ken]

Noam,
When this happens, check your primer bulb. If it is collapsed, then there is a restriction from that point on back to the tank. Look for a kink in the fuel line, or maybe a bad vent hose not venting properly and causing a vacuum in the tank.
You could check poor venting by unscrewing the fill cap and listening for air being sucked in.
You could also have a bad fuel pick-up tube in the tank. Remove it and examine to see that the pick-up end still goes to the bottom - maybe a hole halfway up.
Also, check to see if there is an anti-siphon valve on the connector at the fuel tank just in front of the fuel hose to the motor. Sometimes these valves stick and will not let fuel pass at higher speeds - but there should not be any correlation to the tank being half full.
Good luck, and let us know what you find out [img]smile.gif[/img] ,
Ken K

[chompin]

thank's
i checked for vacuum causes and didn't find any
but i notice that the fuel tank filter is empty
when runing the engine on high rpm (the filter is empty and geting very weak streaming of fuel)
could the it be the pump that bringing air to the fuel system ?

[Ken]

Noam,
If that fuel pump is at least five years old, I would replace it.
Also, check all hose connections from primer bulb to carbs.
Check filter housing for cracks - sounds like air is getting into fuel line at high rpm when the pump demands more fuel.
Good luck [img]smile.gif[/img] ,
Ken K

[chompin]

the fuel pump is the original one since 1997
can i replace only the diaphragm or all the unit ?
the hose connections looks ok .

[Ken]

Noam,
I just looked up a 97 90 hp - the fuel pump assy is less money than it's individual parts - I would replace the entire assy:

http://www.boats.net/yamaha/partsyst...mponent=335093

Individual parts are available - but which ones do you replace ?
Let us know what you find out [img]smile.gif[/img] ,
Ken K

[chompin]

Dear Ken
thank's again !
i have another question, are you familiar of a case when fuel leak from the pump to the engine
through the hole that operates the pump ?

what i found out is that when the pump is disconnected from the engine and i push fuel from the tank through the pump, then fuel spilt from the hole out of the pump !
another phenomenon that i noticed is that the lower plug is very dirty (only the lower one totally destroyed)
and this after the all 3 plugs was replaced a week ago .
the bottom line i think fuel chamber from the pump through the hole on to the engine and upon
the lower plug,
any way i ordered anew pump and i will update you off the result .
here in israel a price off the pump 117$ ( triple then the U S

[Hamfisted]

Chompin sounds like you got it beat! What's your email and I'll send you a service manual for your motor.


Mike

[chompin]

chompin@walla.com

[Ken]

Noam,
If your fuel pump has a ruptured diaphram, then when you pump the primer bulb, you can actually pump fuel directly thru the fuel pump and into the crankcase, down the exhaust, and out onto the ground.
Even a small split in the fuel pump diaphram can dump fuel directly into the crankcase and cause problems.
Good luck, and let us know how you come out [img]smile.gif[/img] ,
Ken K

[chompin]

the pump was replaced and there is still pumping problem

[jb123]

run your motor with an auxillary tank and line to ensure its your pickup tube,lines,fittings,prefilters or junctions.

[chompin]

well,
there was another problem
after the pump was replaced fuel flow improved but not as good enough
i find crack in the filter and replaced it

[chompin]

the good news is that i finely find the problem
the bad news is that i dont have any idea what to do :
the problem is that the upper plug responds very very weak, and that is the cause for backfires and runs leap .

i change plugs .
cheked compretion and all three are good.
there is agood spark from the coil (the upper one was good all so ).
and the carbs all so was checked .
what else can i check

[jb123]

swap your coils and see if the problem stays at that plug or follows the coil. also, your motor has some age, i had an issue years ago with coil leads bleeding out, open all pre-coil wires,run it at night and look for any errant spark.