Logo
FULL SITE MENU

F70LA Running out of fuel

E

evinruderunner

New member
F70LA Yamaha running out of fuel after 8 minutes at idle or 4 minutes at 2000 RPMs. Have Replaced the low pressure fuel pump, same results. Replaced high pressure fuel pump with the new LP fuel pump still installed same results. while VST was off cleaned and checked out theVST. Reinstalled VST, Removed the fuel vent line, ran the motor and we had the same results. When the engine starts to fall off if I pump the fuel bulb the engine picks up and runs fine for the next 8 minute cycle. Not sure if this is a known problem or just something on this motor. Would like to hear your Ideas.
 
Check your fuel pressure against Yamaha’s specifications. If your fuel pressure is factory spec. that would rule out the high pressure fuel pump. When you had the VST off did you check the float level against Yamaha spec.? Fuel filter blockage? Fuel lines in tip top condition, no kinks or bends, clamps tight and not sucking air? Fuel tank vent is venting? You mentioned you checked the vent line but it can’t hurt to recheck. What is your primer bulb doing just before it stalls? I don’t own a Yamaha, I’m just tossing ideas out there to see if anything sticks.
 
High Pressure pump is 44 psi then drops down to 33 psi and holds good until it falls on its self.Fuel tank is venting, rechecked vent line and have blown out the vent line.The primer bulb is not getting sucked flat, its not rock hard nor is it super soft. going to pull the VST again.
 
I had a similar problem with my F50. The Solenoid valve that releases pressure from the VST wasn't releasing pressure fast enough. The pressure would build up to the point that the needle valve couldn't open and the VST would starve of fuel. Time would be about 8 minutes for the VST to run dry from full. After the motor sat for a minute and the pressuer dropped enough, the needle could open, fill the VST with fuel, and you say, cycle again. It would also cycle if I pumped the bulb. I assume pumping the bulb made enough pressure to force open the needle valve and refill the VST. An easy test is remove the vent tube that runs from the VST to the Solenoid value. It has to be the vent from the VST before the Solenoid. Or, as soon as the motor stops release the schrader valve on top of the VST. If lots of pressure comes out then the VST probably isn't venting. If there's no excess pressure and assuming fuel is going to the VST, then I believe it has to the needle valve sticking.
I also read an article of hot fuel causing excess vapour pressure. I use to lift my motor when I was using the electric. When I restarted the motor, the VST running dry would be more noticable. I believe lifting the motor drained all the water which acts like a heatsink cooling the motor and fuel reducing fuel vapour pressure. I think this was all related to the VST not venting properly.
Between relpacing the Solenoid valve and leaving the motor in the water when I'm using the electric, the problem has gone away.
Mine drove me nuts until I found it. Hope this helps.
If you can access a service manual, there may be a fuel curcuit diagram near the front that shows how the Solenoid valve controls venting. Mine had an excellent diagram that makes it easy to troubleshoot fuel flow issues.
 
evinruderunner said:
F70LA Yamaha running out of fuel after 8 minutes at idle or 4 minutes at 2000 RPMs. Have Replaced the low pressure fuel pump, same results. Replaced high pressure fuel pump with the new LP fuel pump still installed same results. while VST was off cleaned and checked out theVST. Reinstalled VST, Removed the fuel vent line, ran the motor and we had the same results. When the engine starts to fall off if I pump the fuel bulb the engine picks up and runs fine for the next 8 minute cycle. Not sure if this is a known problem or just something on this motor. Would like to hear your Ideas.
Click to expand...
Did you ever figure this out mine is doing the same thing
 
Jhunter90 said:
Did you ever figure this out mine is doing the same thing
Click to expand...
If his was running out of fuel for one reason and yours is running out of fuel for a different reason, then the solution for the other motor is not going to be the solution for your motor. Don't go throwing parts at a problem, even if Yamaha likes you to do that.
 
boscoe said:
If his was running out of fuel for one reason and yours is running out of fuel for a different reason, then the solution for the other motor is not going to be the solution for your motor. Don't go throwing parts at a problem, even if Yamaha likes you to do that.
Click to expand...
Im just at a loss with it, and have Replaced the same stuff he did. I ran it on a new auxiliary tank today and same results.
 
Do you have a fuel pressure gauge so you can see what the injector rail pressure is?

Do you have a vacuum gauge so you can see if there is a restriction to fuel flow?

Have you tried to have someone pump the primer ball to see if that changes the situation?
 
I have not ordered the adapter for the rail pressure yet that will be next I guess. Yesterday I put it in an auxiliary tank and it takes off and gets on plane running good and starves out off fuel , I pull the throttle back and have someone pump the primer bulb and off we go. As long as we pump every once in awhile it is good. I drained the vst tank and ran wide open for a long while and thought it was fixed and then it happened again. This motor had sat for along time the vst pump and filter are new , the water separator filter on motor is new and the unline strainer. Im wondering if the needle is getting stuck on the vst. I hook it up to my laptop and everything is normal and no codes
 
If pumping the primer ball keeps the motor running forget about the pressure gauge. Forget about the VST for now.

For some reason it appears that the lift pump is not doing its job of keeping the VST full of fuel.

I would be focusing on that. Could be the pump itself, it could be a restriction between the lift pump and the fuel tank, or it could be an air leak in the system.
 
Last edited:
boscoe said:
If pumping the primer ball keeps the motor running forget about the pressure gauge. Forget about the VST for now.

For some reason it appears that the lift pump is not doing its job of keeping the VST full of fuel.

I would be focusing on that. Could be the pump itself, it could be a restriction between the lift pump and the fuel tank, or it could be an air leak in the system.
Click to expand...
I put an auxiliary tank and a new low pressure mechanical fuel pump on it because I thought the same thing. And I used a hand operated vacuum pump and put vacumm on the line in the motor and it held vacuum. I can pump up the primer bulb hard and dont see any leaks with the motor cowl cover off
 
Back
Top