Rodbolt 17 please help!!

[dorado42]

You had replied to an earlier post about problems with f225 yamaha. The motors run great when I leave the dock, then I run 30 miles offshore, with no problems. When I am drifting (fishing) and then go to start motors, they start and then when I take off, the motors come up to desired rpms, then about 5-7 seconds later, the motors fall back down to about 1200 rpms. If I go pump the primer balls the motors pick up, but the ball seems to stay flat at first when i squeeze it. The motors got to the point they would not idle if I moved a few miles to another spot, and tried to idle for a few minutes before shutting them off..I have replaced primer balls, anti-siphon valves, racor filters, fuel lines, low pressure pumps. Both motors act the same, and have their own fuel tank, fuel lines, racor filters etc. I found a check valve when i removed the vst tank to check the filter (spotless) but i am uncertain what the check valve on port of airbox is....can give me a part number for the valve so i can look at on the boats.net website....also do you have any other ideas why the motors are acting such....PLEASE HELP....going crazy chasing this gremlin

[Bilge Rat]

The primer bulb not inflating sounds like a restriction between it and the tank.
IF you have replaced hoses, bulb, anti-siphon etc. Check the tank vent and fuel pickup.

[dorado42]

I have pulled the pick up tubes...clean filter on bottom of it....vents seem fine...just added 50 gallons and plenty of air coming out of vents...even tried loosening cap when problem occurs and there was no change...racors have probably 30 hours...they had clean gas and no water in them...can they clog this fast...just emptied them again and same ( clean fuel, no water)....still no clue

[rodbolt17]

the check valve at the air box ,upper port side, is for VST venting.
the one that you want to check is between the lift pump inlet and outlet on the VST.
they look the same but have different pressure ratings.

remember below 1200 RPM the lift pump runs 10 seconds on 20 seconds off

[dorado42]

Rodbolt17....that is the one i found....but was unsure about how to verify it was good...any ideas....if i place a clear hose on fuel line going to the primer ball from tank...should the clear hose be totally full of fuel or does it only fill when the low pressure pump cycles on (while boat is idling on trailer)...thanks for your help....you seem to be the man with the right answers

[rodbolt17]

as its going to suck fuel for 10m seconds then not for 20 seconds your going to see it do stupid stuff.
if the lift pump check valve leaks it normally shows up as a VST running out of fuel at low speeds.

that valve is set at 10 PSI, if it leaks the lift pump simply starts recirculating fuel and not filling the VST.

[dorado42]

Well my problem does occur at low speeds.....I did not see any leaks on check valve when I checked the vet filter....can I check it another way or simply just if it leaks...it is an expensive relief valve to take a guess on....do you mean leaks fuel back towards tank or leaks outside the valve itself....

[99yam40]

it will not be seen on the outside.
When he said " if it leaks the lift pump simply starts recirculating fuel and not filling the VST", it sounds like it just dumps it back to the intake side of lift pump.
It should try to hold the pressure to 10 psi or less, anything over would just recirculate back

[rodbolt17]

I completely Disagree with capt solo.
IF you had a fuel restriction it would NOT behave well at or near WOT.
the faster the engine turns the MORE fuel it must suck with the lift pump.
as the lift pump is a single speed single stage pump it will try to pump the same amount regaurdless of engine speed.
that's why it cycles below about 1200 RPM.
helps keep the pump from running in a near stalled condition.

that's why at low speeds if that valve wont hold 10 psi the VST will get lower and lower until it finally runs out of gasoline.
its sole function is to fill the VST at about 10 PSI until the needle closes then it diverts the rest back to the lift pump intake.

if it leaks rather than pusing gas into the VST via that small hole in the needle seat it will start recircing. as it runs 10 sec on and 20 seconds off below 1200 RPM the VST eventually runs dry.
its absolutely normal for the filters on the suction side to remain about 1/2 full when operating.

[dorado42]

Thanks rodbolt17 for all your help, I really appreciate it....is there anyway to bypass the check valve just to see if that is definitely the problem....can I have the actual check valve checked by a certified master mechanic if I remove it from the boat....I agree with your comment that if the problem does not occur WOT then it should not be fuel restriction....I am just trying to avoid spending 250.00 for 2check valves, to test it out....but if you say that is the best option, I will roll the dice and try it....thanks a million "thank you"

[99yam40]

I would think measuring the fuel pressure would give you an idea if the check valve is leaking or not.
But I do not know for sure.

Thinking about it , would turning key on with out running motor cause lift pump to pressure up to 10 psi and stay there if no fuel is being used?

Is there something in the service manual on it?

[rodbolt17]

at key on the lift pump does not run.
its a real pain to test lift pump output pressure.
a quick test??
use a pair of needle nosed pliers and pinch the hose between the check valve and the pump intake T fitting.

like a water system or an electrical diagram, sometimes if you draw it on paper its easier to understand.

[Bilge Rat]

The OP says motors, not one of them. If both are doing the same thing the cause should be something in common to both.
Heat/vapor lock is common to both.
When the problem is found please tell us.

[dorado42]

I agree with the fact that is much more concerning due to problems with BOTH motors....I know people have commented about buying brand new primer balls and they were "bad" right out of the package....does anyone know if this is true about anti siphon valves....it is very hard to believe that ANY problem would occur at the same time in both motors....i have been using the same gas for the last 5 months (since I have owned it)....the racors have been emptied/changed three times and have NEVER had any water in them or barely any dirt/debris....am going to check for fuel restriction with vacum gauge next and probably try running it on seperate gas tank....will post when I know more...thank you ALL for your help!!!

[rodbolt17]

1 it wont vapor lock as the fuel in the rail is pressurized.
IT can percolate vapors into the air box creating an excessively rich A/F mix at the next hot restart.
easy to find simply open the throttles slightly and try again.
if it starts easily with the throttle cracked test the VST vent valve.

google the proper term for "vapor lock".

the misnomer vapor lock is tossed about way to much.

the later engines,mostly mandated by the EPA, were equipped with a charcoal canister and then a canister and a vapor shut off valve.

this stuff aint rocket science folks.


vapor lock occurs when fuel BOILS in the fuel line causing the pumps to not pump and carbs to boil over.
as the VST is water cooled and the fuel in the rail is under pressure I have never seen an EFI with a vapor lock.

why do you think its called a VAPOR separator?
its simply a miniature gasoline tank that supplies the high pressure pump with a steady supply of bubble/vapor free fuel.

its also why EFI automobiles typically have the fuel pump in the tank as oppsed to on the block.
fuel pushes way better than it pulls.

as fuel pressure increases so does its boiling(vapor) point.
however fuel under 6"Hg can boil at about 70*F.

see that little hose,ref #16, that is the return from the rails through the fuel cooler and back to the VST.
what the injectors don't use is returned to the VST on this system.
at higher speeds there is enough fuel moved that the cooler is not nessasary.
if it clogs not only will you lose the water tell tale but after about 40 min of idling it will boil the fuel in the VST,overheat the hp pump and cause a stall.

remember this pump is a blow through type pump. fuel enters at the intake,flows through the pump rotor, over the armature over the brushes and out the outlet side.
fuel is what lubricates AND cools it.