1999 ox66 225 vst tank won't stay full

[family affair]

One LP pump was bad and we replaced all 3. We can use the primer bulb to fill the the VST, but after the engine runs at low speed (<1k rpm) for 5 minutes the VST tank is empty and the engine dies. I also noticed that the fuel level in the clear filter housing drops about 1 inch from the top as this happens and then stabilizes. I never noticed this before.
I can keep the engine running indefinitely if i keep refilling the VST tank with the primer bulb. The bulb seems to pump normally and does not become deflated.
The engine also seems to run at an abnormally low RPM. Neutral 600, in gear 4-500.

Any ideas?

 

[seasick]

everything points to a bad lp pump. Are the replacements Yamaha or OEM?

 

[family affair]

The problem existed before the pumps were replaced.

 

[1st grady]

Are you certain that you are sealing the clear fuel cup completely? There should only be one thread showing at the top of the plastic nut. I had this problem several years ago. Remove the cup and let it dry in the sun for about an hour, it will shrink and allow you to insert the cup all the way into the housing. If the O-ring is not seated against the housing, it will suck air and the engine will stall.

 

[family affair]

We're double checking everything, but i don't see how that would suddenly become a problem.

Should the cup stay completely full at all times or is it normal for the fuel level to drop about an inch from the top while running?

 

[1st grady]

The fuel level will drop about an inch like you have noticed. Make sure that plastic cup nut is all the way up. The cup does swell from the ethanol and will not fit back into the base (at the top). Did you take the VST tank apart? Are you certain that you got the o-ring back into place?

 

[family affair]

We never took the VST apart.

Explain this:
After installing the LP pumps, we started the engine and it ran mush better than before. We then put the boat in the water, it ran great, then died once the VST tank was empty. Today we checked other possible sources for issues and came up with nothing. My Dad checked the LP pumps and they were all loose! I had tightened them all down to the piont that I was concrened about doing damage on initial installation. Somehow running the engine caused the pumps to seat further onto the powerhead requiring retightening of the pumps. We then ran the engine for ~15 minutes and no issues... for now.
I'll report back if we are still having issues.

Thanks!

 

[BobP]

The fuel level in the VST is maintained like the level in a carbureator, by a float and needle valve. I presume all is working correctly and not sticking? How about back pressure, any restrictions in fuel source?

Email Andy at SIM and ask him what he thinks.
SIMYAMAHA.com
(I believe).

 

[Tucker]

The low pressure pumps need to be tight or they won't work. They need engine vacuum to work. Sounds like you found the problem. To check pull the discharge line off the VST and crank the engine with the emergency stop pulled. Fuel should spurt out in pulses.