fuel pump or fuel filters been checked

Took the boat out this past weekend to Havasu. Took a ride up to pirates from the landing and boat ran great all the way up there (was running at about 3200 RPM with an occasional pedal mashing). On the way back to the lake I switched tanks. Id like to add that I've checked all pickups and valves for suction on my tanks. The boat will start to bog down and I can pedal through it for a little bit but eventually it dies out on me completely. I can get it started after it stalls but it does the same thing. I've had this issue since I rebuilt the boat and have tried everything that makes sense to me. Anyone have any input?

I'm feeling one of three problems. Bad fuel hose (inner line collapsed), inadequate tank venting (pulling a negative on the tank) or garbage in the pick up (may have a small screen on the puck up tube that's full of shit).

Good hunting.

GT

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You could have a flapper.  The inner liner tears as the hose is assembled onto the fitting.  It then acts like a check valve when the flow is high enough.  Then when the flow drops, the flapper returns and so does the flow.

To try and pinpoint this to a fuel delivery issue, did you look to see if there is fuel in the carb immediately after it stalls?  Do you need to crank the starter a lot to get it to restart like the pump needs to reprime the carb?

Is the fuel filter on the suction side of the pump or the pressure side?  I see a lot of issues with filters creating too much restriction on the inlet side of the pump, actually starving the pump for flow.  However, you indicated that the problem didn't happen until you switched tanks AND only after replumbing the boat.

Check the hose connections between said tank and the switching valve.

If it is a venting issue, it recovers before you can investigate or try to restart the engine.  That means there is just enough restriction to stall fuel flow.

A little joke from my pump systems class; pumps don't suck, they blow!  That means all pumps work better on the pressure side than on the suction side.

Fuel is pushed into your fuel pump by atmospheric pressure inside your vented fuel tank and a reduction in pressure created by the fuel pump.  In order for flow to occur, this differential in pressure must exist.  If no air can enter the fuel tank, as the fuel level drops, the volume above the fuel level increases, and the pressure inside the tank drops.  Once the pressure (or lack of pressure) in the tank drops, the vacuum in the tank equals the vacuum on the suction side of the pump, flow stops.

There is probably just enough restriction on the vent to where once the engine stalls, atmospheric pressure returns.  Fuel is being consumed faster than air can flow into the tank.

So you think my best bet is to tap some vents in the filler cap somehow?

If I am understanding you properly, It sounds like your problem follows the switching of the vents AND while operating on the good vent, there is no ussue.  The problem will most likely manifest itself only when using high volumes of fuel for extended periods of time.  If that is in fact true, repair the vented cap as they were intended to function.

Kinsler has the following to say regarding fuel tank vent size:

The tank MUST have a vent.  Large amounts of fuel volume are  being removed from the tank.  This volume must be replaced or a vacuum may be formed inside the tank, causing the fuel to be ‘locked’ inside.  Scenario – a straw in a drinking glass:  put a finger on the end of the straw and lift it out of the liquid; the straw retains the liquid until the finger is removed and allows air in.

NOTICE:  Fuel tank and cell manufacturers may NOT offer a tank that is properly configured for your application.

Too large of a vent will not hurt anything, too small may cause the pump to cavitate, resulting in an inconsistent fuel supply or damaged engine parts.  Remember when altering a vent by adding length (hose), installing a roll-over valve, or a filter, the vent may require a larger inside diameter (see ORIFICE THEORY on Pages #202-203).

MINIMUM ALLOWABLE TANK VENT SIZES:

For pump sizes of -00 upto -1  3/8” (.375) inside diameter or bigger.
For pump sizes of -2 upto -4  1/2” (.500) inside diameter or bigger.
For pump sizes of -5 and bigger  3/4” (.750) inside diameter or bigger.

NOTE:  When running multiple pump systems multiply the area of the above vent sizes by the quantity of fuel pumps.

Also would it behoove of me to get a better fuel pump seeing that I probably have the minimum in my boat now?

I would suggest, once you get your vent issue sorted out, install a fuel pressure gauge near your carburetor.  Have someone observe the gauge while you launch hard and keep your foot in it for a minimum of 15 seconds.  If your current pump can maintain 5-7PSI (with a Holley) and your Air Fuel Ratio (AFR) is within reason, I would say you do not need to upgrade.  If the pressure falls off (like it does right now), I would suggest that you find the cause of that before blindly replacing parts.

Good luck,

Joe

Okay great! Thank you for your help. Now a quick "what if" question. Lets say it is venting perfectly and the fuel pump is not the problem. Can there be an issue with ignition of some sort?

I think you can safely eliminate the ignition as the source of this problem.

My $ is on a fuel delivery issue because you said the primary float bowl was empty.  Everything you said points towards that.

I was told that the primary (front) bowl will be empty when at high RPM and since it dies at high RPM it would be empty in that case and it will be running on the rear bowl.
is this correct?

Not correct

Pressure is the result of resistance to flow.  As fuel fills the float bowls, buoyancy pushes the float up which closes the needle and seat valve.  When the needle and seat valve is closed, all fuel flow is restricted.  This causes the pressure in the fuel line to come up.

As the engine consumes fuel, the fuel level in the float bowls drop along with the float and the needle valve opens, allowing fuel flow into the float bowls and the process repeats.

The float bowls should remain full at all times, without the fuel pressure overpowering the float and the needle and seat valve.

If fuel flow into the float bowls is insufficient to keep the fuel level above the jets and other metering orifices in the metering blocks/plates, the engine will lean out.  If it happens while you got your foot in it, you could melt down the engine. 

Do i dare say run it with the gas cap loose? If it still does it its not a vent issue

Sometimes we iver look the easy stuff Flusher

Ya I thought about running it with the cap open but out there on the lake a full throttle I didn't want to take any chances on water getting in the tank or fuel going everywhere in the boat or on people!

What size fueling are you running, from the tanks to the T to the fuel pump? Also if you have a Holley carb they have a brass filter in the bowl right where the fuel line hooks up, it might be plugged, running the boat with the caps open will determine if it is a vent problem, just don't do it in Havasu 😉


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