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Jeff"Yammer"

Fuel system

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I am completely re-rigging my STV this year. I am unhappy with the fuel tank operation when it gets to 1/3 full ( 2/3 empty if you prefer ) During hard decelleration sometimes the pick-up will run dry !! I was thinking of getting a bulk-head through-hull -8AN fitting and trying to mount it in the back of the tank ( the reason for that is because I want the pick-up to be at the same level or higher than the pump. The pick-up in the STV tank comes from the top down to the bottom of the tank, which means my pump has to pull the fuel up from the tank then down to the level of the pump ) , then to a Seebold vapor tank, then to the Kinsler filter/Weldon pump then to the fuel rail.

My question's are ............. (1) do I need to install a check valve before the vapor tank ? (2) Has any one ever put a different pick-up in an STV 28gal tank like what I am describing ?

Thanks ..... Yammer

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Jeff,

First off, Happy Easter!

This has been covered here and on S&F several times in the past, but I will repeat it again since many don't understand how the fuel system works.

If you are not racing, you should not be running the tank empty. You don't do it in a car, and you sure don't do it in a diesel truck or car - and neither of these sees the fuel slosh as much as a boat does. If you are RACING, then use a fuel cell with a sump. If you don't filter your fuel BEFORE it goes into the tank, any debris will get sucked into the fuel system when you run it dry.

If your fuel pump is at or below the fuel level in the tank, your fuel pump is hardly working at all - think about the siphoning action. The only times it has to pull fuel is when the pick up is not submerged. A "duckfoot" or bell mouth can be added to the end of the pick up but should not be closer to the bottom of the tank than the diameter of the "duckfoot/bellmouth".

Another old trick is to add a short length of flexible fuel line to the fuel pick up and to add a weight to the end of the fuel line. The FLEXIBLE fuel line will move around with the sloshing fuel in the bottom of the tank and help reduce sucking up air.

If you are going to race or want to run a race fuel system like you mentioned - so you can run the fuel tank lower, DO NOT put a restriction like a check ball in front of the fuel pump! A vapor tank is really called a vapor elimination tank or surge tank. If you do suck air into the fuel system with a vapor tank, no harm is done. It simply gets returned to the fuel tank.

If you are using a vapor tank, its PRIMARY mission is as a surge tank - to absorb the times when the fuel pick up is not submerged when the boat is under hard acceleration (fore/back, side-to-side), and when the fuel demand is high. The feeder pump should fill the vapor tank, this is its ONLY job. If you match the size of the vapor tank to your needs, the feeder pump does not need to flow quite as much fuel as the primary pump.

The bottom of the surge tank feeds the primary fuel pump, that pressurizes the fuel, sends it to a good filter which feeds the engine.

The primary pump must be matched to the fuel demands of your engine. If the pump is rated for 450 hp for a four-stroke engine, you need a LOT more capacity for a 300 hp two-stroke engine - nearly double the four-stroke flow rating - especially for a high RPM engine.

The return line from the fuel rail should empty into the top of the surge tank. A low pressure check ball (like the 3 psi ones from Kinsler and others) should be mounted on top of the surge tank and used to route fuel back to the top of the tank near the pick up. This will keep the surge tank pressurized at low pressure (so the feeder pump is hardly working) and allows any air (vapor) pulled into the system to return back to the top of the fuel tank.

If you do not use a check ball between the vapor tank and the fuel tank, the feeder pump needs to flow as much fuel as the primary pump. If not, the vapor tank can run dry under large fuel demands. A low pressure check ball is much cheaper than a bigger fuel pump in many cases.

[Realize that fuel has weight. When you accelerate, the fuel surges toward the rear of the boat. In a car the fuel tank is generally mounted in the rear of a car and the engine is in the front. In this case the fuel pump has to overcome the added weight of the fuel under acceleration. - Just someting to think about.]

Also, MAKE SURE your fuel tank has a vent big enough to allow as much air to enter the tank as the amount of fuel being pulled out of it. You would be amazed at the number of guys who don't do this and then have fuel system issues....and blame the fuel pump. A simple length of fuel line mounted to the top of the tank with 1 or 2 coils will flow well and not allow fuel to splash out since the coils act like a trap.

This type of fuel system design is used in many types of racing and allows the tank to be run completely dry before the fuel system sends any air to the engine, since the vapor/surge tank and fuel lines are still full when the tank runs dry.

I hope this helps.

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Here is what I've done so far.

An AN-10 sump fitting mounted in the bottom/back of the tank, then to the Seebold vapor can, then to the Kinsler filter/Weldon pump ( Hey Wayne got any more deals on 2015's hahaha ) to the fuel rail/reg. , back to the vapor can. The vapor can will be vented to the main tank.

This is how our Champboat is rigged and every other Seebold. The main question is .......... Do I need the check valve ( not a ball check .... a check valve ) in front of the vapor tank or not. I have seen them with and without them installed.

And Happy Easter to you Wayne and every one else !!!!!

Your opinions are valued greatly !!

Jeff

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Jeff,

Since you are not using a pump to feed the vapor tank, I assume that the entire vapor tank is below the bottom of the fuel tank - otherwise, it will not fill completely using gravity.

If it is not completely below the fuel tank, a check valve will allow the return from the fuel rail to fill the vapor tank if the fuel pump has enough flow. Make sure the flow of the check valve is NOT a restriction to the fuel pump.

If the vapor tank is mounted level with the bottom of the fuel tank, you are not accomplishing much with the vapor tank that couldn't be done with a baffled sump in the fuel tank.

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